HighCVE-2026-93990

    Expat before 2.8.5 Malformed UTF-16 Acceptance via Unchecked Surrogate (CVE-2026-93990)

    Published Last modified Sources: SUSE, Red Hat, Debian LTS, Debian, BSI, CVE.org, Msrc Update Guide

    What is affected, what to do

    A flaw was found in Expat. This vulnerability allows a remote attacker to send specially crafted UTF-16 encoded XML data.

    Update Libexpat to version 2.8.5.

    Affected
    • Libexpat lt *..2.8.5
    Fixed in
    • Libexpat 2.8.5

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.7

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v4

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: SUSE, Red Hat, Debian LTS, Debian, BSI, CVE.org, Msrc Update Guide

    0.4 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 34.5

    Show all metrics
    Severity
    High
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    80.89 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.7 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.5
    CVSS v4

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.7
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.4 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    34.5
    Elevated — this CVE ranks above at least 10% of all CVEs scored today.
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: none known · automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 21 September 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-176

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    A flaw was found in Expat. This vulnerability allows a remote attacker to send specially crafted UTF-16 encoded XML data. Due to improper validation of surrogate characters, the parser can be tricked into accepting malformed sequences, which can hide legitimate markup characters. This could enable XML injection attacks, potentially leading to information disclosure or other integrity impacts.

    Source: suse_csaf, redhat_csaf, Debian LTS Advisories, Debian Security Advisories, BSI WID Portal, BSI CSAF, NVD (NIST), cvelistv5, Microsoft Security Update Guide

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    libexpatlibexpat
    • < 2.8.5
    • 2.8.5
    Open Sourceexpat
    • ≤ 2.8.4
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianexpatbookworm
    Linuxdebianexpattrixie

    Weakness classes (CWE)

    • CWE-176Improper Handling of Unicode Encoding

      Variant

      The product does not properly handle when an input contains Unicode encoding.

      Entry at MITRE CWE-176

    References & sources

    • github.comGitHub PR #1282Issue tracking, Patch
    • github.comlibexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc834286…Patch
    • github.comlibexpat/libexpatProduct
    • vulncheck.comVulnCheck Advisory: Expat through 2.8.4 Malformed UTF-16 Acceptance via Unchecked SurrogateThird-party advisory
    • github.comlibexpat/libexpat/releases/tag/R_2_8_5Release notes
    • blog.hartwork.orgposts/expat-2-8-5-releasedVendor advisory

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedVulnCheck

      Description added or changed

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionchanged
      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.
      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.
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/93xxx/CVE-2026-93990.json">CVE-2026-93990</a>
      Referenceadded
      —
      https://blog.hartwork.org/posts/expat-2-8-5-released/
      Referenceadded
      —
      https://github.com/libexpat/libexpat/releases/tag/R_2_8_5

      Source identifier: disclosure@vulncheck.com

    2. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: none known, automatable: yes

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentadded
      —
      {"id":"CVE-2026-93990","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-21T15:10:43.564847Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    3. ReceivedVulnCheck

      CVSS V4.0: AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

      CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

      Weakness type: CWE-176

      Description added or changed

      Show 9 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      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.
      CVSSadded
      —
      AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
      CVSSadded
      —
      AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
      Weakness type (CWE)added
      —
      CWE-176
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/93xxx/CVE-2026-93990.json">CVE-2026-93990</a>
      Referenceadded
      —
      https://github.com/libexpat/libexpat
      Referenceadded
      —
      https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a
      Referenceadded
      —
      https://github.com/libexpat/libexpat/pull/1282
      Referenceadded
      —
      https://www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate

      Source identifier: disclosure@vulncheck.com

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