Stack Overflow Vulnerability in Python's Expat Parser Affecting Document Type Definitions
CVE-2026-4224

6MEDIUM

Key Information:

Vendor

Python Software Foundation

Status
Cpython
Vendor
CVE Published:
16 March 2026

What is CVE-2026-4224?

The Expat parser in Python is susceptible to a stack overflow vulnerability when parsing inline document type definitions that contain deeply nested content models with a registered ElementDeclHandler. This flaw can lead to unexpected behavior and potential crashes, making it crucial for developers to review and apply patches to mitigate the risks associated with this parsing issue.

Affected Version(s)

CPython 0 < 3.13.13

CPython 3.14.0 < 3.14.4

CPython 3.15.0a1 < 3.15.0a8

References

https://github.com/python/cpython/commit/eb0e8be3a7e11b87...
patch
https://mail.python.org/archives/list/security-announce@p...
vendor-advisory
https://github.com/python/cpython/issues/145986
issue-tracking

CVSS V4

Score:
6
Severity:
MEDIUM
Confidentiality:
None
Integrity:
None
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
Physical
Privileges Required:
Undefined
User Interaction:
None

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Gil Portnoy
Stan Ulbrych
Bénédikt Tran
Stan Ulbrych
.