GHSA-C6Q7-WQW2-H65W
Vulnerability from github – Published: 2026-10-30 22:51 – Updated: 2026-10-30 22:51
VLAI
Summary
NationalSecurityAgency/emissary has an incomplete fix for CVE-2026-35571
Details
Summary
The URL validation regex ^(https?:/)?/.* added to emissary's NavAction.java allows protocol-relative URLs (//evil.com/...) and single-slash scheme variants (http:/evil.com) to bypass validation, enabling SSRF and open redirect attacks.
Affected Package
- Ecosystem: Maven
- Package: NationalSecurityAgency/emissary
- Affected versions: < e2078417464b
- Patched versions: >= e2078417464b
Severity
Medium
CWE
CWE-918 — Server-Side Request Forgery (SSRF)
Details
The fix in NavAction.java (line 47) introduces a VALID_LINK pattern:
Pattern.compile("^(https?:/)?/.*")
This regex is intended to restrict navigation links to relative paths and HTTP/HTTPS URLs. However, it has multiple bypass vectors.
Protocol-relative URLs (//evil.com/...): The group (https?:/)? is optional, so the pattern reduces to //.* when the group is absent. This matches //evil.com/steal?c=SESSION, which browsers resolve to the current page's protocol plus the attacker's host.
Single-slash scheme (http:/evil.com): The regex group captures http:/ as a valid match, and browsers normalize http:/evil.com to http://evil.com, creating an open redirect to the attacker's server.
Protocol-relative to cloud metadata (//169.254.169.254/...): This enables SSRF to cloud instance metadata endpoints, which is particularly dangerous in AWS/GCP environments where IAM credentials can be retrieved.
The pre-fix code had no validation at all, accepting any URL. The fix blocks javascript:, ftp:, and data: URIs, but the protocol-relative and single-slash bypasses remain exploitable.
PoC
"""
CVE-2026-35571 - NationalSecurityAgency/emissary SSRF via NavAction
The fix in commit e2078417464b adds URL scheme validation using:
Pattern.compile("^(https?:/)?/.*")
This allows protocol-relative URLs (//evil.com) and single-slash variants
(http:/evil.com) to bypass the validation.
Tested against: emissary@0d4dc56 (pre-fix) and patch e207841
Extracted from: src/main/java/emissary/server/mvc/NavAction.java:47
"""
import re
import sys
FIXED_PATTERN = re.compile(r"^(https?:/)?/.*")
def vulnerable_accepts(link: str) -> bool:
"""Pre-fix behaviour: no validation, everything passes."""
return True
def fixed_accepts(link: str) -> bool:
"""Post-fix behaviour: must match VALID_LINK pattern."""
return bool(FIXED_PATTERN.match(link))
TEST_VECTORS = [
("javascript: XSS payload",
"javascript:alert(document.cookie)", True),
("ftp: scheme",
"ftp://evil.com/exfil", True),
("data: URI",
"data:text/html,<script>alert(1)</script>", True),
("Relative path",
"/path/to/resource", False),
("HTTP URL",
"http://trusted.internal:8080/api", False),
("HTTPS URL",
"https://trusted.internal/api", False),
("Protocol-relative URL to attacker host (//evil.com)",
"//evil.com/steal?c=SESSION", True),
("Protocol-relative with embedded credentials",
"//user:pass@evil.com/path", True),
("Protocol-relative to cloud metadata (//169.254.169.254)",
"//169.254.169.254/latest/meta-data/", True),
("Backslash trick (\\\\evil.com)",
"\\\\evil.com/path", True),
("Tab in scheme (java\\tscript:...)",
"java\tscript:alert(1)", True),
("Path starting with attacker-like host (/evil.com)",
"/evil.com", False),
("Triple slash (///evil.com)",
"///evil.com/etc/passwd", True),
("http:/ single-slash to attacker host",
"http:/evil.com/steal", True),
("https:/ single-slash to attacker host",
"https:/evil.com/steal", True),
("URL-encoded javascript scheme (%6a...)",
"%6aavascript:alert(1)", True),
("CRLF injection in path",
"/legit\r\nLocation: http://evil.com", True),
]
def main():
print("=" * 72)
