Integer overflow in Libexpat
CVE-2026-118370
libexpat through 2.9.0 contains a signed integer overflow vulnerability in storeAtts() in lib/xmlparse.c that allows attackers to trigger out-of-bounds writes via oversized attribute counts. Attackers can supply a multi-gigabyte document with a start tag carrying over 2^30 attributes to overflow attIndex and nPrefixes, crashing applications or corrupting heap memory.
Vulnerability class: Integer Overflow
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.004 (38.3th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 38th percentile — 38.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
The 30-day trend appears once there are two daily scores.
Affected products
Weakness classification (CWE)
References
- github.com/libexpat/libexpat/pull/1450 (issue-tracking, patch)
- github.com/libexpat/libexpat/commit/1bac53c4fc460a03cd9860f39501c7979f6750ae (patch)
- github.com/libexpat/libexpat/commit/621cb2c78b253ee023a86bc91d6ddb048481c04c (patch)
- github.com/libexpat/libexpat/blob/a76b1174c1ea52abb639a4c69d52b630a8a106a2/expa… (technical-description)
- github.com/libexpat/libexpat (product)
- www.vulncheck.com/advisories/libexpat-through-2.9.0-integer-overflow-in-storeat… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-118370?
- CVE-2026-118370 is a medium-severity vulnerability in Libexpat, classified under Integer Overflow or Wraparound. CVSS score: 5.9/10. Published 2026-10-28.
- How severe is CVE-2026-118370?
- Medium severity. CVSS v3 base score is 5.9 out of 10.