GHSA-89v8-rhwq-hf77

Описание

Summary

An attacker who can supply expressions to asteval.Interpreter.eval() can raise SystemExit, KeyboardInterrupt, GeneratorExit, or BaseException from inside the sandbox. These exceptions are subclasses of BaseException but not Exception, so they bypass the except Exception: safety net in both run() and eval(). The exception propagates verbatim to the calling application, terminating the process or disrupting signal and cleanup handlers.

This is distinct from prior vulnerabilities CVE-2025-24359 (format string injection) and GHSA-vp47-9734-prjw (AST mutation TOCTOU), both fixed in 1.0.6. This vector is present in all versions including 1.0.6 and current HEAD.

Affected Code

asteval/astutils.py, lines 89–108 — FROM_PY exposes dangerous classes to sandbox users:

asteval/asteval.py, line 322 — run() exception handler:

asteval/asteval.py, line 370 — eval() exception handler:

asteval/asteval.py, line 264 — raise_exception() raises the class directly:

Root Cause

Python's exception hierarchy has two distinct branches under BaseException:

FROM_PY exposes all four non-Exception classes to sandbox users. When a user writes raise SystemExit("msg"), the on_raise() handler calls:

which executes raise SystemExit(msg). This propagates through both except Exception: guards unchecked and surfaces in the calling application.

Proof of Concept

Output (tested on asteval 1.0.6, Python 3.11/3.12):

Real-world server scenario

Impact

Any application that:

• Accepts user-supplied expressions via asteval
• Relies on except Exception: at the top level (standard practice)
• Does not wrap aeval.eval() in except BaseException: (non-standard, unexpected requirement)

...is vulnerable to attacker-triggered process termination (DoS).

CVSS breakdown: Network-reachable (AV:N), no special conditions (AC:L), no credentials (PR:N), no interaction (UI:N), scope unchanged (S:U), no confidentiality/integrity impact (C:N/I:N), high availability impact — process termination (A:H).

Additional Note: File Read Capability (Acknowledged Limitation)

Independently of this vulnerability, asteval exposes a read-only open() wrapper (_open in astutils.py) that allows reading arbitrary files with the permissions of the calling process:

This is documented in doc/motivation.rst as a known design choice ("If reading from disk must be forbidden, you will want to overwrite the open() function from the symbol table"). It is included here for completeness, not as a separate advisory claim.

Recommended Fix

Option A — Remove dangerous classes from FROM_PY (minimal, preferred):

Option B — Block non-Exception raises in on_raise():

Note: Option B also fixes a secondary bug on the same line — ' '.join(out.args) crashes with TypeError when args contain non-strings (e.g., raise SystemExit(0) with integer code). The fix uses str(a) for a in out.args.

Option C — Catch BaseException in run() and eval() (broadest, requires care):

Option A is the simplest and least likely to introduce regressions. Option B additionally addresses the str.join crash on integer args.

Disclosure Timeline

Researcher

Independent security researcher. No bug bounty program exists for this project. CVE assignment requested via GitHub Security Advisory submission.

References

• Prior CVE: CVE-2025-24359 (format string injection, fixed 1.0.6)
• Prior advisory: GHSA-vp47-9734-prjw (AST mutation TOCTOU, fixed 1.0.6)
• Python exception hierarchy: https://docs.python.org/3/library/exceptions.html#exception-hierarchy
• asteval documentation: https://lmfit.github.io/asteval/