---
id: CVE-2026-93990
title: >-
  Expat before 2.8.5 fails to validate that a high surrogate in UTF-16 input is
  followed by a low surrogate, allowing malformed UTF-16 sequences to be
  accepted
summary: >-
  Expat before 2.8.5 fails to validate that a high surrogate in UTF-16 input is
  followed by a low surrogate, allowing malformed UTF-16 sequences to be
  accepted. Attackers can supply UTF-16 encoded XML containing lone high
  surrogates that c…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N'
cwe:
  - CWE-176
  - CWE-91
vendor: libexpat
product: libexpat
affected:
  - libexpat <= 2.8.4
patched:
  - hardened_images
published: '2026-09-19'
updated: '2026-09-28'
sourceUpdated: '2026-09-28T17:17:53.037'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-93990'
references:
  - url: 'https://blog.hartwork.org/posts/expat-2-8-5-released/'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/libexpat/libexpat'
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a
    label: disclosure@vulncheck.com
  - url: 'https://github.com/libexpat/libexpat/pull/1282'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/libexpat/libexpat/releases/tag/R_2_8_5'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate
    label: disclosure@vulncheck.com
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-93990.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-93990'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2538967'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-93990'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-93990'
  - url: 'https://access.redhat.com/errata/RHSA-2026:71655'
  - url: 'https://access.redhat.com/errata/RHSA-2026:74001'
  - url: 'https://access.redhat.com/errata/RHSA-2026:72663'
  - url: 'https://access.redhat.com/errata/RHSA-2026:72448'
  - url: 'https://access.redhat.com/errata/RHSA-2026:79357'
tags:
  - nvd
  - cve.org
  - csaf
  - vex
  - red-hat
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-09-21T15:10:43.564847Z'
ingestedAt: '2026-09-19T23:10:34.247Z'
epss: 0.00403
epssPercentile: 0.34409
---

## Overview

Expat before 2.8.5 fails to validate that a high surrogate in UTF-16 input is followed by a low surrogate, allowing malformed UTF-16 sequences to be accepted. Attackers can supply UTF-16 encoded XML containing lone high surrogates that consume the following code unit, causing Expat to pass unpaired surrogates to applications built with XML_UNICODE and to silently replace input characters in other builds.

## Remediation

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

## Vendor advisories

- **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 2026-10-09 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-93990.json)
- **RHSA-2026:71655** · Red Hat · fixed in: Red Hat Hardened Images · released 2026-09-24 · [advisory](https://access.redhat.com/errata/RHSA-2026:71655)
- **RHSA-2026:74001** · Red Hat · fixed in: Red Hat Enterprise Linux AppStream (v. 10), Red Hat Enterprise Linux BaseOS (v. 10) · released 2026-09-30 · [advisory](https://access.redhat.com/errata/RHSA-2026:74001)
- **RHSA-2026:72663** · Red Hat · fixed in: Red Hat Enterprise Linux AppStream (v. 9), Red Hat Enterprise Linux BaseOS (v. 9) · released 2026-09-29 · [advisory](https://access.redhat.com/errata/RHSA-2026:72663)
- **RHSA-2026:72448** · Red Hat · fixed in: Red Hat Enterprise Linux BaseOS (v. 8) · released 2026-09-28 · [advisory](https://access.redhat.com/errata/RHSA-2026:72448)
- **RHSA-2026:79357** · Red Hat · fixed in: Red Hat Cost Management On-Premise 1 · released 2026-10-08 · [advisory](https://access.redhat.com/errata/RHSA-2026:79357)