Crawl4AI: SSRF filter bypass in Docker server via IPv6 transition forms (NAT64 / 6to4 / unspecified / v4-mapped)
Summary
The Docker API server's SSRF protection (validate_webhook_url / validate_url_destination in deploy/docker/utils.py) used an explicit IPv4/IPv6 CIDR blocklist that missed several address families. An attacker could reach internal services and cloud metadata endpoints (e.g. 169.254.169.254) despite the filter by encoding an internal IPv4 address inside an IPv6 transition form, or by using the IPv6 unspecified address.
Because the Docker API is unauthenticated by default (jwt_enabled: false), no credentials are required.
Affected paths
The blocklist was applied to crawl URLs (POST /crawl, /md, /html, /screenshot, /pdf, /execute_js) and webhook URLs (/crawl/job, /llm/job). All shared the same incomplete check.
Bypasses
The following all resolve to (or route to) blocked internal addresses but were NOT caught:
- IPv6 unspecified
:: - NAT64
64:ff9b::a9fe:a9fe (embeds 169.254.169.254) - 6to4
2002:a9fe:a9fe:: (embeds 169.254.169.254) - IPv4-mapped
::ffff:169.254.169.254 - IPv4-compatible
::a9fe:a9fe
The error message also echoed the resolved internal IP, acting as a minor DNS/oracle leak.
Impact
Server-Side Request Forgery: an unauthenticated attacker can make the server fetch internal-network URLs and cloud instance-metadata endpoints, potentially exposing internal services and cloud credentials.
Fix
The blocklist is replaced by a single rule: reject any resolved IP where not ip.is_global, evaluated on the address AND every embedded IPv4 transition form (v4-mapped, NAT64 64:ff9b::/96, 6to4 2002::/16, v4-compat ::/96). Error messages are now opaque and no longer echo the resolved IP.
Workarounds
- Upgrade to the patched version.
- Enable authentication (
CRAWL4AI_API_TOKEN). - Restrict the container's outbound network access (egress firewall / no metadata route).
Credits
Internal security audit (Crawl4AI maintainers).
