What’s Vulnerable › Vulnerabilities

CVE-2026-66046: Expat Denial of Service via storeAtts() Quadratic Complexity

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-08-18 · updated 2026-10-22
• Weakness: CWE-407

Description

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: through 2.8.3

References

• https://github.com/libexpat/libexpat/pull/1321
• https://www.vulncheck.com/advisories/expat-denial-of-service-via-storeatts-quadratic-complexity

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