CVE-2026-66046: Expat through 2.8.3 contains a denial of service vulnerability caused by quadratic algorithmic complexity in t

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HIGH 7.5REMOTEUPGRADE

CVE-2026-66046 is a high-severity flaw (CVSS 7.5) affecting libexpat. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Expat through 2.8.3 contains a denial of service vulnerability caused by quadratic algorithmic complexity in the storeAtts() function in xmlparse.c, where processing N specified attributes with non-normalized values triggers an O(N^2) linear scan of elementType->defaultAtts to determine CDATA status. A remote unauthenticated attacker can supply a single well-formed XML document of a few megabytes to an application parsing untrusted XML to cause e

Why this priority

High severity
Public exploit reference
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	0.7% (top 46%)
CISA exploited list	No
Weakness	CWE-407
Published	2026-08-18
Last modified	2026-10-22

Publisher (CNA): VulnCheck · Vendor: libexpat project

Am I affected?

Timeline

2026-08-18Published CVE record published
2026-10-22 22:10 UTCUpdated Record last modified

libexpat project libexpat UPGRADE

Affected: <= 2.8.3

Fixed in: newer than 2.8.3

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to newer than 2.8.3 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libexpat

Note

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

Exact fixes by distribution

Fedora 46 · python-fontconfig · FIXED · 0.6.3-1.fc46 · FEDORA-2026-f92ca8b01f · source

sudo dnf upgrade --advisory=FEDORA-2026-f92ca8b01f

Fedora ELN · valgrind · FIXED · 3.28.0-1.eln159 · FEDORA-2026-c593ef1d15 · source

sudo dnf upgrade --advisory=FEDORA-2026-c593ef1d15

Ubuntu 20.04 · expat · FIXED · 2.2.9-1ubuntu0.8+esm3

sudo apt update && sudo apt install --only-upgrade expat dpkg --compare-versions "$(dpkg-query -W -f='${Version}' expat)" ge "2.2.9-1ubuntu0.8+esm3" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 22.04 · expat · FIXED · 2.4.7-1ubuntu0.9

sudo apt update && sudo apt install --only-upgrade expat dpkg --compare-versions "$(dpkg-query -W -f='${Version}' expat)" ge "2.4.7-1ubuntu0.9" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 24.04 · expat · FIXED · 2.6.1-2ubuntu0.6

sudo apt update && sudo apt install --only-upgrade expat dpkg --compare-versions "$(dpkg-query -W -f='${Version}' expat)" ge "2.6.1-2ubuntu0.6" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 20.04 · apache2 · NOT AFFECTED

Ubuntu 22.04 · apache2 · NOT AFFECTED

Debian 12 · expat · FIXED · 2.5.0-1+deb12u4

sudo apt update && sudo apt install --only-upgrade expat dpkg --compare-versions "$(dpkg-query -W -f='${Version}' expat)" ge "2.5.0-1+deb12u4" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · expat · FIXED · 2.9.0-1~deb13u1

sudo apt update && sudo apt install --only-upgrade expat dpkg --compare-versions "$(dpkg-query -W -f='${Version}' expat)" ge "2.9.0-1~deb13u1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 10 · expat · FIXED · expat-2.7.3-1.el10_2.5

sudo dnf upgrade --advisory=RHSA-2026:74001 rpm -q expat

RHEL 8 · expat · FIXED · expat-2.5.0-4.el8_10

sudo dnf upgrade --advisory=RHSA-2026:72448 rpm -q expat

RHEL 9 · expat · FIXED · expat-2.5.0-6.el9_8.5

sudo dnf upgrade --advisory=RHSA-2026:72663 rpm -q expat

RHEL 10 · firefox · NO FIX YET

RHEL 10 · thunderbird · NO FIX YET

Alpine 3.21 · expat · FIXED · expat-2.8.4-r0

apk add --upgrade expat apk info -v expat

Alpine 3.22 · expat · FIXED · expat-2.8.4-r0

apk add --upgrade expat apk info -v expat

Alpine 3.23 · expat · FIXED · expat-2.8.4-r0

apk add --upgrade expat apk info -v expat

Alpine 3.24 · expat · FIXED · expat-2.8.4-r0

apk add --upgrade expat apk info -v expat

Amazon Linux 2023 · see advisory · FIXED · ALAS2023-2026-2157 · source

sudo dnf update --advisory=ALAS2023-2026-2157

Amazon Linux 2 · see advisory · FIXED · ALAS2-2026-3965 · source

sudo yum update --advisory=ALAS2-2026-3965

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Write-up: github.com/libexpat/libexpat/pull/1321

Advisories and coverage

Official advisories

Amazon Linux 2 ALAS: ALAS2-2026-3965 (important): expat 2026-10-12
Amazon Linux 2 ALAS: ALAS2-2026-3880 (important): thunderbird 2026-08-31
Amazon Linux 2 ALAS: ALAS2FIREFOX-2026-064 (important): firefox 2026-08-31

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.

References

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