CVE-2026-56403: libexpat before 2.8.2 has an integer overflow in storeAtts.

mediumCVSS 6.9CVE-2026-56403

CSIRTS triage

vendor: libexpat product: libexpatOtheraffected: before 2.8.2

What: The vulnerability is an integer overflow in the storeAtts function.
Who is affected: Deployments using libexpat versions before 2.8.2.
Urgency: Remediation is medium urgency 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.

⚡ Watch libexpat

Get an email when a new libexpat advisory drops — max one per day, one-click unsubscribe.

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-2026-56403

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-2026-564030.10% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 1% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-56403	coverage & exploitation status	NVD · CVE.org

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.

mediumCVE-2026-56409: xmlwf in libexpat before 2.8.2 has an integer overflow for the output filename when -d outputD…msrc · 2026-06-09
mediumCVE-2026-56406: libexpat before 2.8.2 has an integer overflow in XML_ParseBuffer because it lacked a check tha…msrc · 2026-06-09
mediumCVE-2026-56405: libexpat before 2.8.2 has an integer overflow in getAttributeId.msrc · 2026-06-09
mediumCVE-2026-56411: xmlwf in libexpat before 2.8.2 has an integer overflow in endDoctypeDecl via NOTATION declarat…msrc · 2026-06-09
mediumCVE-2026-56131: libexpat before 2.8.2 lacks handler call depth tracking for calls to XML_ResumeParser from wit…msrc · 2026-06-09
mediumCVE-2026-50219: libexpat before 2.8.2 lacks handler call depth tracking for calls to XML_GetBuffer, XML_Parse,…msrc · 2026-06-09