CVE-2026-53753: Crawl4AI: AST Sandbox Escape via gi_frame.f_back Chain - Pre-Auth RCE in Docker API

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 2.9%, percentile 87: FIRST’s model estimates a 2.9% chance of exploitation activity in the next 30 days, higher than 87% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes crawl4ai (PyPI) <= 0.8.6, fixed in 0.8.7. Check your lockfile, including transitive dependencies.

What it is

Crawl4AI is an open-source LLM friendly web crawler & scraper. Prior to 0.8.7, the safeevalexpression() function in the computed fields feature uses an AST validator that only blocks attributes starting with underscore. Python generator and frame object attributes (giframe, fback, fbuiltins) 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.

From the GitHub advisory

Summary

The safeevalexpression() function in the computed fields feature uses an AST validator that only blocks attributes starting with underscore. Python generator and frame object attributes (giframe, fback, fbuiltins) 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.

Attack Vector

An attacker sends a POST /crawl request with a JsonCssExtractionStrategy schema containing a malicious computed field expression that: 1. Creates a generator to access giframe 2. Walks the frame chain via fback 3. Reaches fbuiltins containing the real import 4. Imports os and executes arbitrary commands

Weakness type: CWE-94, CWE-913.

Product: crawl4ai (PyPI) · Assigned by: GitHub_M · Published: 2026-06-23

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 2.90% probability of exploitation activity in the next 30 days; percentile 87.

Timeline

• 2026-06-16 — GitHub-reviewed advisory GHSA-qxjp-w3pj-48m7 published (critical)
• 2026-06-23 — CVE record last updated
• 2026-06-23 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-qxjp-w3pj-48m7 github.com
2. CVE.org record CVE-2026-53753 cve.org
3. NVD entry CVE-2026-53753 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com
6. pypi.org pypi.org

Critical PyPI advisories · Today’s Security Digest