What’s Vulnerable › Vulnerabilities

CVE-2025-6069: HTMLParser quadratic complexity when processing malformed inputs

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-06-17 · updated 2026-07-31
• Weakness: CWE-1333

Description

The html.parser.HTMLParser class had worse-case quadratic complexity when processing certain crafted malformed inputs potentially leading to amplified denial-of-service.

Affected products

• Python: before 3.9.24; from 3.10.0 before 3.10.19; from 3.11.0 before 3.11.14; from 3.12.0 before 3.12.12; from 3.13.0 before 3.13.6; from 3.14.0a1 before 3.14.0b3 — fixed in 3.10.19, 3.11.14, 3.12.12, 3.13.6, 3.14.0b3, 3.9.24

How to fix

Upgrade to a fixed version: Python 3.10.19, 3.11.14, 3.12.12, 3.13.6. Check the vendor advisory for mitigations.

References

• https://github.com/python/cpython/commit/4455cbabf991e202185a25a631af206f60bbc949
• https://github.com/python/cpython/commit/6eb6c5dbfb528bd07d77b60fd71fd05d81d45c41
• https://github.com/python/cpython/commit/8d1b3dfa09135affbbf27fb8babcf3c11415df49
• https://github.com/python/cpython/commit/ab0893fd5c579d9cea30841680e6d35fc478afb5
• https://github.com/python/cpython/commit/d851f8e258c7328814943e923a7df81bca15df4b
• https://github.com/python/cpython/commit/f3c6f882cddc8dc30320d2e73edf019e201394fc
• https://github.com/python/cpython/commit/fdc9d214c01cb4588f540cfa03726bbf2a33fc15
• https://github.com/python/cpython/issues/135462
• https://github.com/python/cpython/pull/135464
• https://mail.python.org/archives/list/security-announce@python.org/thread/K5PIYLR6EP3WR7ZOKKYQUWEDNQVUXOYM/

Get alerted when CVE-2025-6069 or the next vulnerability affects your stack