# CVE-2026-126189: Soup Sieve: Quadratic-time denial of service when matching `:has()`

Status: published. Published: 2026-11-12. Updated: 2026-11-12.

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 4.0.1, Soup Sieve repeatedly searches descendant subtrees for each candidate evaluated by :has() and repeatedly reevaluates a parent containing :has(> ...) for each child, causing quadratic matching on deeply nested or wide attacker-influenced HTML or XML. Applications using ordinary fixed selectors through select(), select_one(), or Beautiful Soup CSS APIs against untrusted documents can have a worker occupied for seconds or minutes, while applications that process only trusted documents are not affected. Selectors compiled with NOCACHE continue to use the old uncached behavior. This issue is fixed in version 4.0.1.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- facelessuser / soupsieve: < 4.0.1 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 3.1: 5.3 (medium); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L; source: CNA: GitHub_M

- CVSS 3.1: 5.3 (medium); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L; source: NVD: security-advisories@github.com

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-407

## Exploit evidence

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

## References

- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-8jvp-3x96-xpqw (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/facelessuser/soupsieve/pull/352 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/facelessuser/soupsieve/commit/0469c148f5501ec2127e0d65b77c2a1c395fa379 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/facelessuser/soupsieve/commit/305fbf6b8c1f6ff05fec55d1e5bb28d80d22de44 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/facelessuser/soupsieve/releases/tag/4.0.1 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/facelessuser/soupsieve/commit/0469c148f5501ec2127e0d65b77c2a1c395fa379 (security-advisories@github.com)

- https://github.com/facelessuser/soupsieve/commit/305fbf6b8c1f6ff05fec55d1e5bb28d80d22de44 (security-advisories@github.com)

- https://github.com/facelessuser/soupsieve/pull/352 (security-advisories@github.com)

- https://github.com/facelessuser/soupsieve/releases/tag/4.0.1 (security-advisories@github.com)

- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-8jvp-3x96-xpqw (security-advisories@github.com)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-126189; retrieved 2026-11-12; source updated 2026-11-12

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126189; retrieved 2026-11-12; source updated 2026-11-12

Revision: b90a1f3004b77c961d005c0675b5c5ec60b1cc9390f57dbf49a0a72d4aa0da1d

Canonical: https://y2.dev/cve/CVE-2026-126189

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