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

High 8.7 EPSS 0.6% Python (CPython)

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

CVE-2026-15308CVE identifier
8.7CVSS 4.0 base score (high)
0.6%EPSS: chance of exploitation in the next 30 days (higher than 51% of CVEs)
2026-07-09published

What do these terms mean? CVE · CVSS score · exploited (KEV) · EPSS · backported fix · all terms

What to do now

Check whether you run Python (CPython) and which version (commands below).
Install the fixed version for your system: see “Patch it” below.
Restart long-running Python services (WSGI/ASGI servers, Celery, etc.)

Affected and fixed versions

Affected:

Python Software Foundation CPython before 3.10.21
Python Software Foundation CPython 3.11.0 to before 3.11.16
Python Software Foundation CPython 3.12.0 to before 3.12.14
Python Software Foundation CPython 3.13.0 to before 3.13.15
Python Software Foundation CPython 3.14.0 to before 3.14.7
Python Software Foundation CPython 3.15.0a1 to before 3.15.0b4

Patch it

Use the commands for your system. Distribution packages often keep an older version number with the fix backported, so compare with the fixed package versions in the table below, not only with the upstream version.

Debian

sudo apt update
sudo apt install --only-upgrade python3.13 libpython3.13

Ubuntu

sudo apt update
sudo apt install --only-upgrade python3.14 libpython3.14

RHEL, AlmaLinux, Rocky, Oracle

sudo dnf upgrade --refresh python3 (RHEL 9 also python3.11/python3.12)

Fedora

sudo dnf upgrade --refresh python3

SUSE / openSUSE

sudo zypper refresh
sudo zypper update python313 python3-base

Amazon Linux

sudo dnf upgrade --releasever=latest python3 (or python3.11/3.12/3.13)

Arch Linux

sudo pacman -Syu

Alpine

apk update && apk upgrade python3

FreeBSD

pkg update && pkg upgrade python311 python312 python313 (pythonXY)

General update steps for every system are on the Linux and Unix & BSD pages; after updating, see which services to restart.

Fix status in Linux distributions

Release	Package	Status	Fixed version / advisory
Debian 12	`python3.11`	fixed	`3.11.2-6+deb12u9`
Debian 13	`python3.13`	not fixed yet
Debian testing	`python3.14`	fixed	`3.14.7-1`
Red Hat Enterprise Linux 9	`python3.12-0:3.12.13-3.el9_8.1`	fixed	`RHSA-2026:39771`
Red Hat Enterprise Linux 9	`python3.9-0:3.9.25-7.el9_8.2`	fixed	`RHSA-2026:39798`
Red Hat Enterprise Linux 9	`python3.14-0:3.14.5-1.el9_8.1`	fixed	`RHSA-2026:41949`
Red Hat Enterprise Linux 8	`python3-0:3.6.8-77.el8_10`	fixed	`RHSA-2026:39320`
Red Hat Enterprise Linux 8	`python3.12-0:3.12.13-3.el8_10`	fixed	`RHSA-2026:39893`
Red Hat Enterprise Linux 7	`python`	out of support scope
Red Hat Enterprise Linux 7	`python3`	out of support scope
Red Hat Enterprise Linux 6	`python`	out of support scope
Red Hat Enterprise Linux 10	`python3.12-0:3.12.13-2.el10_2.1`	fixed	`RHSA-2026:39183`
Red Hat Enterprise Linux 10	`python3.14-0:3.14.5-1.el10_2.1`	fixed	`RHSA-2026:40856`

From the Debian security tracker, Ubuntu CVE tracker and Red Hat Security Data API, refreshed every few hours.

Sources

Page generated automatically from official sources on 2026-11-03 18:05 UTC. Always confirm with your vendor's advisory.