Crawl4AI: SSRF via proxy settings in the Docker server bypasses the crawl-URL SSRF check

Summary

The Docker API server applied its SSRF destination check to the crawl target URL only, not to the proxy address. An unauthenticated request could supply a proxy pointing at an internal IP and route the browser through it, reaching internal services and cloud-metadata endpoints, while using a perfectly valid crawl URL. The Docker API is unauthenticated by default.

Affected paths

/crawl, /crawl/stream, and /crawl/job accept a browser_config (and crawler_config). The following all feed Chromium's egress and were unchecked:

• browser_config.proxy_config.server
• browser_config.proxy (deprecated field)
• crawler_config.proxy_config.server
• --proxy-server / --proxy-pac-url / --proxy-bypass-list / --host-resolver-rules flags in browser_config.extra_args

Attack

An attacker sends /crawl with a benign, validation-passing URL but a proxy_config.server pointing at an internal IP. Chromium routes all requests through that proxy. For plain-HTTP targets the proxy receives the full request and can return any content, which is then returned verbatim in the crawl result (results[0].html / cleaned_html / markdown). In a real deployment the proxy would be an attacker-controlled server pointing at cloud metadata (e.g. AWS IMDSv1 at 169.254.169.254) to retrieve IAM credential tokens.

Impact

Unauthenticated server-side request forgery to internal services and cloud-metadata endpoints, with the response returned to the attacker.

Fix

Every proxy destination is validated with the same global-routability check used for crawl URLs (reject any resolved address that is not is_global, including IPv6 transition forms) before the browser is constructed; proxy/DNS-redirecting flags are stripped from extra_args. A legitimate public proxy still works. Honors CRAWL4AI_ALLOW_INTERNAL_URLS.

Workarounds

• Upgrade to the patched version (0.8.9).
• Enable authentication (CRAWL4AI_API_TOKEN).
• Restrict the container's outbound network access (egress firewall / no metadata route).

Credits

Geo (geo-chen) - reported the proxy_config.server SSRF with a clear PoC.