What’s Vulnerable › Vulnerabilities

CVE-2026-15308: Incremental HTMLParser feed() allows CPU-exhaustion DoS via repeated unterminated markup declarations

• Severity: high (CVSS 7.5 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: 2026-07-09 · updated 2026-10-25
• Weakness: CWE-400

Description

The incremental HTML parser (html.parser.HTMLParser) allows for CPU denial-of-service through repeated unterminated markup declarations when processing uncontrolled data.

Affected products

• Python: before 3.10.21; from 3.11.0 before 3.11.16; from 3.12.0 before 3.12.14; from 3.13.0 before 3.13.15; from 3.14.0 before 3.14.7; 3.15.0-alpha1, 3.15.0-alpha2, 3.15.0-alpha3, 3.15.0-alpha4, 3.15.0-alpha5, 3.15.0-alpha6 (+5) — fixed in 3.10.21, 3.11.16, 3.12.14, 3.13.15, 3.14.7

How to fix

Upgrade to a fixed version: Python 3.10.21, 3.11.16, 3.12.14, 3.13.15. Check the vendor advisory for mitigations.

References

• https://github.com/python/cpython/commit/07efb08123ba9367a7107325adb9d5626dca1ca9
• https://github.com/python/cpython/commit/1e7956f1a722df9aabc509c30f8fbdc3a2b4fdc7
• https://github.com/python/cpython/commit/785df8f743800661961528970f8598edcd291c14
• https://github.com/python/cpython/commit/7933f4bf7131aa4140750f9404f5de0aa2969ced
• https://github.com/python/cpython/commit/bcf98ddbc40ec9b3ee87da0124a5660b19b7e606
• https://github.com/python/cpython/commit/c2390b9376e35a701ed3acc597b8fc87546c9b00
• https://github.com/python/cpython/commit/e9f92ac0b298292e7ff998e52cb8ccacfb27a0bd
• https://github.com/python/cpython/issues/153030
• https://github.com/python/cpython/pull/153031
• https://mail.python.org/archives/list/security-announce@python.org/thread/F6453LWKSHKCTWFLCOURWPLETNUIW2Z5/
• http://www.openwall.com/lists/oss-security/2026/07/09/4

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