CVE-2026-53753

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

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

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

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2026-53753 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.8.7); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-53753? CVE-2026-53753 is a critical-severity code injection vulnerability in crawl4ai (pip), affecting versions <= 0.8.6. It is fixed in 0.8.7. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-53753? CVE-2026-53753 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of crawl4ai are affected by CVE-2026-53753? crawl4ai (pip) versions <= 0.8.6 is affected.
4. Is there a fix for CVE-2026-53753? Yes. CVE-2026-53753 is fixed in 0.8.7. Upgrade to this version or later.
5. Is CVE-2026-53753 exploitable, and should I be worried? Whether CVE-2026-53753 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-53753 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-53753? Upgrade crawl4ai to 0.8.7 or later.