CVE-2026-15308: The incremental HTML parser (html.parser.HTMLParser) allows for CPU denial-of-service through repeated untermi

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HIGH 7.5REMOTEUPGRADE

CVE-2026-15308 is a high-severity flaw (CVSS 7.5) affecting python. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 50%)
CISA exploited list	No
Weakness	CWE-400
Published	2026-07-09
Last modified	2026-10-25

Publisher (CNA): Python Software Foundation · Vendor: python

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-09 17:16 UTCPublished CVE record published
2026-10-25 17:10 UTCUpdated Record last modified

python python UPGRADE

Affected: < 3.10.21 · >= 3.11.0 and < 3.11.16 · >= 3.12.0 and < 3.12.14 · >= 3.13.0 and < 3.13.15 · >= 3.14.0 and < 3.14.7 · 3.15.0 alpha1 · 3.15.0 alpha2 · 3.15.0 alpha3

Fixed in: 3.10.21, 3.11.16, 3.12.14, 3.13.15, 3.14.7

Safe to downgrade to: a release before 3.11.0

1. Check installed version

apt-cache policy python || rpm -q python || python --version

2. UPGRADE to 3.10.21 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade python

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade python

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.11.0 (Debian / Ubuntu)

apt-cache madison python sudo apt install python=<VERSION_BEFORE_3.11.0> sudo apt-mark hold python

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list python sudo dnf downgrade python-<VERSION_BEFORE_3.11.0> sudo dnf versionlock add python

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Advisories and coverage

Official advisories

AlmaLinux Errata: [ALSA-2026:41949] Important: python3.14 security update 2026-07-21
AlmaLinux Errata: [ALSA-2026:39771] Important: python3.12 security update 2026-07-15
AlmaLinux Errata: [ALSA-2026:39798] Important: python3.9 security, bug fix, and enhancement update 2026-07-15

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-15308?

python: upgrade to 3.10.21, 3.11.16, 3.12.14, 3.13.15, 3.14.7. Exact commands for each OS are on this page.

Is CVE-2026-15308 being exploited?

CVE-2026-15308 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-15308 affect?

CVE-2026-15308 affects python. Severity is high (CVSS 7.5). The incremental HTML parser (html.parser.HTMLParser) allows for CPU denial-of-service through repeated unterminated markup declarations when processing uncontrolled data.

How urgent is CVE-2026-15308?

Priority 44/100. High severity; Remotely exploitable, no login, no user action

Software pages

Python vulnerabilities

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References

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