print("CVE-2026-35571 -- Emissary NavAction SSRF / Open-Redirect PoC")
print("Pattern under test: ^(https?:/)?/.*")
print("=" * 72)
print()
bypasses_found = []
results_table = []
for desc, url, should_block in TEST_VECTORS:
vuln_result = vulnerable_accepts(url)
fixed_result = fixed_accepts(url)
if should_block and fixed_result:
status = "BYPASS"
bypasses_found.append((desc, url))
elif should_block and not fixed_result:
status = "BLOCKED (good)"
elif not should_block and fixed_result:
status = "ALLOWED (good)"
elif not should_block and not fixed_result:
status = "FALSE-POS"
else:
status = "UNKNOWN"
results_table.append((desc, url, vuln_result, fixed_result, status))
for desc, url, vuln, fixed, status in results_table:
display_url = repr(url) if len(url) > 50 else url
print(f"[{status:>15}] {desc}")
print(f"{'':>17} URL: {display_url}")
print(f"{'':>17} pre-fix={vuln} post-fix={fixed}")
print()
print("=" * 72)
print(f"Total vectors tested : {len(TEST_VECTORS)}")
print(f"Bypasses found : {len(bypasses_found)}")
print()
if bypasses_found:
print("VULNERABILITY CONFIRMED -- the following vectors bypass the fix:")
print()
for desc, url in bypasses_found:
print(f" - {desc}")
print(f" URL: {repr(url)}")
print()
print("Key finding: The regex ^(https?:/)?/.* allows protocol-relative")
print("URLs (//evil.com/...) because the optional group (https?:/) can be")
print("absent, leaving //.* which matches //attacker.com. This enables")
print("SSRF to arbitrary external hosts via navigation links.")
print()
print("Additionally, http:/evil.com and https:/evil.com match the regex")
print("(group 1 captures 'http:/' or 'https:/') and browsers normalise")
print("these to http://evil.com, enabling open redirect / SSRF.")
sys.exit(0)
else:
print("No bypasses found -- fix appears complete.")
sys.exit(1)
if __name__ == "__main__":
main()
Steps to reproduce:
1. Run python3 poc.py
2. Observe that protocol-relative and single-slash URLs bypass the regex
Expected output:
VULNERABILITY CONFIRMED
Protocol-relative URLs (//evil.com/) and single-slash variants (http:/evil.com) bypass the regex validation.
Impact
An attacker can craft navigation links that bypass URL validation to redirect users to attacker-controlled servers (open redirect) or make the emissary server issue requests to internal services (SSRF). The //169.254.169.254/latest/meta-data/ bypass is particularly dangerous in cloud environments, enabling retrieval of IAM credentials.
Suggested Remediation
Replace the regex with one that explicitly rejects protocol-relative URLs. Alternatively, parse the URL with java.net.URI and validate the scheme and host against an allowlist rather than relying on regex.
References
- Incomplete fix commit: https://github.com/NationalSecurityAgency/emissary/commit/e2078417464b9004620dde28dcbca2f73ea06c13
- Original CVE: CVE-2026-35571
Severity
4.8 (Medium)
{
"affected": [
{
"package": {
"ecosystem": "Maven",
"name": "gov.nsa.emissary:emissary"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "8.51.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-54442"
],
"database_specific": {
"cwe_ids": [
"CWE-918"
],
"github_reviewed": true,
"github_reviewed_at": "2026-10-30T22:51:19Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "### Summary\n\nThe URL validation regex `^(https?:/)?/.*` added to emissary\u0027s `NavAction.java` allows protocol-relative URLs (`//evil.com/...`) and single-slash scheme variants (`http:/evil.com`) to bypass validation, enabling SSRF and open redirect attacks.\n\n### Affected Package\n\n- **Ecosystem:** Maven\n- **Package:** NationalSecurityAgency/emissary\n- **Affected versions:** \u003c e2078417464b\n- **Patched versions:** \u003e= e2078417464b\n\n### Severity\n\nMedium\n\n### CWE\n\nCWE-918 \u2014 Server-Side Request Forgery (SSRF)\n\n### Details\n\nThe fix in `NavAction.java` (line 47) introduces a `VALID_LINK` pattern:\n\n```java\nPattern.compile(\"^(https?:/)?