CVE-2026-109239: Crawl4AI SSRF Vulnerability

CVE-2026-109239 Overview

CVE-2026-109239 is a blind Server-Side Request Forgery (SSRF) vulnerability in Crawl4AI versions before 0.9.4. The flaw resides in the RobotsParser.can_fetch function, which fetches robots.txt without enforcing Docker server egress controls. Unauthenticated attackers can enable check_robots_txt and abuse HTTP redirects or DNS rebinding to reach internal network ranges, loopback interfaces, and cloud-metadata endpoints. The weakness is tracked under CWE-918 and allows service discovery against resources that should be unreachable from the crawler. The issue was fixed in Crawl4AI 0.9.4.

Critical Impact
Unauthenticated attackers can pivot through a Dockerized Crawl4AI deployment to probe internal services, loopback endpoints, and cloud instance-metadata APIs.

Affected Products

Crawl4AI versions prior to 0.9.4
Dockerized Crawl4AI deployments with check_robots_txt enabled
Cloud-hosted Crawl4AI instances exposed to untrusted URL input

Discovery Timeline

2026-10-12 - CVE-2026-109239 published to NVD
2026-10-16 - Last updated in NVD database

Technical Details for CVE-2026-109239

Vulnerability Analysis

Crawl4AI is an open-source web crawling framework commonly deployed as a Docker service that accepts URLs to fetch and parse. When check_robots_txt is enabled, the crawler calls RobotsParser.can_fetch to retrieve the target site's robots.txt before issuing the main request. This preliminary fetch does not apply the same egress filtering that governs the primary crawl path. Attackers can submit URLs that trigger robots.txt lookups against attacker-controlled hosts, which then redirect to internal addresses or resolve to private IP space through DNS rebinding.

The response body is not returned to the attacker, which keeps the attack blind. However, response timing, status codes, and connection behavior reveal whether internal services are reachable. This is sufficient for enumerating internal HTTP services, Kubernetes APIs, and cloud instance-metadata endpoints such as 169.254.169.254.

Root Cause

The root cause is missing SSRF protection in the robots.txt fetch path. The RobotsParser.can_fetch method issues outbound HTTP requests before egress filtering is applied, trusting the resolved host and following redirects without validating the destination against a denylist of private, loopback, link-local, and metadata ranges.

Attack Vector

An unauthenticated attacker submits a crawl request to a Crawl4AI endpoint with check_robots_txt enabled. The attacker-controlled domain either returns an HTTP 3xx redirect pointing to an internal address or uses short-TTL DNS rebinding to resolve to a private IP during the robots.txt fetch. The crawler establishes a connection to the internal target, allowing the attacker to infer reachability. For technical specifics, see the GitHub Security Advisory GHSA-f77g-77vp-r96v and the VulnCheck Blind SSRF Advisory.

Detection Methods for CVE-2026-109239

Indicators of Compromise

Outbound connections from the Crawl4AI container to RFC1918 ranges, 127.0.0.0/8, or 169.254.169.254
robots.txt fetch requests resolving to short-TTL DNS records that change between resolution and connection
HTTP 3xx redirect chains in crawler logs terminating at private or link-local addresses
Repeated can_fetch calls targeting diverse attacker-controlled hostnames in a short window

Detection Strategies

Correlate crawler process network telemetry with destination IP classification to flag private-range egress
Alert on DNS responses to the crawler returning multiple A records that include both public and private addresses
Monitor Crawl4AI access logs for crawl submissions where check_robots_txt is true and the target domain is newly observed

Monitoring Recommendations

Enable egress flow logging on the Docker host or VPC and baseline normal crawler destinations
Capture DNS query logs from the container to identify rebinding patterns
Forward Crawl4AI application logs to a central platform for retention and correlation with network data

How to Mitigate CVE-2026-109239

Immediate Actions Required

Upgrade Crawl4AI to version 0.9.4 or later
Disable check_robots_txt on exposed deployments until the upgrade is applied
Restrict container egress to the public internet only, blocking RFC1918, loopback, and 169.254.169.254
Require authentication on any Crawl4AI endpoint reachable from untrusted networks

Patch Information

The vulnerability is resolved in Crawl4AI 0.9.4. The fix enforces egress validation during the RobotsParser.can_fetch path so that redirects and resolved hosts targeting internal, loopback, or cloud-metadata addresses are rejected. Refer to GitHub Security Advisory GHSA-f77g-77vp-r96v for the authoritative fix details.

Workarounds

Deploy an egress proxy that enforces an allowlist of permitted destination domains
Configure Docker or Kubernetes network policies to drop traffic to private and link-local ranges
Set IMDSv2 with hop-limit 1 on AWS to prevent containerized SSRF from reaching instance metadata
Resolve target hostnames once in application code and pin the IP for the subsequent HTTP connection to mitigate DNS rebinding

bash

# Example Docker egress restriction using iptables on the host
iptables -I DOCKER-USER -s 172.17.0.0/16 -d 169.254.169.254 -j DROP
iptables -I DOCKER-USER -s 172.17.0.0/16 -d 10.0.0.0/8 -j DROP
iptables -I DOCKER-USER -s 172.17.0.0/16 -d 192.168.0.0/16 -j DROP
iptables -I DOCKER-USER -s 172.17.0.0/16 -d 127.0.0.0/8 -j DROP