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PYSEC-2026-3733
Vulnerability from pysec - Published: 2026-08-25 02:16 - Updated: 2026-09-01 08:54NLTK before 3.10.3 contains a server-side request forgery vulnerability in nltk.pathsec.urlopen (and callers nltk.data.load, nltk.downloader.Downloader.index/download) when an HTTP proxy is configured. pathsec.urlopen validates the requested hostname locally, but proxy-handler inheritance disables the safe HTTP/HTTPS handlers so the actual fetch is performed by the proxy against a destination that is never re-validated. An attacker can supply a validated public URL that the proxy forwards to an internal loopback-only service, allowing disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker-chosen package content.
| Name | purl | nltk | pkg:pypi/nltk |
|---|
{
"affected": [
{
"ecosystem_specific": {},
"package": {
"ecosystem": "PyPI",
"name": "nltk",
"purl": "pkg:pypi/nltk"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.10.3"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"0.8",
"0.9",
"0.9.3",
"0.9.4",
"0.9.5",
"0.9.6",
"0.9.7",
"0.9.8",
"0.9.9",
"2.0.1",
"2.0.1rc1",
"2.0.1rc2-git",
"2.0.1rc3",
"2.0.1rc4",
"2.0.2",
"2.0.3",
"2.0.4",
"2.0.5",
"2.0b4",
"2.0b5",
"2.0b6",
"2.0b7",
"2.0b8",
"2.0b9",
"3.0.0",
"3.0.0b1",
"3.0.0b2",
"3.0.1",
"3.0.2",
"3.0.3",
"3.0.4",
"3.0.5",
"3.1",
"3.10.0",
"3.10.1",
"3.10.2",
"3.2",
"3.2.1",
"3.2.2",
"3.2.3",
"3.2.4",
"3.2.5",
"3.3",
"3.4",
"3.4.1",
"3.4.2",
"3.4.3",
"3.4.4",
"3.4.5",
"3.5",
"3.5b1",
"3.6",
"3.6.1",
"3.6.2",
"3.6.3",
"3.6.4",
"3.6.5",
"3.6.6",
"3.6.7",
"3.7",
"3.8",
"3.8.1",
"3.9",
"3.9.1",
"3.9.2",
"3.9.3",
"3.9.4",
"3.9b1"
]
}
],
"aliases": [
"CVE-2026-78682",
"GHSA-6ww7-3frv-cqxh"
],
"details": "NLTK before 3.10.3 contains a server-side request forgery vulnerability in nltk.pathsec.urlopen (and callers nltk.data.load, nltk.downloader.Downloader.index/download) when an HTTP proxy is configured. pathsec.urlopen validates the requested hostname locally, but proxy-handler inheritance disables the safe HTTP/HTTPS handlers so the actual fetch is performed by the proxy against a destination that is never re-validated. An attacker can supply a validated public URL that the proxy forwards to an internal loopback-only service, allowing disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker-chosen package content.",
"id": "PYSEC-2026-3733",
"modified": "2026-09-01T08:54:51.899146Z",
"published": "2026-08-25T02:16:52.890Z",
"references": [
{
"type": "ADVISORY",
"url": "https://www.vulncheck.com/advisories/nltk-before-ssrf-protection-bypass-via-proxy"
},
{
"type": "EVIDENCE",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-6ww7-3frv-cqxh"
}
],
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
"type": "CVSS_V4"
}
]
}
BREW-ACRONYM-CVE-2026-78682 (PYSEC-2026-3733)
Vulnerability from osv_homebrew – Published: 2026-09-02 08:43 – Updated: 2026-09-17 18:47 – Source websiteSummary
Current NLTK source reopens SSRF in proxied environments. pathsec.urlopen() validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.
Details
- Vulnerability type: Server-side request forgery
- Affected component:
nltk.pathsec.urlopen,nltk.data.load,nltk.downloader.Downloader.index,nltk.downloader.Downloader.download - Affected versions: Current source
v3.10.0-rc2; published3.9.4was a negative control and did not reproduce. - Patched versions: Not yet patched
- Root cause: Proxy-handler inheritance disables
_SafeHTTPHandlerand_SafeHTTPSHandler, so the validated hostname no longer matches the actual egress destination.
The hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies ProxyHandler instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via pathsec.urlopen(), nltk.data.load(), Downloader.index(), and Downloader.download().
PoC
Preconditions
- The runtime has an HTTP proxy configured and the caller relies on pathsec to keep network fetches SSRF-safe.
Steps
1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.
2. Configure a proxy that forwards a validated public URL to that internal loopback service.
3. Call pathsec.urlopen() or nltk.data.load() on the public URL and observe the internal response is returned.
4. Instantiate Downloader(server_index_url=...), call index() and download(), and observe internal-only content is parsed and installed.
