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High, CVSS 7.5amended

Soup Sieve: Exponential ReDoS in selector compilation via \9-run in CSS_ESCAPES/IDENTIFIER

CVE-2026-126191GHSA-6655-6qjp-jx7x

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (soupsieve 3.0.2), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
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
  • CVE-2026-126191: 0.43% estimated exploitation activity in the next 30 days; percentile 38.25 (FIRST EPSS, 2026-11-14). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary The IDENTIFIER/CSS_ESCAPES regex (css_parser.py:119-124) exhibits exponential backtracking on a run of ambiguous digit-escapes (`\9`) in an attribute-name or tag position of an otherwise well-formed selector. It fires at compile time (before the SELECTOR_LIMIT guard) via the public `soupsieve.compile()` and BeautifulSoup `.select()`.

PoC (verified on soupsieve 2.9.2) import soupsieve as sv sv.compile('[a' + '\\9'*26 + '=x]')

~57 bytes -> ~10s; ~16x time per +4 chars

Novelty Distinct from the patched GHSA-836r-... (VALUE regex) and GHSA-2wc2-... (comma-list) advisories, which are fixed in 2.9.2; this is a different regex/vector.

Reachability CSS selectors are usually developer-controlled, so exposure requires an application that compiles UNTRUSTED selector strings (web-scraping-as-a-service, no-code/query builders, user-defined extraction rules). Where selectors are user-supplied, a ~60-byte string hangs the worker.

Note This issue was fixed in version…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from osv record GHSA-6655-6qjp-jx7x

  2. Revision 1 · created ·

    Published from ghsa record GHSA-6655-6qjp-jx7x

Agent brief

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# Patch Next advisory brief: CVE-2026-126191

Advisory: Soup Sieve: Exponential ReDoS in selector compilation via \9-run in CSS_ESCAPES/IDENTIFIER
IDs: CVE-2026-126191, GHSA-6655-6qjp-jx7x
Record: https://patchnext.com/advisories/cve-2026-126191 (revision 2, source checked 14 Nov 2026, 07:50 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 3.0.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI soupsieve: affected < 3.0.2; fixed in 3.0.2

## 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-6655-6qjp-jx7x (GitHub-reviewed advisory GHSA-6655-6qjp-jx7x lists affected ranges: PyPI soupsieve >=0 <=2.10)
- Verified: https://osv.dev/vulnerability/GHSA-6655-6qjp-jx7x (OSV record GHSA-6655-6qjp-jx7x lists affected ranges: PyPI soupsieve >=0 <3.0.2 (fixed))

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