Soup Sieve: Polynomial-time ReDoS (O(n²)) in the whitespace/comment trimming regex RE_WS_END (triggers on VALID selectors)

Exploitation

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

CVSS

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

EPSS

• CVE-2026-85999: 0.61% estimated exploitation activity in the next 30 days; percentile 49.21 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Before tokenizing, selectoriter trims leading/trailing whitespace and comments by running two regexes over the whole raw selector with .search(). The trailing one, REWSEND = re.compile(r'{WSC}$'), is anchored only at the end ($), not the start. Because .search() retries the pattern at every offset, a long run of whitespace or CSS comments that is not sitting exactly at the end of the string makes each retry greedily consume the run and then fail $, producing O(n²) time. This triggers on perfectly valid selectors — e.g. a descendant combinator with a long whitespace gap, a + " "n + b — so no malformed input is required. A single valid ~20 KB selector stalls the interpreter for ~10 s of CPU. Trust model (Q0) The selector string is the input, reaching this code via soupsieve.compile(), the soupsieve.select/iselect/match/filter helpers, and BeautifulSoup's soup.select(selector) / soup.selectone(selector). Exploitable wherever an application passes a user-controlled CSS selector…

Published by the source

17 Sep 2026, 20:32 UTC

Last modified by the source

1 Oct 2026, 17:56 UTC

Source last checked

26 Oct 2026, 13:52 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-j934-xhv5-fg8f

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-j934-xhv5-fg8f

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-j934-xhv5-fg8f

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# Patch Next advisory brief: CVE-2026-85999

Advisory: Soup Sieve: Polynomial-time ReDoS (O(n²)) in the whitespace/comment trimming regex `RE_WS_END` (triggers on VALID selectors)
IDs: CVE-2026-85999, GHSA-j934-xhv5-fg8f
Record: https://patchnext.com/advisories/cve-2026-85999 (revision 3, source checked 26 Oct 2026, 13:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: soupsieve 2.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI soupsieve: affected < 2.9.0; fixed in 2.9.0

## 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-j934-xhv5-fg8f (GitHub-reviewed advisory GHSA-j934-xhv5-fg8f lists affected ranges: PyPI soupsieve >=0 <2.9.0)
- Verified: https://osv.dev/vulnerability/GHSA-j934-xhv5-fg8f (OSV record GHSA-j934-xhv5-fg8f lists affected ranges: PyPI soupsieve >=0 <2.9.0 (fixed))

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