Minimal reproducible excerpt
{'urlopen': 'PROXY_TEXT_SECRET', 'data_load': 'PROXY_TEXT_SECRET', 'downloaded_file': 'INTERNAL_ZIP_SECRET'}
Impact
Consumers that trust pathsec as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy's network view.
Remediation
Preserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across pathsec.urlopen, nltk.data.load, and downloader fetches with a configured proxy.
References
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226
- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250
Fix + attack demonstration (verified)
NLTK cannot pin the egress through a proxy, so it stops pretending to: under ENFORCE a proxied fetch is refused rather than performed unvalidated. Operators who trust their proxy opt back in with NLTK_ALLOW_PROXIED_URLOPEN=1 or nltk.pathsec.ALLOW_PROXIED_FETCH=True; under ENFORCE=False the refusal degrades to a warning. This closes the whole class (environment proxies and explicit ProxyHandler alike), because NLTK declines any fetch whose egress it cannot validate.
Attack demonstration (reproduced; captured output)
A loopback HTTP server stands in for the internal target; http_proxy points at it; NLTK is asked for a public IP URL.
Before the fix — the internal secret is exfiltrated through the proxy:
validate_network_url(public): PASSED
*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: 'INTERNAL_ONLY_SECRET'
After the fix — five scenarios, isolated subprocesses:
| Scenario | Result |
|---|---|
| proxied (env) + ENFORCE | PermissionError — blocked |
| proxied + opt-in | returns secret — escape hatch works |
| explicit ProxyHandler (not env) + ENFORCE | PermissionError — blocked (whole class) |
| no proxy (direct) | internal IP still refused — pinning intact |
| proxied + ENFORCE=False | returns secret + warns |
Tests
nltk/test/unit/test_pathsec.py: 64 passed. Added an end-to-end regression (test_proxied_fetch_does_not_reach_internal_target) plus test_env_proxy_fails_closed_under_enforce; the prior test_env_proxy_skips_pinning_handlers (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.
Note
The upfront validate_network_url() and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged — this only closes the proxied downgrade they didn't cover.
{
"affected": [
{
"ecosystem_specific": {
"fix": "bump",
"range_state": "fixed",
"resource": "nltk",
"resource_purl": "pkg:pypi/nltk@3.10.3",
"upstream_fixed_in": "3.10.3"
},
"package": {
"ecosystem": "Homebrew",
"name": "acronym",
"purl": "pkg:brew/acronym"
},
"ranges": [
{
"events": [
{
"introduced": "2.0.0"
},
{
"fixed": "2.0.0_5"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"database_specific": {
"confidence": "high",
"source": "matched",
"strategy": "registry",
"upstream_evidence": [
{
"ecosystem": "PyPI",
"key": "pkg:pypi/nltk@3.10.3",
"name": "nltk",
"resource": "nltk",
"strategy": "registry",
"subject_version": "3.10.3"
}
]
},
"details": "### Summary\n\nCurrent NLTK source reopens SSRF in proxied environments. `pathsec.urlopen()` validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.\n\n### Details\n\n- **Vulnerability type:** Server-side request forgery\n- **Affected component:** `nltk.pathsec.urlopen`, `nltk.data.load`, `nltk.downloader.Downloader.index`, `nltk.downloader.Downloader.download`\n- **Affected versions:** Current source `v3.10.0-rc2`; published `3.9.4` was a negative control and did not reproduce.\n- **Patched versions:** Not yet patched\n- **Root cause:** Proxy-handler inheritance disables `_SafeHTTPHandler` and `_SafeHTTPSHandler`, so the validated hostname no longer matches the actual egress destination.\n\nThe hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies `ProxyHandler` instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via `pathsec.urlopen()`, `nltk.data.load()`, `Downloader.index()`, and `Downloader.download()`.\n\n### PoC\n\n**Preconditions**\n- The runtime has an HTTP proxy configured and the caller relies on `pathsec` to keep network fetches SSRF-safe.\n\n**Steps**\n1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.\n2. Configure a proxy that forwards a validated public URL to that internal loopback service.\n3. Call `pathsec.urlopen()` or `nltk.data.load()` on the public URL and observe the internal response is returned.\n4. Instantiate `Downloader(server_index_url=...)`, call `index()` and `download()`, and observe internal-only content is parsed and installed.\n\n**Minimal reproducible excerpt**\n\n```text\n{\u0027urlopen\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027data_load\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027downloaded_file\u0027: \u0027INTERNAL_ZIP_SECRET\u0027}\n```\n\n### Impact\n\nConsumers that trust `pathsec` as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy\u0027s network view.\n\n### Remediation\n\nPreserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across `pathsec.urlopen`, `nltk.data.load`, and downloader fetches with a configured proxy.