CVE-2026-56406: libexpat before 2.8.2 has an integer overflow in XML_ParseBuffer because it lacked a check that was present in XML_Parse.

mediumCVSS 6.9CVE-2026-56406

CSIRTS triage

vendor: XMLSoft product: libexpatOtheraffected: before 2.8.2

What: There is an integer overflow in XML_ParseBuffer due to a missing check.
Who is affected: Users of libexpat versions prior to 2.8.2.
Urgency: Remediation is important due to the medium severity rating.
Action: Upgrade to libexpat version 2.8.2 or later.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: medium — CVSS 6.9
Published: 2026-06-09
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-56406

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-564060.14% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 3% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-56406	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

medium[UPDATE] [medium] libexpat: Multiple vulnerabilities allow unspecified attackcert-bund
unknownDSA-6404-1 expat - security updatedebian

Recent advisories for libexpat

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

medium[UPDATE] [medium] libexpat: Multiple vulnerabilities allow unspecified attackcert-bund · 2026-08-24
mediumCVE-2026-76957: libexpat before 2.8.4 lacks handler call depth tracking with custom encoding callbacks. Thus, …nvd · 2026-08-20
mediumCVE-2026-76956: In libexpat 2.8.2 and 2.8.3 before 2.8.4, misinterpretation of getentropy's return code leads …nvd · 2026-08-20
mediumCVE-2026-72522: libexpat before 2.8.3 has an out-of-bounds read and resultant infinite loop because low surrog…msrc · 2026-08-11
highCVE-2026-76956: In libexpat 2.8.2 and 2.8.3 before 2.8.4, misinterpretation of getentropy's return code leads …msrc · 2026-08-11
mediumCVE-2026-76957: libexpat before 2.8.4 lacks handler call depth tracking with custom encoding callbacks. Thus, …msrc · 2026-08-11