# CVE-2026-118370: libexpat through 2.9.0 Integer Overflow in storeAtts via attIndex and nPrefixes

Status: published. Published: 2026-10-28. Updated: 2026-11-02.

libexpat through 2.9.0 contains a signed integer overflow vulnerability in storeAtts() in lib/xmlparse.c that allows attackers to trigger out-of-bounds writes via oversized attribute counts. Attackers can supply a multi-gigabyte document with a start tag carrying over 2^30 attributes to overflow attIndex and nPrefixes, crashing applications or corrupting heap memory.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/libexpat/libexpat/pull/1450
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/libexpat/libexpat/commit/1bac53c4fc460a03cd9860f39501c7979f6750ae
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/libexpat/libexpat/commit/621cb2c78b253ee023a86bc91d6ddb048481c04c
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: disclosure@vulncheck.com) — https://github.com/libexpat/libexpat/commit/1bac53c4fc460a03cd9860f39501c7979f6750ae
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: disclosure@vulncheck.com) — https://github.com/libexpat/libexpat/commit/621cb2c78b253ee023a86bc91d6ddb048481c04c
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: disclosure@vulncheck.com) — https://github.com/libexpat/libexpat/pull/1450
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: disclosure@vulncheck.com) — https://www.vulncheck.com/advisories/libexpat-through-2.9.0-integer-overflow-in-storeatts-via-attindex-and-nprefixes

## Affected products and versions

- libexpat / libexpat (libexpat/libexpat): ≥ 0; ≤ 2.9.0 (affected, semver). Source: CNA: VulnCheck

## NVD configuration conditions

Affected CPE matches depend on the following AND/OR and negation conditions:

```json
[
  {
    "negate": false,
    "nodes": [
      {
        "cpeMatch": [
          {
            "criteria": "cpe:2.3:a:libexpat_project:libexpat:*:*:*:*:*:*:*:*",
            "versionEndIncluding": "2.9.0",
            "vulnerable": true
          }
        ],
        "negate": false,
        "operator": "OR"
      }
    ],
    "operator": "OR"
  }
]
```

## Severity and prioritization

- CVSS 4.0: 8.2 (high); CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 5.9 (medium); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H; source: CNA: VulnCheck

- CVSS 4.0: 8.2 (high); CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: disclosure@vulncheck.com

- CVSS 3.1: 8.2 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H; source: NVD: nvd@nist.gov

- CVSS 3.1: 5.9 (medium); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H; source: NVD: disclosure@vulncheck.com

- CISA KEV: No listing in collected sources

- EPSS: 0.44% as of 2026-11-02; percentile 38.42%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-190

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/libexpat/libexpat/pull/1450 (CNA: VulnCheck; patch, issue-tracking)

- https://github.com/libexpat/libexpat/commit/1bac53c4fc460a03cd9860f39501c7979f6750ae (CNA: VulnCheck; patch)

- https://github.com/libexpat/libexpat/commit/621cb2c78b253ee023a86bc91d6ddb048481c04c (CNA: VulnCheck; patch)

- https://github.com/libexpat/libexpat/blob/a76b1174c1ea52abb639a4c69d52b630a8a106a2/expat/lib/xmlparse.c#L4138-L4140 (CNA: VulnCheck; technical-description)

- https://github.com/libexpat/libexpat (CNA: VulnCheck; product)

- https://www.vulncheck.com/advisories/libexpat-through-2.9.0-integer-overflow-in-storeatts-via-attindex-and-nprefixes (CNA: VulnCheck; third-party-advisory)

- https://github.com/libexpat/libexpat (disclosure@vulncheck.com; Product)

- https://github.com/libexpat/libexpat/blob/a76b1174c1ea52abb639a4c69d52b630a8a106a2/expat/lib/xmlparse.c#L4138-L4140 (disclosure@vulncheck.com; Product)

- https://github.com/libexpat/libexpat/commit/1bac53c4fc460a03cd9860f39501c7979f6750ae (disclosure@vulncheck.com; Patch)

- https://github.com/libexpat/libexpat/commit/621cb2c78b253ee023a86bc91d6ddb048481c04c (disclosure@vulncheck.com; Patch)

- https://github.com/libexpat/libexpat/pull/1450 (disclosure@vulncheck.com; Patch)

- https://www.vulncheck.com/advisories/libexpat-through-2.9.0-integer-overflow-in-storeatts-via-attindex-and-nprefixes (disclosure@vulncheck.com; Patch, Third Party Advisory)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-118370; retrieved 2026-11-02; source updated 2026-11-02

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-118370; retrieved 2026-11-02; source updated 2026-11-02

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-118370; retrieved 2026-11-02; source updated Not reported

Revision: f9be18f526386e44dc70f5babe46555ee43befd8cc012db00fe330f7ced7ce7d

Canonical: https://y2.dev/cve/CVE-2026-118370

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