Vulnerability in Libexpat Project
CVE-2026-66046
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.007 (53.0th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 53rd percentile — 53.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.006 · newest: 0.007 · change: +0.002
Affected products
Weakness classification (CWE)
References
- Pull Request (issue-tracking, Exploit, Patch, Vendor Advisory)
- www.vulncheck.com/advisories/expat-denial-of-service-via-storeatts-quadratic-co… (Exploit, Patch, Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-66046?
- CVE-2026-66046 is a high-severity vulnerability in Libexpat Project, classified under Inefficient Algorithmic Complexity. CVSS score: 7.5/10. Published 2026-08-18.
- How severe is CVE-2026-66046?
- High severity. CVSS v3 base score is 7.5 out of 10.