\n\n### References\n\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226\n- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250\n\n---\n\n## Fix + attack demonstration (verified)\n\n\nNLTK cannot pin the egress through a proxy, so it stops pretending to: under `ENFORCE` a proxied fetch is **refused** rather than performed unvalidated. Operators who trust their proxy opt back in with `NLTK_ALLOW_PROXIED_URLOPEN=1` or `nltk.pathsec.ALLOW_PROXIED_FETCH=True`; under `ENFORCE=False` the refusal degrades to a warning. This closes the **whole class** (environment proxies and explicit `ProxyHandler` alike), because NLTK declines any fetch whose egress it cannot validate.\n\n## Attack demonstration (reproduced; captured output)\nA loopback HTTP server stands in for the internal target; `http_proxy` points at it; NLTK is asked for a **public** IP URL.\n\n**Before the fix** \u2014 the internal secret is exfiltrated through the proxy:\n```\nvalidate_network_url(public): PASSED\n*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: \u0027INTERNAL_ONLY_SECRET\u0027\n```\n\n**After the fix** \u2014 five scenarios, isolated subprocesses:\n| Scenario | Result |\n|---|---|\n| proxied (env) + ENFORCE | `PermissionError` \u2014 **blocked** |\n| proxied + opt-in | returns secret \u2014 escape hatch works |\n| explicit `ProxyHandler` (not env) + ENFORCE | `PermissionError` \u2014 **blocked** (whole class) |\n| no proxy (direct) | internal IP still refused \u2014 pinning intact |\n| proxied + `ENFORCE=False` | returns secret **+ warns** |\n\n## Tests\n`nltk/test/unit/test_pathsec.py`: 64 passed. Added an end-to-end regression (`test_proxied_fetch_does_not_reach_internal_target`) plus `test_env_proxy_fails_closed_under_enforce`; the prior `test_env_proxy_skips_pinning_handlers` (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.\n\n## Note\nThe upfront `validate_network_url()` and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged \u2014 this only closes the proxied downgrade they didn\u0027t cover.",
"id": "BREW-acronym-CVE-2026-78682",
"modified": "2026-09-17T18:47:55Z",
"published": "2026-09-02T08:43:54Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-6ww7-3frv-cqxh"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-78682"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/767333a005a1cd3d82d2029215f2dbe66a5844d9"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3733.yaml"
},
{
"type": "WEB",
"url": "https://www.vulncheck.com/advisories/nltk-before-ssrf-protection-bypass-via-proxy"
}
],
"schema_version": "1.7.3",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "NLTK: pathsec SSRF protection can be bypassed when a proxy is configured",
"upstream": [
"PYSEC-2026-3733",
"CVE-2026-78682",
"GHSA-6ww7-3frv-cqxh"
]
}
BREW-GPTLINE-CVE-2026-78682 (PYSEC-2026-3733)
Vulnerability from osv_homebrew – Published: 2026-09-02 09:04 – Updated: 2026-09-17 19:32 – Source websiteSummary
Current NLTK source reopens SSRF in proxied environments. pathsec.urlopen() validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.
Details
- Vulnerability type: Server-side request forgery
- Affected component:
nltk.pathsec.urlopen,nltk.data.load,nltk.downloader.Downloader.index,nltk.downloader.Downloader.download - Affected versions: Current source
v3.10.0-rc2; published3.9.4was a negative control and did not reproduce. - Patched versions: Not yet patched
- Root cause: Proxy-handler inheritance disables
_SafeHTTPHandlerand_SafeHTTPSHandler, so the validated hostname no longer matches the actual egress destination.
The hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies ProxyHandler instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via pathsec.urlopen(), nltk.data.load(), Downloader.index(), and Downloader.download().
PoC
Preconditions
- The runtime has an HTTP proxy configured and the caller relies on pathsec to keep network fetches SSRF-safe.
Steps
1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.
2. Configure a proxy that forwards a validated public URL to that internal loopback service.
3. Call pathsec.urlopen() or nltk.data.load() on the public URL and observe the internal response is returned.
4. Instantiate Downloader(server_index_url=...), call index() and download(), and observe internal-only content is parsed and installed.
Minimal reproducible excerpt
{'urlopen': 'PROXY_TEXT_SECRET', 'data_load': 'PROXY_TEXT_SECRET', 'downloaded_file': 'INTERNAL_ZIP_SECRET'}
Impact
Consumers that trust pathsec as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy's network view.
Remediation
Preserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across pathsec.urlopen, nltk.data.load, and downloader fetches with a configured proxy.
References
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226
- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250
Fix + attack demonstration (verified)
NLTK cannot pin the egress through a proxy, so it stops pretending to: under ENFORCE a proxied fetch is refused rather than performed unvalidated. Operators who trust their proxy opt back in with NLTK_ALLOW_PROXIED_URLOPEN=1 or nltk.pathsec.ALLOW_PROXIED_FETCH=True; under ENFORCE=False the refusal degrades to a warning. This closes the whole class (environment proxies and explicit ProxyHandler alike), because NLTK declines any fetch whose egress it cannot validate.
Attack demonstration (reproduced; captured output)
A loopback HTTP server stands in for the internal target; http_proxy points at it; NLTK is asked for a public IP URL.
