Soup Sieve: Polynomial-time denial of service in the descendant combinator

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary Matching a selector that uses the descendant combinator (a b) takes time that grows polynomially with how deeply the document is nested, and each extra descendant combinator in the selector raises the degree by one. A selector with k descendant combinators takes O(n^(k+1)) time on a chain of n nested elements. An attacker who controls the HTML or XML that an application passes to Soup Sieve (directly, or through Beautiful Soup's select(), selectone(), css.select(), etc.) can send a very small, deeply nested document that keeps a CPU busy for seconds to minutes against an ordinary selector such as section div div. Details To match a b, Soup Sieve checks each element against b and then walks up through its ancestors looking for one that matches a. When the selector has more descendant combinators, the rest of the selector is tried again from each ancestor on that walk, and so on for every combinator. Nothing was reused between walks, so the same ancestors were matched against…

Published by the source

13 Nov 2026, 00:49 UTC

Last modified by the source

13 Nov 2026, 00:49 UTC

Source last checked

13 Nov 2026, 01:40 UTC

1. Revision 1 · created · 13 Nov 2026

   Published from ghsa record GHSA-mmhr-3c5m-f64f

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# Patch Next advisory brief: CVE-2026-126190

Advisory: Soup Sieve: Polynomial-time denial of service in the descendant combinator
IDs: CVE-2026-126190, GHSA-mmhr-3c5m-f64f
Record: https://patchnext.com/advisories/cve-2026-126190 (revision 1, source checked 13 Nov 2026, 01:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: soupsieve 4.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI soupsieve: affected < 4.0.0; fixed in 4.0.0

## Suggested inspection (read-only)
- `python -m pip show soupsieve`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on soupsieve.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-mmhr-3c5m-f64f (GitHub-reviewed advisory GHSA-mmhr-3c5m-f64f lists affected ranges: PyPI soupsieve >=0 <4.0.0)

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