Soup Sieve: Regular Expression Denial of Service (ReDoS) via Selector Parser

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-2026-49477: 0.64% estimated exploitation activity in the next 30 days; percentile 50.92 (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 The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the VALUE regex pattern in cssparser.py enters exponential backtracking. A payload of only 300 bytes causes the regex engine to hang for over 3 seconds, enabling a trivial Regular Expression Denial of Service (ReDoS) attack. To be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated. Any application that passes untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup's .select() / .selectone() is affected. Details Affected code: soupsieve/cssparser.py, line ~121 - REVALUES / VALUE regex pattern The soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings ("value" or 'value') and unquoted…

Published by the source

9 Jul 2026, 13:37 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:46 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-836r-79rf-4m37

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-836r-79rf-4m37

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# Patch Next advisory brief: CVE-2026-49477

Advisory: Soup Sieve: Regular Expression Denial of Service (ReDoS) via Selector Parser
IDs: CVE-2026-49477, GHSA-836r-79rf-4m37
Record: https://patchnext.com/advisories/cve-2026-49477 (revision 2, source checked 26 Oct 2026, 13:46 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 2.8.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI soupsieve: affected <= 2.8.3; fixed in 2.8.4

## 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://osv.dev/vulnerability/GHSA-836r-79rf-4m37 (OSV record GHSA-836r-79rf-4m37 lists affected ranges: PyPI soupsieve >=0 <2.8.4 (fixed))
- Verified: https://github.com/advisories/GHSA-836r-79rf-4m37 (GitHub-reviewed advisory GHSA-836r-79rf-4m37 lists affected ranges: PyPI soupsieve >=0 <=2.8.3)

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