CVE-2026-66046
Expat Denial of Service via storeAtts() Quadratic Complexity
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.
Fixes and workarounds
Fix
The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches.
Source: CNA: VulnCheckAffected products and versions
| Product | Versions | Authority |
|---|---|---|
| libexpat libexpat project libexpat/libexpat
github |
≥ 0; ≤ 2.8.3 (affected, semver)
Other versions: affected |
CNA: VulnCheck |
| SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0) Siemens |
≥ V3.1.7; < * (affected, custom)
Other versions: unknown |
ADP: siemens-SADP |
| SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0) Siemens |
≥ V3.1.7; < * (affected, custom)
Other versions: unknown |
ADP: siemens-SADP |
| SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0) Siemens |
≥ V3.1.7; < * (affected, custom)
Other versions: unknown |
ADP: siemens-SADP |
| SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0) Siemens |
≥ V3.1.7; < * (affected, custom)
Other versions: unknown |
ADP: siemens-SADP |
| SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0) Siemens |
≥ V3.1.7; < * (affected, custom)
Other versions: unknown |
ADP: siemens-SADP |
Version ranges are source assertions. Check the vendor advisory and your installed configuration before determining exposure.
CVSS assessments
| Version / score | Vector | Authority |
|---|---|---|
| 4.0 / 8.7 high |
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N | CNA: VulnCheck |
| 3.1 / 7.5 high |
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H | CNA: VulnCheck |
Exploit evidence
No exploit evidence in collected sources. This is not proof of no exploitation.
References
- https://github.com/libexpat/libexpat/pull/1321CNA: VulnCheck · issue-tracking, patch
- https://www.vulncheck.com/advisories/expat-denial-of-service-via-storeatts-quadratic-complexityCNA: VulnCheck · third-party-advisory
- https://cert-portal.siemens.com/productcert/html/ssa-457936.htmlADP: siemens-SADP
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