/.*\")\n```\n\nThis regex is intended to restrict navigation links to relative paths and HTTP/HTTPS URLs. However, it has multiple bypass vectors.\n\nProtocol-relative URLs (`//evil.com/...`): The group `(https?:/)?` is optional, so the pattern reduces to `//.*` when the group is absent. This matches `//evil.com/steal?c=SESSION`, which browsers resolve to the current page\u0027s protocol plus the attacker\u0027s host.\n\nSingle-slash scheme (`http:/evil.com`): The regex group captures `http:/` as a valid match, and browsers normalize `http:/evil.com` to `http://evil.com`, creating an open redirect to the attacker\u0027s server.\n\nProtocol-relative to cloud metadata (`//169.254.169.254/...`): This enables SSRF to cloud instance metadata endpoints, which is particularly dangerous in AWS/GCP environments where IAM credentials can be retrieved.\n\nThe pre-fix code had no validation at all, accepting any URL. The fix blocks `javascript:`, `ftp:`, and `data:` URIs, but the protocol-relative and single-slash bypasses remain exploitable.\n\n### PoC\n\n```python\n\"\"\"\nCVE-2026-35571 - NationalSecurityAgency/emissary SSRF via NavAction\n\nThe fix in commit e2078417464b adds URL scheme validation using:\n Pattern.compile(\"^(https?:/)?/.*\")\n\nThis allows protocol-relative URLs (//evil.com) and single-slash variants\n(http:/evil.com) to bypass the validation.\n\nTested against: emissary@0d4dc56 (pre-fix) and patch e207841\nExtracted from: src/main/java/emissary/server/mvc/NavAction.java:47\n\"\"\"\n\nimport re\nimport sys\n\nFIXED_PATTERN = re.compile(r\"^(https?:/)?/.*\")\n\n\ndef vulnerable_accepts(link: str) -\u003e bool:\n \"\"\"Pre-fix behaviour: no validation, everything passes.\"\"\"\n return True\n\n\ndef fixed_accepts(link: str) -\u003e bool:\n \"\"\"Post-fix behaviour: must match VALID_LINK pattern.\"\"\"\n return bool(FIXED_PATTERN.match(link))\n\n\nTEST_VECTORS = [\n (\"javascript: XSS payload\",\n \"javascript:alert(document.cookie)\", True),\n (\"ftp: scheme\",\n \"ftp://evil.com/exfil\", True),\n (\"data: URI\",\n \"data:text/html,\u003cscript\u003ealert(1)\u003c/script\u003e\", True),\n\n (\"Relative path\",\n \"/path/to/resource\", False),\n (\"HTTP URL\",\n \"http://trusted.internal:8080/api\", False),\n (\"HTTPS URL\",\n \"https://trusted.internal/api\", False),\n\n (\"Protocol-relative URL to attacker host (//evil.com)\",\n \"//evil.com/steal?c=SESSION\", True),\n (\"Protocol-relative with embedded credentials\",\n \"//user:pass@evil.com/path\", True),\n (\"Protocol-relative to cloud metadata (//169.254.169.254)\",\n \"//169.254.169.254/latest/meta-data/\", True),\n (\"Backslash trick (\\\\\\\\evil.com)\",\n \"\\\\\\\\evil.com/path\", True),\n (\"Tab in scheme (java\\\\tscript:...)\",\n \"java\\tscript:alert(1)\", True),\n (\"Path starting with attacker-like host (/evil.com)\",\n \"/evil.com\", False),\n (\"Triple slash (///evil.com)\",\n \"///evil.com/etc/passwd\", True),\n (\"http:/ single-slash to attacker host\",\n \"http:/evil.com/steal\", True),\n (\"https:/ single-slash to attacker host\",\n \"https:/evil.com/steal\", True),\n (\"URL-encoded javascript scheme (%6a...)\",\n \"%6aavascript:alert(1)\", True),\n (\"CRLF injection in path\",\n \"/legit\\r\\nLocation: http://evil.com\", True),\n]\n\n\ndef main():\n print(\"=\" * 72)\n print(\"CVE-2026-35571 -- Emissary NavAction SSRF / Open-Redirect PoC\")\n print(\"Pattern under test: ^(https?:/)?/.