CVE-2026-66046: Libexpat Project Libexpat

High severity, CVSS 7.5. EPSS: 0.7% chance of exploitation in the next 30 days.

Expat through 2.8.3 contains a denial of service vulnerability caused by quadratic algorithmic complexity in the storeAtts() function in xmlparse.c, where processing N specified attributes with non-normalized values triggers an O(N^2) linear scan of elementType->defaultAtts to determine CDATA status. A remote unauthenticated attacker can supply a single well-formed XML document of a few megabytes to an application parsing untrusted XML to cause excessive CPU consumption, resulting in denial of service without requiring authentication, external entity resolution, or non-default parser options.

Affected products

• Libexpat Project Libexpat: up to and including 2.8.3

Published 2026-08-18. Last modified 2026-09-18.