Before the fix — the internal secret is exfiltrated through the proxy:
validate_network_url(public): PASSED
*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: 'INTERNAL_ONLY_SECRET'
After the fix — five scenarios, isolated subprocesses:
| Scenario | Result |
|---|---|
| proxied (env) + ENFORCE | PermissionError — blocked |
| proxied + opt-in | returns secret — escape hatch works |
| explicit ProxyHandler (not env) + ENFORCE | PermissionError — blocked (whole class) |
| no proxy (direct) | internal IP still refused — pinning intact |
| proxied + ENFORCE=False | returns secret + warns |
Tests
nltk/test/unit/test_pathsec.py: 64 passed. Added an end-to-end regression (test_proxied_fetch_does_not_reach_internal_target) plus test_env_proxy_fails_closed_under_enforce; the prior test_env_proxy_skips_pinning_handlers (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.
Note
The upfront validate_network_url() and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged — this only closes the proxied downgrade they didn't cover.
| URL | Type | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|||||||||||||||||||||||
{
"affected": [
{
"ecosystem_specific": {
"fix": "bump",
"range_state": "fixed",
"resource": "nltk",
"resource_purl": "pkg:pypi/nltk@3.10.3",
"upstream_fixed_in": "3.10.3"
},
"package": {
"ecosystem": "Homebrew",
"name": "gptline",
"purl": "pkg:brew/gptline"
},
"ranges": [
{
"events": [
{
"introduced": "1.0.8"
},
{
"fixed": "1.0.8_23"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"database_specific": {
"confidence": "high",
"source": "matched",
"strategy": "registry",
"upstream_evidence": [
{
"ecosystem": "PyPI",
"key": "pkg:pypi/nltk@3.10.3",
"name": "nltk",
"resource": "nltk",
"strategy": "registry",
"subject_version": "3.10.3"
}
]
},
"details": "### Summary\n\nCurrent NLTK source reopens SSRF in proxied environments. `pathsec.urlopen()` validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.\n\n### Details\n\n- **Vulnerability type:** Server-side request forgery\n- **Affected component:** `nltk.pathsec.urlopen`, `nltk.data.load`, `nltk.downloader.Downloader.index`, `nltk.downloader.Downloader.download`\n- **Affected versions:** Current source `v3.10.0-rc2`; published `3.9.4` was a negative control and did not reproduce.\n- **Patched versions:** Not yet patched\n- **Root cause:** Proxy-handler inheritance disables `_SafeHTTPHandler` and `_SafeHTTPSHandler`, so the validated hostname no longer matches the actual egress destination.\n\nThe hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies `ProxyHandler` instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via `pathsec.urlopen()`, `nltk.data.load()`, `Downloader.index()`, and `Downloader.download()`.\n\n### PoC\n\n**Preconditions**\n- The runtime has an HTTP proxy configured and the caller relies on `pathsec` to keep network fetches SSRF-safe.\n\n**Steps**\n1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.\n2. Configure a proxy that forwards a validated public URL to that internal loopback service.\n3. Call `pathsec.urlopen()` or `nltk.data.load()` on the public URL and observe the internal response is returned.\n4. Instantiate `Downloader(server_index_url=...)`, call `index()` and `download()`, and observe internal-only content is parsed and installed.\n\n**Minimal reproducible excerpt**\n\n```text\n{\u0027urlopen\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027data_load\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027downloaded_file\u0027: \u0027INTERNAL_ZIP_SECRET\u0027}\n```\n\n### Impact\n\nConsumers that trust `pathsec` as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy\u0027s network view.\n\n### Remediation\n\nPreserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across `pathsec.urlopen`, `nltk.data.load`, and downloader fetches with a configured proxy.\n\n### References\n\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226\n- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250\n\n---\n\n## Fix + attack demonstration (verified)\n\n\nNLTK cannot pin the egress through a proxy, so it stops pretending to: under `ENFORCE` a proxied fetch is **refused** rather than performed unvalidated. Operators who trust their proxy opt back in with `NLTK_ALLOW_PROXIED_URLOPEN=1` or `nltk.pathsec.ALLOW_PROXIED_FETCH=True`; under `ENFORCE=False` the refusal degrades to a warning. This closes the **whole class** (environment proxies and explicit `ProxyHandler` alike), because NLTK declines any fetch whose egress it cannot validate.\n\n## Attack demonstration (reproduced; captured output)\nA loopback HTTP server stands in for the internal target; `http_proxy` points at it; NLTK is asked for a **public** IP URL.\n\n**Before the fix** \u2014 the internal secret is exfiltrated through the proxy:\n```\nvalidate_network_url(public): PASSED\n*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: \u0027INTERNAL_ONLY_SECRET\u0027\n```\n\n**After the fix** \u2014 five scenarios, isolated subprocesses:\n| Scenario | Result |\n|---|---|\n| proxied (env) + ENFORCE | `PermissionError` \u2014 **blocked** |\n| proxied + opt-in | returns secret \u2014 escape hatch works |\n| explicit `ProxyHandler` (not env) + ENFORCE | `PermissionError` \u2014 **blocked** (whole class) |\n| no proxy (direct) | internal IP still refused \u2014 pinning intact |\n| proxied + `ENFORCE=False` | returns secret **+ warns** |\n\n## Tests\n`nltk/test/unit/test_pathsec.py`: 64 passed. Added an end-to-end regression (`test_proxied_fetch_does_not_reach_internal_target`) plus `test_env_proxy_fails_closed_under_enforce`; the prior `test_env_proxy_skips_pinning_handlers` (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.\n\n## Note\nThe upfront `validate_network_url()` and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged \u2014 this only closes the proxied downgrade they didn\u0027t cover.",
