---
id: CVE-2026-66046
title: >-
  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) li…
summary: >-
  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) li…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-407
vendor: libexpat_project
product: libexpat
affected:
  - libexpat <= 2.8.3
published: '2026-08-18'
updated: '2026-09-18'
sourceUpdated: '2026-09-18T15:07:24.370'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-66046'
references:
  - url: 'https://github.com/libexpat/libexpat/pull/1321'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/expat-denial-of-service-via-storeatts-quadratic-complexity
    label: disclosure@vulncheck.com
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-66046.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-66046'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2517901'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-66046'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-66046'
  - url: 'https://access.redhat.com/errata/RHSA-2026:65568'
tags:
  - nvd
  - csaf
  - vex
  - red-hat
epss: 0.0061
epssPercentile: 0.47881
ingestedAt: '2026-09-18T14:43:13.027Z'
patched:
  - hardened_images
---

## Overview

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 excessive CPU consumption, resulting in denial of service without requiring authentication, external entity resolution, or non-default parser options.

## Affected

- `libexpat <= 2.8.3`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## Vendor advisories

- **RHSA-2026:65568** · Red Hat · fixed in: Red Hat Hardened Images · released 2026-09-08 · [advisory](https://access.redhat.com/errata/RHSA-2026:65568)
- **Red Hat VEX** · Important · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, … · updated 2026-09-21 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-66046.json)