# CVE-2026-93990: Expat before 2.8.5 Malformed UTF-16 Acceptance via Unchecked Surrogate

Status: published. Published: 2026-09-19. Updated: 2026-11-08.

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

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/libexpat/libexpat/pull/1282
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a

## Affected products and versions

- libexpat / libexpat (libexpat/libexpat): ≥ 0; < 2.8.5 (affected, semver); 2.8.5 (unaffected, semver). Source: CNA: VulnCheck

## Severity and prioritization

- CVSS 4.0: 8.7 (high); CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N; source: CNA: VulnCheck

- CISA KEV: No listing in collected sources

- EPSS: 0.40% as of 2026-11-08; percentile 34.85%

- SSVC: exploitation none; automatable yes; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-176

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/libexpat/libexpat/pull/1282 (CNA: VulnCheck; issue-tracking, patch)

- https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a (CNA: VulnCheck; patch)

- https://github.com/libexpat/libexpat/releases/tag/R_2_8_5 (CNA: VulnCheck; release-notes)

- https://blog.hartwork.org/posts/expat-2-8-5-released/ (CNA: VulnCheck; vendor-advisory)

- https://github.com/libexpat/libexpat (CNA: VulnCheck; product)

- https://www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate (CNA: VulnCheck; third-party-advisory)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-93990; retrieved 2026-11-08; source updated 2026-11-08

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-93990; retrieved 2026-11-08; source updated Not reported

Revision: 137f03f4ac460f1d09b27c2406de31c342e515a5738e441c10ee979fa1577333

Canonical: https://y2.dev/cve/CVE-2026-93990

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