What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-19 · updated 2026-09-28
• Weakness: CWE-176

Description

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.

Affected products

• libexpat: before 2.8.5 — fixed in 2.8.5

How to fix

Upgrade to a fixed version: libexpat 2.8.5. Check the vendor advisory for mitigations.

References

• https://blog.hartwork.org/posts/expat-2-8-5-released/
• https://github.com/libexpat/libexpat
• https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a
• https://github.com/libexpat/libexpat/pull/1282
• https://github.com/libexpat/libexpat/releases/tag/R_2_8_5
• https://www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate

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