"id": "BREW-gptline-CVE-2026-78682",
"modified": "2026-09-17T19:32:01Z",
"published": "2026-09-02T09:04:32Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-6ww7-3frv-cqxh"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-78682"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/767333a005a1cd3d82d2029215f2dbe66a5844d9"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3733.yaml"
},
{
"type": "WEB",
"url": "https://www.vulncheck.com/advisories/nltk-before-ssrf-protection-bypass-via-proxy"
}
],
"schema_version": "1.7.3",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "NLTK: pathsec SSRF protection can be bypassed when a proxy is configured",
"upstream": [
"PYSEC-2026-3733",
"CVE-2026-78682",
"GHSA-6ww7-3frv-cqxh"
]
}
BREW-SAFETY-CVE-2026-78682 (PYSEC-2026-3733)
Vulnerability from osv_homebrew – Published: 2026-09-02 09:53 – Updated: 2026-09-17 17:35 – Source websiteSummary
Current NLTK source reopens SSRF in proxied environments. pathsec.urlopen() validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.
Details
- Vulnerability type: Server-side request forgery
- Affected component:
nltk.pathsec.urlopen,nltk.data.load,nltk.downloader.Downloader.index,nltk.downloader.Downloader.download - Affected versions: Current source
v3.10.0-rc2; published3.9.4was a negative control and did not reproduce. - Patched versions: Not yet patched
- Root cause: Proxy-handler inheritance disables
_SafeHTTPHandlerand_SafeHTTPSHandler, so the validated hostname no longer matches the actual egress destination.
The hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies ProxyHandler instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via pathsec.urlopen(), nltk.data.load(), Downloader.index(), and Downloader.download().
PoC
Preconditions
- The runtime has an HTTP proxy configured and the caller relies on pathsec to keep network fetches SSRF-safe.
Steps
1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.
2. Configure a proxy that forwards a validated public URL to that internal loopback service.
3. Call pathsec.urlopen() or nltk.data.load() on the public URL and observe the internal response is returned.
4. Instantiate Downloader(server_index_url=...), call index() and download(), and observe internal-only content is parsed and installed.
Minimal reproducible excerpt
{'urlopen': 'PROXY_TEXT_SECRET', 'data_load': 'PROXY_TEXT_SECRET', 'downloaded_file': 'INTERNAL_ZIP_SECRET'}
Impact
Consumers that trust pathsec as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy's network view.
Remediation
Preserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across pathsec.urlopen, nltk.data.load, and downloader fetches with a configured proxy.
References
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226
- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250
Fix + attack demonstration (verified)
NLTK cannot pin the egress through a proxy, so it stops pretending to: under ENFORCE a proxied fetch is refused rather than performed unvalidated. Operators who trust their proxy opt back in with NLTK_ALLOW_PROXIED_URLOPEN=1 or nltk.pathsec.ALLOW_PROXIED_FETCH=True; under ENFORCE=False the refusal degrades to a warning. This closes the whole class (environment proxies and explicit ProxyHandler alike), because NLTK declines any fetch whose egress it cannot validate.
Attack demonstration (reproduced; captured output)
A loopback HTTP server stands in for the internal target; http_proxy points at it; NLTK is asked for a public IP URL.
Before the fix — the internal secret is exfiltrated through the proxy:
validate_network_url(public): PASSED
*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: 'INTERNAL_ONLY_SECRET'
After the fix — five scenarios, isolated subprocesses:
| Scenario | Result |
|---|---|
| proxied (env) + ENFORCE | PermissionError — blocked |
| proxied + opt-in | returns secret — escape hatch works |
| explicit ProxyHandler (not env) + ENFORCE | PermissionError — blocked (whole class) |
| no proxy (direct) | internal IP still refused — pinning intact |
| proxied + ENFORCE=False | returns secret + warns |
Tests
nltk/test/unit/test_pathsec.py: 64 passed. Added an end-to-end regression (test_proxied_fetch_does_not_reach_internal_target) plus test_env_proxy_fails_closed_under_enforce; the prior test_env_proxy_skips_pinning_handlers (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.
