CVE-2026-55244

BaseException subclasses can escape Asteval, causing DoS via untrusted expressions.

• CVSS 5.0
• 248
• Design Defects
• Remote

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.

CVE

CVE-2026-55244

CWE

248

CVSS base score

5.0

Published

2026-09-14

OWASP

A03 Injection

Orthogonal defect classification

Checking

Code defect classification

Missing Check

Category

Design Defects

Subcategory

Inadequate Error Handling

Accessibility scope

Remote

Impact

Denial of Service (DoS)

Affected component

asteval

Fixed by upgrading

Yes

Solution

Upgrade asteval to version 1.0.9 or later.

from asteval import Interpreter

def evaluate_expression(expr):
    """Evaluate an untrusted Python expression using asteval."""
    # VULNERABLE: BaseException subclasses are exposed and can escape safety handlers
    interp = Interpreter()
    return interp.eval(expr)

from asteval import Interpreter

def evaluate_expression(expr):
    """Evaluate an untrusted Python expression using a hardened asteval interpreter."""
    # FIX: remove BaseException subclasses from the interpreter's symbol table
    interp = Interpreter()
    for name in list(interp.symtable):
        if isinstance(interp.symtable[name], BaseException):
            del interp.symtable[name]
    return interp.eval(expr)

__VAITP_MODEL_REFUSED__

@misc{vaitp:cve202655244,
  title        = {{BaseException subclasses can escape Asteval, causing DoS via untrusted expressions.}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2026},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2026-55244},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2026-55244/}}
}