Expat through 2.8.4 Malformed UTF-16 Acceptance via Unchecked Surrogate (CVE-2026-93990) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-93990

Expat through 2.8.4 Malformed UTF-16 Acceptance via Unchecked Surrogate

Answer in brief

CVE-2026-93990 records a Unknown severity vulnerability in Expat through 2.8.4 Malformed UTF-16 Acceptance via Unchecked Surrogate. The current sources do not mark it as known exploited. The current feed maps libexpat/libexpat (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 19, 2026Updated Sep 19, 2026Source checked Sep 20, 2026First seen by HOL Sep 19, 2026Material review Sep 19, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Not reported

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps libexpat/libexpat (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
libexpat/libexpatgeneric	0	Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for libexpat/libexpat.
Review the reported fix. Monitor this advisory for an available fix and review any installs of the affected package.

Evidence timeline and material changes

Published upstream
Sep 19, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 19, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 19, 2026

Sources and claim methodology

Upstream source description

Expat through 2.8.4 fails to validate low surrogates following high surrogates in UTF-16 input, allowing malformed UTF-16 sequences to be accepted. Attackers can craft UTF-16 encoded XML with lone high surrogates that consume following code units, hiding markup characters from the parser and enabling XML injection attacks.

Quoted source text, attributed separately from HOL analysis.