Note
The upfront validate_network_url() and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged — this only closes the proxied downgrade they didn't cover.
| URL | Type | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|||||||||||||||||||||||
{
"affected": [
{
"ecosystem_specific": {
"fix": "bump",
"range_state": "fixed",
"resource": "nltk",
"resource_purl": "pkg:pypi/nltk@3.10.3",
"upstream_fixed_in": "3.10.3"
},
"package": {
"ecosystem": "Homebrew",
"name": "safety",
"purl": "pkg:brew/safety"
},
"ranges": [
{
"events": [
{
"introduced": "3.3.1"
},
{
"fixed": "3.8.1_2"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"database_specific": {
"confidence": "high",
"source": "matched",
"strategy": "registry",
"upstream_evidence": [
{
"ecosystem": "PyPI",
"key": "pkg:pypi/nltk@3.10.3",
"name": "nltk",
"resource": "nltk",
"strategy": "registry",
"subject_version": "3.10.3"
}
]
},
"details": "### Summary\n\nCurrent NLTK source reopens SSRF in proxied environments. `pathsec.urlopen()` validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.\n\n### Details\n\n- **Vulnerability type:** Server-side request forgery\n- **Affected component:** `nltk.pathsec.urlopen`, `nltk.data.load`, `nltk.downloader.Downloader.index`, `nltk.downloader.Downloader.download`\n- **Affected versions:** Current source `v3.10.0-rc2`; published `3.9.4` was a negative control and did not reproduce.\n- **Patched versions:** Not yet patched\n- **Root cause:** Proxy-handler inheritance disables `_SafeHTTPHandler` and `_SafeHTTPSHandler`, so the validated hostname no longer matches the actual egress destination.\n\nThe hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies `ProxyHandler` instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via `pathsec.urlopen()`, `nltk.data.load()`, `Downloader.index()`, and `Downloader.download()`.\n\n### PoC\n\n**Preconditions**\n- The runtime has an HTTP proxy configured and the caller relies on `pathsec` to keep network fetches SSRF-safe.\n\n**Steps**\n1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.\n2. Configure a proxy that forwards a validated public URL to that internal loopback service.\n3. Call `pathsec.urlopen()` or `nltk.data.load()` on the public URL and observe the internal response is returned.\n4. Instantiate `Downloader(server_index_url=...)`, call `index()` and `download()`, and observe internal-only content is parsed and installed.\n\n**Minimal reproducible excerpt**\n\n```text\n{\u0027urlopen\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027data_load\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027downloaded_file\u0027: \u0027INTERNAL_ZIP_SECRET\u0027}\n```\n\n### Impact\n\nConsumers that trust `pathsec` as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy\u0027s network view.\n\n### Remediation\n\nPreserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across `pathsec.urlopen`, `nltk.data.load`, and downloader fetches with a configured proxy.\n\n### References\n\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226\n- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250\n\n---\n\n## Fix + attack demonstration (verified)\n\n\nNLTK cannot pin the egress through a proxy, so it stops pretending to: under `ENFORCE` a proxied fetch is **refused** rather than performed unvalidated. Operators who trust their proxy opt back in with `NLTK_ALLOW_PROXIED_URLOPEN=1` or `nltk.pathsec.ALLOW_PROXIED_FETCH=True`; under `ENFORCE=False` the refusal degrades to a warning. This closes the **whole class** (environment proxies and explicit `ProxyHandler` alike), because NLTK declines any fetch whose egress it cannot validate.\n\n## Attack demonstration (reproduced; captured output)\nA loopback HTTP server stands in for the internal target; `http_proxy` points at it; NLTK is asked for a **public** IP URL.\n\n**Before the fix** \u2014 the internal secret is exfiltrated through the proxy:\n```\nvalidate_network_url(public): PASSED\n*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: \u0027INTERNAL_ONLY_SECRET\u0027\n```\n\n**After the fix** \u2014 five scenarios, isolated subprocesses:\n| Scenario | Result |\n|---|---|\n| proxied (env) + ENFORCE | `PermissionError` \u2014 **blocked** |\n| proxied + opt-in | returns secret \u2014 escape hatch works |\n| explicit `ProxyHandler` (not env) + ENFORCE | `PermissionError` \u2014 **blocked** (whole class) |\n| no proxy (direct) | internal IP still refused \u2014 pinning intact |\n| proxied + `ENFORCE=False` | returns secret **+ warns** |\n\n## Tests\n`nltk/test/unit/test_pathsec.py`: 64 passed. Added an end-to-end regression (`test_proxied_fetch_does_not_reach_internal_target`) plus `test_env_proxy_fails_closed_under_enforce`; the prior `test_env_proxy_skips_pinning_handlers` (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.\n\n## Note\nThe upfront `validate_network_url()` and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged \u2014 this only closes the proxied downgrade they didn\u0027t cover.",
"id": "BREW-safety-CVE-2026-78682",
"modified": "2026-09-17T17:35:56Z",
