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

> Attackers can inject malformed UTF-16 sequences into XML, causing data corruption or silent character replacement in applications using the Expat library.

- **Severity:** High 8.7 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Sep 2026
- **Updated:** 28 Sep 2026
- **Also known as:** ALPINE-CVE-2026-93990, AZL-103316, AZL-105099, AZL-105149, AZL-105260, BELL-CVE-2026-93990, CLSA-2026-1790172968, CLSA-2026-1790177624, CLSA-2026-1790252195, CLSA-2026-1790252568, CLSA-2026-1790918446, CLSA-2026-1790919520, CLSA-2026-1790919725, DEBIAN-CVE-2026-93990, ECHO-41d9-8113-ce4d, GHSA-rfp9-ffqc-h4x8, OESA-2026-4175, OESA-2026-4176, OESA-2026-4177, OESA-2026-4178, RHSA-2026:71655, UBUNTU-CVE-2026-93990

## What is CVE-2026-93990?

CVE-2026-93990 is a high-severity vulnerability in libexpat, published on 19 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-176 (Improper Handling of Unicode Encoding)

## Which versions are affected by CVE-2026-93990?


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `expat` | Red Hat:hummingbird:1 | All versions | 0:2.8.4-0.2.hum1 |
| `expat-devel` | Red Hat:hummingbird:1 | All versions | 0:2.8.4-0.2.hum1 |
| `expat-static` | Red Hat:hummingbird:1 | All versions | 0:2.8.4-0.2.hum1 |

## How do I fix CVE-2026-93990?

Upgrade to a fixed version.

## Is CVE-2026-93990 being exploited?

There is no known exploitation of CVE-2026-93990 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-93990 and fix it if it is affected.

What it is: CVE-2026-93990 is a high-severity vulnerability in libexpat, published on 19 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-93990 in the wild and no public exploit code.

Affected product: libexpat. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-93990/ (Markdown: https://insights.sbomify.com/CVE-2026-93990/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-93990
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-93990
- OSV: https://osv.dev/vulnerability/CVE-2026-93990

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-93990/
Summaries are generated and may contain mistakes; the official description above is the record.