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Fencer Vulns
medium 4.9

CVE-2026-76957 json

libexpat before 2.8.4 lacks handler call depth tracking with custom encoding callbacks.

published 2026-08-20 2 mo ago modified 2026-09-08 2 mo ago also EUVD-2026-63196

cvss
4.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.1%
EPSS 0.1% chance of exploitation in 30 days, percentile 1%
public exploits
0
none known

Summary

libexpat before 2.8.4 lacks handler call depth tracking with custom encoding callbacks. Thus, a use-after-free can occur. NOTE: this is similar to CVE-2026-50219, CVE-2026-56131 and CVE-2026-56412.

Patch / fix

  • libexpat project libexpat 2.8.4
  • expat Debian 12 2.5.0-1+deb12u3
  • expat Debian 13 2.9.0-1~deb13u1
  • expat Debian 14 2.8.4-1

debian advisories DEBIAN-CVE-2026-76957

  • expat Ubuntu 22.04 LTS 2.4.7-1ubuntu0.9
  • expat Ubuntu 24.04 LTS 2.6.1-2ubuntu0.6
  • expat Ubuntu 26.04 LTS 2.7.4-1ubuntu0.2
  • expat Ubuntu Pro 14.04 LTS 2.1.0-4ubuntu1.4+esm13
  • expat Ubuntu Pro 16.04 LTS 2.1.0-7ubuntu0.16.04.5+esm14
  • expat Ubuntu Pro 18.04 LTS 2.2.5-3ubuntu0.9+esm5
  • expat Ubuntu Pro 20.04 LTS 2.2.9-1ubuntu0.8+esm3

ubuntu advisories UBUNTU-CVE-2026-76957

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • expat hummingbird 1 2.8.4-0.1.hum1
  • expat hummingbird 1 2.8.3-0.1.1.hum1

red hat advisories RHSA-2026:65568RHSA-2026:60451

  • expat Linux Micro 6.1 2.8.5-slfo.1.1_1.1
  • MozillaFirefox SLES for SAP 15 SP6, SLES for SAP 15 SP5, SLES for SAP 15 SP4 +8 more 153.5.0-150400.157.16.1
  • MozillaFirefox SLES for SAP applications 16.0, SLES 16.0 153.5.0-160000.1.1
  • MozillaThunderbird SLE Workstation Extension 15 SP7, SLE Module for Package Hub 15 SP7 153.5.0-150400.13.9.1
  • expat SLE Module for Basesystem 15 SP7 2.8.5-150700.3.15.1
  • expat Linux Micro 6.0 2.8.5-159998.1.1
  • expat SLES for SAP 15 SP6, SLES for SAP 15 SP5, SLES for SAP 15 SP4 +10 more 2.8.5-150400.3.40.1
  • expat SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 2.8.4-160000.1.1

suse advisories SUSE-SU-2026:24554-1SUSE-SU-2026:4834-1SUSE-SU-2026:4806-1SUSE-SU-2026:24260-1SUSE-SU-2026:4736-1SUSE-SU-2026:4619-1

  • MozillaFirefox openSUSE Leap 16.0 153.5.0-160000.1.1
  • expat openSUSE Leap 16.0 2.8.4-160000.1.1
  • expat openSUSE Tumbleweed 2.8.4-1.1

opensuse advisories openSUSE-SU-2026:22139-1openSUSE-SU-2026:21945-1openSUSE-SU-2026:11733-1

  • expat Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 2.8.4-r0

alpine advisories ALPINE-CVE-2026-76957

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • expat Azure Linux 3.0 2.8.3-2

azure linux advisories AZL-96851

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-76957 updated cvss 4.9

libexpat: libexpat: Memory corruption vulnerability allows arbitrary code execution or denial of service

  • Red Hat Enterprise Linux 6 expat, compat-expat1 Red Hat Enterprise Linux 7 expat, firefox Red Hat Enterprise Linux 8 expat, firefox, xmlrpc-c, mingw-expat, thunderbird, xmlrpc-c-c++, +4 more Red Hat Enterprise Linux 9 expat Red Hat Enterprise Linux 10 expat, firefox, thunderbird Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8, openshift/ose-rhel-coreos-9
    +1 more products
    Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat OpenShift AI (RHOAI) rhoai/odh-dashboard-rhel9
    under investigation
  • Red Hat Hardened Images expat 2.8.4-0.1.hum1, expat 2.8.3-0.1.1.hum1
    fixed: vendor fix RHSA-2026:60451
  • Red Hat OpenShift AI (RHOAI) rhoai/odh-mlflow-rhel9
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
2 mo ago

Exploit prediction

0.11% chance of exploitation in the next 30 days, higher than 1% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 4.9 CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
NVD 3.1 7.8 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Affected products

  • libexpat project libexpat

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.