XML injection via unchecked UTF-16 surrogate pairs
Published Sep 19, 2026 · Updated Sep 19, 2026
Improper UTF-16 handling in Expat libexpat 2.8.4 and earlier allows remote attackers to inject XML via malformed surrogate pairs. The UTF-16 tokenizers classify a high surrogate as a four-byte character without verifying that the following code unit is in the U+DC00–U+DFFF low-surrogate range. Unauthenticated network reachability depends on a consuming application accepting attacker-supplied UTF-16 XML; the malformed pair can hide markup from Expat and alter the XML structure seen downstream.
Summary
What happened
Improper UTF-16 handling in Expat libexpat 2.8.4 and earlier allows remote attackers to inject XML via malformed surrogate pairs. The UTF-16 tokenizers classify a high surrogate as a four-byte character without verifying that the following code unit is in the U+DC00–U+DFFF low-surrogate range. Unauthenticated network reachability depends on a consuming application accepting attacker-supplied UTF-16 XML; the malformed pair can hide markup from Expat and alter the XML structure seen downstream.
The record
- CVE
- CVE-2026-93990
- Published
- Sep 19, 2026
- Updated
- Sep 19, 2026
- Vendor
- libexpat Project
- Product
- libexpat
- Classifications
- CWE-176
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Sep 19, 2026CVE publishedPublication date reported by the CVE source.
- Sep 19, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=0 <=2.8.4
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Activity data is delayed. The latest report is Sep 28, 2026.
Weakness, pattern, technique
Public exploit references
- UTF-16 malformed-surrogate regression reproducerproof of concept · validated
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Technologies
Your stack
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