Soup Sieve: General sibling combinator inside :has() matches untrusted HTML in quadratic time

Exploitation

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

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary Matching a selector that puts a general-sibling combinator inside :has(), such as div:has(a ~ b), against untrusted HTML runs in O(n²) time in the number of sibling elements. A single small document with a few thousand siblings ties the process up for a minute of CPU, so an application that runs a fixed selector of this shape over HTML it doesn't control can be stalled by one request. :has is powerful, but much in the same way poor regular expressions can be crafted to cause catastrophic backtracking with greedy capturing, has can be used to create non-ideal scenarios that can cause large portions of the tree to be evaluated for one or many elements if not used thoughtfully. This behavior is not exclusive to Soup Sieve, but browsers make note of these potential performance issues themselves: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:has#performanceconsiderations. With all of that said, there is room for improvements that can in some cases greatly…

Published by the source

13 Nov 2026, 00:45 UTC

Last modified by the source

13 Nov 2026, 00:45 UTC

Source last checked

13 Nov 2026, 01:40 UTC

1. Revision 1 · created · 13 Nov 2026

   Published from ghsa record GHSA-4j63-6c4x-wfg4

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# Patch Next advisory brief: CVE-2026-126192

Advisory: Soup Sieve: General sibling combinator inside :has() matches untrusted HTML in quadratic time
IDs: CVE-2026-126192, GHSA-4j63-6c4x-wfg4
Record: https://patchnext.com/advisories/cve-2026-126192 (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: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: soupsieve 2.10.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI soupsieve: affected <= 2.9.2; fixed in 2.10.0

## 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-4j63-6c4x-wfg4 (GitHub-reviewed advisory GHSA-4j63-6c4x-wfg4 lists affected ranges: PyPI soupsieve >=0 <=2.9.2)

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