Vulnerability Database

351,760

Total vulnerabilities in the database

CVE-2026-53753 — Crawl4AI

Improper Control of Dynamically-Managed Code Resources

Summary

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.

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 gi_frame
  2. Walks the frame chain via f_back
  3. Reaches f_builtins containing the real __import__
  4. Imports os and executes arbitrary commands

Impact

Unauthenticated remote code execution inside the Docker container. An attacker can execute arbitrary system commands, read/write files, and exfiltrate secrets.

Fix Details

  1. Removed eval() from computed field expression path entirely -- expressions now log a warning and return default value
  2. Deleted _safe_eval_expression() function and _SAFE_EVAL_BUILTINS (dead security-sensitive code)
  3. function key with Python callables still works for SDK users
  4. Replaced eval() in /config/dump with JSON-based input validated by Pydantic
  5. Fixed hook_manager sandbox: stripped __builtins__, __loader__, __spec__ from injected modules; removed getattr, setattr, type, __build_class__ from allowed builtins

Workarounds

  1. Upgrade to the patched version (recommended)
  2. Enable JWT authentication via CRAWL4AI_API_TOKEN environment variable
  3. Restrict network access to the Docker API

Credits

  • Song Binglin (q1uf3ng) - reported the AST sandbox escape
  • by111 (August829) - reported the hook sandbox __builtins__ escape and hardcoded JWT secret bypass
  • jannahopp - PR #1855 proposing eval removal
  • ntohidi - PR #1886 proposing allowlist approach

CVSS v3:

  • Severity: Critical
  • Score: 9.8
  • AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Frequently Asked Questions

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.