CVE-2026-76641Network attack vector

Service interrupt via dtdCopy entry over-read

Published Aug 20, 2026 · Updated Aug 20, 2026

Out-of-bounds read in libexpat's DTD-copying logic allows attackers to crash affected XML-processing applications via crafted external-entity input. In dtdCopy, hash entries for defaultAttForName are allocated as sizeof(NAMED) even after defineAttribute and storeAtts expect the larger NAME_AND_DEFAULT_ATTRIBUTE layout, so storeAtts reads attIndex past each entry and can follow a wild pointer. Upstream states the bad commit reached only unreleased master and downstream trees that backported the earlier incomplete fix, so official Expat release tarballs through 2.8.3 are not affected unless a distributor imported that backport.

CVSS severity8.7
High
EPSS probability0.35%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds read in libexpat's DTD-copying logic allows attackers to crash affected XML-processing applications via crafted external-entity input. In dtdCopy, hash entries for defaultAttForName are allocated as sizeof(NAMED) even after defineAttribute and storeAtts expect the larger NAME_AND_DEFAULT_ATTRIBUTE layout, so storeAtts reads attIndex past each entry and can follow a wild pointer. Upstream states the bad commit reached only unreleased master and downstream trees that backported the earlier incomplete fix, so official Expat release tarballs through 2.8.3 are not affected unless a distributor imported that backport.

The record

CVE
CVE-2026-76641
Published
Aug 20, 2026
Updated
Aug 20, 2026
Vendor
Expat Project
Product
libexpat
Classifications
CWE-125, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 20, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 20, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <=2.8.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Expat Project · libexpatversion=0 <=2.8.3

Published CVSS scores

8.7VulnCheckCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
7.5VulnCheckCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-125Out-of-bounds Read
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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