libexpat Heap Buffer Over-read in xmlparse.c via XML_ParseBuffer (CVE-2026-77214)

What is affected, what to do

A flaw was found in expat. A remote attacker can exploit this vulnerability through repeated parse buffer operations with unvalidated buffer lengths, causing a heap buffer over-read.

Update expat to version 2.9.0.

Affected

• Libexpat lte *..2.8.5

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Metrics

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. A remote attacker can exploit this vulnerability through repeated parse buffer operations with unvalidated buffer lengths, causing a heap buffer over-read. This issue primarily leads to information disclosure by leaking adjacent heap memory contents, which could assist in bypassing security mitigations such as Address Space Layout Randomization (ASLR), or result in a Denial of Service (DoS) by crashing the application.

Source: suse_csaf, Debian Security Advisories, Debian LTS Advisories, redhat_csaf, Microsoft Security Update Guide, BSI WID Portal, cvelistv5, NVD (NIST), BSI CSAF, HKCERT (CC BY 3.0 HK) · HKCERT rating: medium

Vendors and products

• Debian Expat
• Libexpat Libexpat
• Open Source Expat

Weakness classes (CWE)

• CWE-125Out-of-bounds Read

  Base

  The product reads data past the end, or before the beginning, of the intended buffer.

  Entry at MITRE CWE-125

References & sources

• github.comPull RequestIssue tracking
• github.comPatch CommitPatch
• vulncheck.comadvisories/libexpat-heap-buffer-over-read-in-xmlparse-c-via-xml…Third-party 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. 10.10.2026Record modifiedCISA ADP

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

   Show the original change

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   SSVC assessment	added	—	{"id":"CVE-2026-77214","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-10T02:25:58.906885Z"}

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

2. 07.10.2026ReceivedVulnCheck

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

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

   Weakness type: CWE-125

   Description added or changed

   New references:

   • https://github.com/libexpat/libexpat/commit/13c5f63a7f1c52c2feee3b16a1134d4fb68e9ea0
   • https://github.com/libexpat/libexpat/pull/1393
   • https://www.vulncheck.com/advisories/libexpat-heap-buffer-over-read-in-xmlparse-c-via-xml-parsebuffer
   Show 8 original changes

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   Description	added	—	libexpat before commit 13c5f63 contains a heap buffer over-read vulnerability in xmlparse.c. XML_ParseBuffer advances the parse buffer end with parser->m_bufferEnd += len using a caller-supplied length that is not validated against the allocated buffer size, so repeated XML_ParseBuffer calls move m_bufferEnd past the end of the heap allocation and subsequent parsing reads out of bounds. Reaching this path requires a parse buffer to already be present; otherwise XML_ParseBuffer returns XML_ERROR_NO_BUFFER. A buffer is present after a prior call to XML_GetBuffer, either directly (the common case) or indirectly through a prior XML_Parse call that allocates the buffer internally. The over-read discloses adjacent heap memory to the calling application, recovering heap pointers, libc function pointers, and code pointers sufficient to defeat ASLR and build further exploitation primitives.
   CVSS	added	—	AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:L/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
   CVSS	added	—	AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L
   Weakness type (CWE)	added	—	CWE-125
   Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/77xxx/CVE-2026-77214.json">CVE-2026-77214</a>
   Reference	added	—	https://github.com/libexpat/libexpat/commit/13c5f63a7f1c52c2feee3b16a1134d4fb68e9ea0
   Reference	added	—	https://github.com/libexpat/libexpat/pull/1393
   Reference	added	—	https://www.vulncheck.com/advisories/libexpat-heap-buffer-over-read-in-xmlparse-c-via-xml-parsebuffer

   Source identifier: disclosure@vulncheck.com