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SSRF filter bypass in Docker server via IPv6 transition forms (NAT64 / 6to4 / unspecified / v4-mapped)

High
unclecode published GHSA-4qqr-vv2q-cmr5 Jun 4, 2026

Package

pip crawl4ai (pip)

Affected versions

<= 0.8.7

Patched versions

0.8.8

Description

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).

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

CVE ID

CVE-2026-53754

Weaknesses

Server-Side Request Forgery (SSRF)

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. Learn more on MITRE.