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

> Attackers can cause denial of service via CPU exhaustion in Soup Sieve by submitting crafted CSS selectors that trigger quadratic regex backtracking.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 17 Sep 2026
- **Also known as:** AZL-103325, CLEANSTART-2026-PS59368, CLEANSTART-2026-YC00032, DEBIAN-CVE-2026-86000, ECHO-7d38-76d0-5832, GHSA-gjv8-xp57-g29c, MINI-7mgj-5fv7-m59w, MINI-97p4-gm7m-v852, MINI-f55x-5c7r-f7wv, MINI-h2p7-pq3j-55mw, MINI-jg94-8w33-p84q, MINI-q5jh-c334-fqq5, MINI-xpxc-4w76-cqhw, PYSEC-2026-4170, UBUNTU-CVE-2026-86000

## What is CVE-2026-86000?

CVE-2026-86000 is a medium-severity denial of service vulnerability in facelessuser soupsieve, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-400 (Uncontrolled Resource Consumption), CWE-1333 (Inefficient Regular Expression Complexity)

## Which versions are affected by CVE-2026-86000?

3 open source packages ship an affected version, including soupsieve.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `soupsieve` | PyPI | All versions | 2.9.0 |
| `soupsieve` | PyPI | All versions | 2.9.0 |
| `soupsieve` | PyPI | All versions | 2.9.0 |

## How do I fix CVE-2026-86000?

Upgrade soupsieve to 2.9.0 or later.

## Is CVE-2026-86000 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-86000 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-86000 and fix it if it is affected.

What it is: CVE-2026-86000 is a medium-severity denial of service vulnerability in facelessuser soupsieve, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-86000 exists.

Affected packages and fixed versions:
- soupsieve (PyPI): affected all versions; fixed in 2.9.0
- soupsieve (PyPI): affected all versions; fixed in 2.9.0
- soupsieve (PyPI): affected all versions; fixed in 2.9.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-86000/ (Markdown: https://insights.sbomify.com/CVE-2026-86000/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-86000
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-86000
- OSV: https://osv.dev/vulnerability/CVE-2026-86000

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-86000/
Summaries are generated and may contain mistakes; the official description above is the record.