Vulnerability in Libexpat
CVE-2026-93990
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
EPSS exploit prediction
EPSS: 0.004 (34.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 35th percentile — 34.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000
Affected products
Weakness classification (CWE)
References
- github.com/libexpat/libexpat/pull/1282 (issue-tracking, patch)
- github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a (patch)
- github.com/libexpat/libexpat (product)
- www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-vi… (third-party-advisory)
- github.com/libexpat/libexpat/releases/tag/R_2_8_5 (release-notes)
- blog.hartwork.org/posts/expat-2-8-5-released/ (vendor-advisory)
Frequently asked questions
- What is CVE-2026-93990?
- CVE-2026-93990 is a high-severity vulnerability in Libexpat, classified under CWE-176. CVSS score: 7.5/10. Published 2026-09-19.
- How severe is CVE-2026-93990?
- High severity. CVSS v3 base score is 7.5 out of 10.