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CVE-2026-56403 libexpat project CVE debrief

CVE-2026-56403 is a medium severity vulnerability in libexpat before version 2.8.2. The issue is an integer overflow in the storeAtts function. This vulnerability has a CVSS score of 6.9. Affected systems and applications using libexpat versions prior to 2.8.2 are at risk. The vulnerability was published on June 21, 2026, and no later modifications were recorded on the same day. The primary concern for defenders is the potential for local attacks with high complexity, which could lead to confidentiality, integrity, and limited availability impacts. Therefore, defenders should prioritize patching or mitigating this vulnerability.

Vendor
libexpat project
Product
libexpat
CVSS
MEDIUM 6.9
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-21
Original CVE updated
2026-06-22
Advisory published
2026-06-21
Advisory updated
2026-06-22

Who should care

Defenders of systems and applications that utilize libexpat, especially those using versions prior to 2.8.2, should be concerned about CVE-2026-56403. This includes IT administrators, cybersecurity professionals, and developers responsible for maintaining or updating software that incorporates libexpat. Given the medium severity and potential for local attacks with high complexity, prioritizing patching or applying mitigations is crucial to reduce risk.

Technical summary

CVE-2026-56403 is a vulnerability in libexpat, a widely used XML parsing library, specifically affecting versions before 2.8.2. The vulnerability is caused by an integer overflow in the storeAtts function. According to the CVSS v3.1 vector (AV:L/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L), the vulnerability allows for local attacks with high complexity, leading to high impacts on confidentiality and integrity, and low impact on availability. The Common Weakness Enumeration (CWE) for this vulnerability is CWE-190, which involves integer overflows.

Defensive priority

Medium priority due to local attack vector with high complexity and potential high impacts on confidentiality and integrity.

Recommended defensive actions

  • Inventory and assess the use of libexpat within your environment, focusing on versions prior to 2.8.2.
  • Review and apply the official patch or upgrade to libexpat version 2.8.2 or later.
  • Implement compensating controls to limit exposure, such as restricting local access to vulnerable systems.
  • Monitor for any unusual activity that could indicate exploitation attempts.
  • Verify the integrity of libexpat installations and ensure no workarounds or unofficial patches are in use.

Evidence notes

The primary evidence for CVE-2026-56403 comes from the NVD and CVE.org records. The vulnerability affects libexpat versions before 2.8.2, with the specific issue being an integer overflow in the storeAtts function. The CVSS score is 6.9, indicating medium severity. The CVE was published and last modified on June 21, 2026. There is a reference to a GitHub pull request (https://github.com/libexpat/libexpat/pull/1232) related to the fix for this issue. Defenders should verify libexpat versions in their environment and check for official patches or updates.

Official resources

This CVE debrief is AI-assisted and based on the supplied source corpus.