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.

• 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
• https://cert-portal.siemens.com/productcert/html/ssa-457936.html