VulnerabilityPublished 19 Sep 2026Updated 8 Nov 2026

CVE-2026-93990: libexpat · Improper Handling of Unicode Encoding

Affects libexpatSource CVE Database V5

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Not known to be exploited · CVSS 8.7 High : upgrade to 2.8.5 soon.

Data as of 8 Nov 2026, 14:21 UTC

CVSS 4.0 base score

8.7High

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (VulnCheck)	CVSS 4.0	8.7
CNA (VulnCheck)	CVSS 3.1	7.5

Exploitation: No evidence of active exploitation and no public proof of concept
Automatable: Attackers can reliably automate steps 1–4 of the kill chain
Technical impact: Limited control over the software or limited information exposure

CISA SSVC assessment, 2026-09-21.

Patch

Available – fixed in 2.8.5

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

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: High
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 19 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 8 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade libexpat to 2.8.5.

Affected software

Product	Affected	Fixed in
libexpatlibexpat	Affected <2.8.5	Fixed in 2.8.5
GitHublibexpat`pkg:github/libexpat`Other threats	Affected <2.8.5	Fixed in 2.8.5

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade libexpat to version 2.8.5 or later, where the issue is fixed. No other mitigations are indicated or required.

Description

CVE-2026-93990 is a high-severity vulnerability in libexpat versions before 2.8.5 where improper handling of UTF-16 encoding allows malformed sequences with lone high surrogates to be accepted. This can cause unpaired surrogates to be passed to applications or silent replacement of input characters, potentially impacting XML processing.

Impact

The vulnerability can cause applications using libexpat to process malformed UTF-16 XML input incorrectly, potentially leading to unexpected behavior or data corruption due to unpaired surrogate characters or silent character replacement. The CVSS v4.0 score of 8.7 indicates a high impact with network attack vector, low complexity, no privileges or user interaction required, but high impact on integrity.

Summary

Libexpat versions prior to 2.8.5 do not properly validate that a high surrogate in UTF-16 input is followed by a low surrogate. This flaw allows attackers to supply malformed UTF-16 encoded XML containing lone high surrogates. In applications built with XML_UNICODE, this results in unpaired surrogates being passed to the application, while in other builds, input characters may be silently replaced. The vulnerability was fixed in version 2.8.5.

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Exploitation evidence

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Affected software

Description

AI analysisMachine-generated, updated 8 Nov 2026, 14:21 UTC

Technical details

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