*\")\n print(\"=\" * 72)\n print()\n\n bypasses_found = []\n results_table = []\n\n for desc, url, should_block in TEST_VECTORS:\n vuln_result = vulnerable_accepts(url)\n fixed_result = fixed_accepts(url)\n\n if should_block and fixed_result:\n status = \"BYPASS\"\n bypasses_found.append((desc, url))\n elif should_block and not fixed_result:\n status = \"BLOCKED (good)\"\n elif not should_block and fixed_result:\n status = \"ALLOWED (good)\"\n elif not should_block and not fixed_result:\n status = \"FALSE-POS\"\n else:\n status = \"UNKNOWN\"\n\n results_table.append((desc, url, vuln_result, fixed_result, status))\n\n for desc, url, vuln, fixed, status in results_table:\n display_url = repr(url) if len(url) \u003e 50 else url\n print(f\"[{status:\u003e15}] {desc}\")\n print(f\"{\u0027\u0027:\u003e17} URL: {display_url}\")\n print(f\"{\u0027\u0027:\u003e17} pre-fix={vuln} post-fix={fixed}\")\n print()\n\n print(\"=\" * 72)\n print(f\"Total vectors tested : {len(TEST_VECTORS)}\")\n print(f\"Bypasses found : {len(bypasses_found)}\")\n print()\n\n if bypasses_found:\n print(\"VULNERABILITY CONFIRMED -- the following vectors bypass the fix:\")\n print()\n for desc, url in bypasses_found:\n print(f\" - {desc}\")\n print(f\" URL: {repr(url)}\")\n print()\n print(\"Key finding: The regex ^(https?:/)?/.* allows protocol-relative\")\n print(\"URLs (//evil.com/...) because the optional group (https?:/) can be\")\n print(\"absent, leaving //.* which matches //attacker.com. This enables\")\n print(\"SSRF to arbitrary external hosts via navigation links.\")\n print()\n print(\"Additionally, http:/evil.com and https:/evil.com match the regex\")\n print(\"(group 1 captures \u0027http:/\u0027 or \u0027https:/\u0027) and browsers normalise\")\n print(\"these to http://evil.com, enabling open redirect / SSRF.\")\n sys.exit(0)\n else:\n print(\"No bypasses found -- fix appears complete.\")\n sys.exit(1)\n\n\nif __name__ == \"__main__\":\n main()\n\n```\n\n**Steps to reproduce:**\n1. Run `python3 poc.py`\n2. Observe that protocol-relative and single-slash URLs bypass the regex\n\n**Expected output:**\n```\nVULNERABILITY CONFIRMED\nProtocol-relative URLs (//evil.com/) and single-slash variants (http:/evil.com) bypass the regex validation.\n```\n\n### Impact\n\nAn attacker can craft navigation links that bypass URL validation to redirect users to attacker-controlled servers (open redirect) or make the emissary server issue requests to internal services (SSRF). The `//169.254.169.254/latest/meta-data/` bypass is particularly dangerous in cloud environments, enabling retrieval of IAM credentials.\n\n### Suggested Remediation\n\nReplace the regex with one that explicitly rejects protocol-relative URLs. Alternatively, parse the URL with `java.net.URI` and validate the scheme and host against an allowlist rather than relying on regex.\n\n### References\n\n- Incomplete fix commit: https://github.com/NationalSecurityAgency/emissary/commit/e2078417464b9004620dde28dcbca2f73ea06c13\n- Original CVE: CVE-2026-35571",
"id": "GHSA-c6q7-wqw2-h65w",
"modified": "2026-10-30T22:51:19Z",
"published": "2026-10-30T22:51:19Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/NationalSecurityAgency/emissary/security/advisories/GHSA-c6q7-wqw2-h65w"
},
{
"type": "WEB",
"url": "https://github.com/NationalSecurityAgency/emissary/security/advisories/GHSA-cpm7-cfpx-3hvp"
},
{
"type": "WEB",
"url": "https://github.com/NationalSecurityAgency/emissary/commit/e2078417464b9004620dde28dcbca2f73ea06c13"
},
{
"type": "PACKAGE",
"url": "https://github.com/NationalSecurityAgency/emissary"
},
{
"type": "WEB",
"url": "https://github.com/NationalSecurityAgency/emissary/releases/tag/8.39.0"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "NationalSecurityAgency/emissary has an incomplete fix for CVE-2026-35571"
}
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