CVE-2026-15308Network attack vector

CPU exhaustion via quadratic incremental markup parsing

Published Jul 9, 2026 · Updated Jul 9, 2026

CPU denial of service in affected CPython releases allows remote attackers to exhaust CPU through incremental HTML parsing. HTMLParser.feed() repeatedly concatenates and rescans its growing raw-data buffer when an unterminated tag, comment, or declaration spans many feed calls, producing quadratic work. Reachability requires an application to incrementally parse attacker-controlled HTML; the bounded consequence is a CPU-driven service hang or severe slowdown, not data access or modification.

CVSS severity8.7
High
EPSS probability0.64%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CPU denial of service in affected CPython releases allows remote attackers to exhaust CPU through incremental HTML parsing. HTMLParser.feed() repeatedly concatenates and rescans its growing raw-data buffer when an unterminated tag, comment, or declaration spans many feed calls, producing quadratic work. Reachability requires an application to incrementally parse attacker-controlled HTML; the bounded consequence is a CPU-driven service hang or severe slowdown, not data access or modification.

The record

CVE
CVE-2026-15308
Published
Jul 9, 2026
Updated
Jul 9, 2026
Vendor
Python Software Foundation
Product
CPython
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 9, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 9, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <3.15.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Python Software Foundation · CPythonversion=0 <3.15.0

Published CVSS scores

7.5National Institute of Standards and TechnologyCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7Python Software FoundationCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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