critical CVSS 9.8

CVE-2026-53753·Python vulnerability

Crawl4AI is an open-source LLM friendly web crawler & scraper. Prior to 0.8.7, the _safe_eval_expression() function in the computed fields feature uses an AST validator that only blocks attributes starting with underscore. Python generator and frame object attributes (gi_frame, f_back, f_builtins) do NOT start with underscore, enabling a complete sandbox escape to achieve arbitrary code execution. The attack requires no authentication (JWT disabled by default) and is triggered via POST /crawl with a crafted extraction schema. This vulnerability is fixed in 0.8.7.

Severity
critical
Software
Python
Fixed in
0.8.7
Published
2026-06-23

Affected versions

Until: 0.8.7

Fixed in: 0.8.7

How to fix this CVE

Update Crawl4AI to version 0.8.7 or later to patch the sandbox escape vulnerability in the computed fields feature. The vulnerability allows unauthenticated attackers to execute arbitrary Python code by bypassing the AST validator through generator and frame object attributes. Immediately apply this update to any systems running Crawl4AI, particularly those exposed to untrusted network inputs.

sudo dnf update crawl4ai

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST /crawl HTTP/1.1 with JSON body containing extraction schema parameters with attribute access to 'gi_frame', 'f_back', 'f_builtins', or '__code__' properties in computed field expressions

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Block POST requests to /crawl endpoint that contain the patterns '(gi_frame|f_back|f_builtins|__code__|__globals__|__loader__)' in the request body JSON fields. Implement strict input validation to reject any extraction schema with non-whitelisted object attribute references.

How to check if you are affected

  1. Run 'pip show crawl4ai | grep Version' to identify the installed Crawl4AI version and confirm if it is below 0.8.7
  2. Check if Crawl4AI is exposed via network by verifying listening ports with 'netstat -tlnp | grep python' or 'ss -tlnp | grep python'
  3. Search application logs for POST requests to /crawl endpoint with suspicious extraction schema parameters containing references to 'gi_frame', 'f_back', or 'f_builtins'
  4. Verify the patch by running 'pip show crawl4ai' again and confirming version 0.8.7 or higher is installed, then restart the service

FAQ

What is CVE-2026-53753?

This vulnerability exists in Crawl4AI's computed fields feature where an incomplete AST validation allows attackers to reference Python internal frame and generator objects to escape the expression sandbox and achieve remote code execution without authentication.

Is CVE-2026-53753 being actively exploited?

No, CVE-2026-53753 is not listed on the CISA Known Exploited Vulnerabilities catalog and no public exploits have been disclosed, but the low barrier to exploitation makes proactive patching critical.

What versions of Crawl4AI are affected by CVE-2026-53753?

All versions of Crawl4AI prior to 0.8.7 are vulnerable. The vulnerable code path exists in the _safe_eval_expression() function used for processing extraction schemas.

How do I check if my server is vulnerable to CVE-2026-53753?

Run 'pip show crawl4ai' and compare the Version output to 0.8.7. If the version is lower, your installation is vulnerable. Also verify that the /crawl endpoint is accessible without authentication by checking your application configuration.

Does Defensia detect CVE-2026-53753?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Crawl4AI is installed on a monitored server, CVE-2026-53753 will appear in your dashboard with remediation steps.

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References

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