"published": "2026-09-02T09:53:14Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-6ww7-3frv-cqxh"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-78682"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/767333a005a1cd3d82d2029215f2dbe66a5844d9"
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{
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"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3733.yaml"
},
{
"type": "WEB",
"url": "https://www.vulncheck.com/advisories/nltk-before-ssrf-protection-bypass-via-proxy"
}
],
"schema_version": "1.7.3",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "NLTK: pathsec SSRF protection can be bypassed when a proxy is configured",
"upstream": [
"PYSEC-2026-3733",
"CVE-2026-78682",
"GHSA-6ww7-3frv-cqxh"
]
}
CVE-2026-78682 (GCVE-0-2026-78682)
Vulnerability from cvelistv5 – Published: 2026-08-25 01:30 – Updated: 2026-10-08 15:25- CWE-918 - Server-Side Request Forgery (SSRF)
| URL | Tags |
|---|---|
| https://github.com/nltk/nltk/security/advisories/… | vendor-advisory |
| https://www.vulncheck.com/advisories/nltk-before-… | third-party-advisory |
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],
"title": "NLTK before 3.10.3 SSRF Protection Bypass via Proxy",
"x_generator": {
"engine": "vulncheck-endgame"
}
}
},
"cveMetadata": {
"assignerOrgId": "83251b91-4cc7-4094-a5c7-464a1b83ea10",
"assignerShortName": "VulnCheck",
"cveId": "CVE-2026-78682",
"datePublished": "2026-08-25T01:30:38.486Z",
"dateReserved": "2026-08-25T01:17:12.263Z",
"dateUpdated": "2026-10-08T15:25:10.277Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
GHSA-6WW7-3FRV-CQXH
Vulnerability from github – Published: 2026-09-08 16:41 – Updated: 2026-09-08 16:41Summary
Current NLTK source reopens SSRF in proxied environments. pathsec.urlopen() validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.
Details
- Vulnerability type: Server-side request forgery
- Affected component:
nltk.pathsec.urlopen,nltk.data.load,nltk.downloader.Downloader.index,nltk.downloader.Downloader.download - Affected versions: Current source
v3.10.0-rc2; published3.9.4was a negative control and did not reproduce. - Patched versions: Not yet patched
- Root cause: Proxy-handler inheritance disables
_SafeHTTPHandlerand_SafeHTTPSHandler, so the validated hostname no longer matches the actual egress destination.
The hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies ProxyHandler instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via pathsec.urlopen(), nltk.data.load(), Downloader.index(), and Downloader.download().
PoC
Preconditions
- The runtime has an HTTP proxy configured and the caller relies on pathsec to keep network fetches SSRF-safe.
Steps
1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.
2. Configure a proxy that forwards a validated public URL to that internal loopback service.
3. Call pathsec.urlopen() or nltk.data.load() on the public URL and observe the internal response is returned.
4. Instantiate Downloader(server_index_url=...), call index() and download(), and observe internal-only content is parsed and installed.
Minimal reproducible excerpt
{'urlopen': 'PROXY_TEXT_SECRET', 'data_load': 'PROXY_TEXT_SECRET', 'downloaded_file': 'INTERNAL_ZIP_SECRET'}
Impact
Consumers that trust pathsec as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy's network view.
Remediation
Preserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across pathsec.urlopen, nltk.data.load, and downloader fetches with a configured proxy.
References
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226
- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250
Fix + attack demonstration (verified)
NLTK cannot pin the egress through a proxy, so it stops pretending to: under ENFORCE a proxied fetch is refused rather than performed unvalidated. Operators who trust their proxy opt back in with NLTK_ALLOW_PROXIED_URLOPEN=1 or nltk.pathsec.ALLOW_PROXIED_FETCH=True; under ENFORCE=False the refusal degrades to a warning. This closes the whole class (environment proxies and explicit ProxyHandler alike), because NLTK declines any fetch whose egress it cannot validate.
Attack demonstration (reproduced; captured output)
A loopback HTTP server stands in for the internal target; http_proxy points at it; NLTK is asked for a public IP URL.
Before the fix — the internal secret is exfiltrated through the proxy:
validate_network_url(public): PASSED
*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: 'INTERNAL_ONLY_SECRET'
After the fix — five scenarios, isolated subprocesses:
| Scenario | Result |
|---|---|
| proxied (env) + ENFORCE | PermissionError — blocked |
| proxied + opt-in | returns secret — escape hatch works |
| explicit ProxyHandler (not env) + ENFORCE | PermissionError — blocked (whole class) |
| no proxy (direct) | internal IP still refused — pinning intact |
| proxied + ENFORCE=False | returns secret + warns |
Tests
nltk/test/unit/test_pathsec.py: 64 passed. Added an end-to-end regression (test_proxied_fetch_does_not_reach_internal_target) plus test_env_proxy_fails_closed_under_enforce; the prior test_env_proxy_skips_pinning_handlers (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.
