CVE-2026-15308 — cpython
Fix: python/cpython@07efb08CVE-2026-15308 is a Uncontrolled Resource Consumption vulnerability. A fix is available — see the affected versions and patch details below.
Incremental HTMLParser feed() allows CPU-exhaustion DoS via repeated unterminated markup declarations
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-15308.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
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
pythonpythonDetection & mitigation playbook
VulnerabilityDetect
Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).
Fix
Upgrade the affected component to the fixed release for CVE-2026-15308, or apply your distribution's backported patch — distro builds are often patched at an older version number, so check your vendor's advisory rather than the upstream version alone.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Important vulnerability in Python's `html.parser.HTMLParser` can lead to a denial of service. By supplying specially crafted, malformed HTML with repeated unterminated markup declarations, an attacker can cause excessive CPU consumption, rendering the system unresponsive. Red Hat products utilizing this parser to…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.Source: Red Hat security advisory for CVE-2026-15308 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 10 | python3.12-0:3.12.13-2.el10_2.1 | RHSA-2026:39183 |
| Red Hat Enterprise Linux 10 | python3.14-0:3.14.5-1.el10_2.1 | RHSA-2026:40856 |
| Red Hat Enterprise Linux 10.0 Extended Update Support | python3.12-0:3.12.9-2.el10_0.10 | RHSA-2026:47939 |
| Red Hat Enterprise Linux 8 | python3-0:3.6.8-77.el8_10 | RHSA-2026:39320 |
| Red Hat Enterprise Linux 8 | python3.12-0:3.12.13-3.el8_10 | RHSA-2026:39893 |
| Red Hat Enterprise Linux 8.4 Advanced Mission Critical Update Support | python3-0:3.6.8-39.el8_4.12 | RHSA-2026:50816 |
| Red Hat Enterprise Linux 8.6 Advanced Mission Critical Update Support | python3-0:3.6.8-47.el8_6.14 | RHSA-2026:50065 |
| Red Hat Enterprise Linux 8.8 Telecommunications Update Service | python3-0:3.6.8-51.el8_8.16 | RHSA-2026:50064 |
Frequently Asked Questions
Is CVE-2026-15308 in your dependencies?
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