High severity · published Aug 18, 2026
CVE-2026-66046
Expat Denial of Service via storeAtts() Quadratic Complexity
7.5
CVSS
0.7%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.7% · higher than 54% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
LI
libexpat · libexpat project
through 2.8.3 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-66046
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-66046
issue-tracking
https://github.com/libexpat/libexpat/pull/1321
third-party-advisory
https://www.vulncheck.com/advisories/expat-denial-of-service-via-storeatts-quadratic-complexity
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