Note
The upfront validate_network_url() and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged — this only closes the proxied downgrade they didn't cover.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 3.10.2"
},
"package": {
"ecosystem": "PyPI",
"name": "nltk"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.10.3"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-78682"
],
"database_specific": {
"cwe_ids": [
"CWE-441",
"CWE-918"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-08T16:41:40Z",
"nvd_published_at": null,
"severity": "HIGH"
},
"details": "### Summary\n\nCurrent NLTK source reopens SSRF in proxied environments. `pathsec.urlopen()` validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.\n\n### Details\n\n- **Vulnerability type:** Server-side request forgery\n- **Affected component:** `nltk.pathsec.urlopen`, `nltk.data.load`, `nltk.downloader.Downloader.index`, `nltk.downloader.Downloader.download`\n- **Affected versions:** Current source `v3.10.0-rc2`; published `3.9.4` was a negative control and did not reproduce.\n- **Patched versions:** Not yet patched\n- **Root cause:** Proxy-handler inheritance disables `_SafeHTTPHandler` and `_SafeHTTPSHandler`, so the validated hostname no longer matches the actual egress destination.\n\nThe hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies `ProxyHandler` instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via `pathsec.urlopen()`, `nltk.data.load()`, `Downloader.index()`, and `Downloader.download()`.\n\n### PoC\n\n**Preconditions**\n- The runtime has an HTTP proxy configured and the caller relies on `pathsec` to keep network fetches SSRF-safe.\n\n**Steps**\n1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.\n2. Configure a proxy that forwards a validated public URL to that internal loopback service.\n3. Call `pathsec.urlopen()` or `nltk.data.load()` on the public URL and observe the internal response is returned.\n4. Instantiate `Downloader(server_index_url=...)`, call `index()` and `download()`, and observe internal-only content is parsed and installed.\n\n**Minimal reproducible excerpt**\n\n```text\n{\u0027urlopen\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027data_load\u0027: \u0027PROXY_TEXT_SECRET\u0027, \u0027downloaded_file\u0027: \u0027INTERNAL_ZIP_SECRET\u0027}\n```\n\n### Impact\n\nConsumers that trust `pathsec` as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy\u0027s network view.\n\n### Remediation\n\nPreserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across `pathsec.urlopen`, `nltk.data.load`, and downloader fetches with a configured proxy.\n\n### References\n\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889\n- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226\n- https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250\n\n---\n\n## Fix + attack demonstration (verified)\n\n\nNLTK cannot pin the egress through a proxy, so it stops pretending to: under `ENFORCE` a proxied fetch is **refused** rather than performed unvalidated. Operators who trust their proxy opt back in with `NLTK_ALLOW_PROXIED_URLOPEN=1` or `nltk.pathsec.ALLOW_PROXIED_FETCH=True`; under `ENFORCE=False` the refusal degrades to a warning. This closes the **whole class** (environment proxies and explicit `ProxyHandler` alike), because NLTK declines any fetch whose egress it cannot validate.\n\n## Attack demonstration (reproduced; captured output)\nA loopback HTTP server stands in for the internal target; `http_proxy` points at it; NLTK is asked for a **public** IP URL.\n\n**Before the fix** \u2014 the internal secret is exfiltrated through the proxy:\n```\nvalidate_network_url(public): PASSED\n*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: \u0027INTERNAL_ONLY_SECRET\u0027\n```\n\n**After the fix** \u2014 five scenarios, isolated subprocesses:\n| Scenario | Result |\n|---|---|\n| proxied (env) + ENFORCE | `PermissionError` \u2014 **blocked** |\n| proxied + opt-in | returns secret \u2014 escape hatch works |\n| explicit `ProxyHandler` (not env) + ENFORCE | `PermissionError` \u2014 **blocked** (whole class) |\n| no proxy (direct) | internal IP still refused \u2014 pinning intact |\n| proxied + `ENFORCE=False` | returns secret **+ warns** |\n\n## Tests\n`nltk/test/unit/test_pathsec.py`: 64 passed. Added an end-to-end regression (`test_proxied_fetch_does_not_reach_internal_target`) plus `test_env_proxy_fails_closed_under_enforce`; the prior `test_env_proxy_skips_pinning_handlers` (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.\n\n## Note\nThe upfront `validate_network_url()` and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged \u2014 this only closes the proxied downgrade they didn\u0027t cover.",
"id": "GHSA-6ww7-3frv-cqxh",
"modified": "2026-09-08T16:41:40Z",
"published": "2026-09-08T16:41:40Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-6ww7-3frv-cqxh"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-78682"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/767333a005a1cd3d82d2029215f2dbe66a5844d9"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3733.yaml"
},
{
"type": "WEB",
"url": "https://www.vulncheck.com/advisories/nltk-before-ssrf-protection-bypass-via-proxy"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "NLTK: pathsec SSRF protection can be bypassed when a proxy is configured"
}
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