# CVE-2026-34827: Rack algorithmic complexity denial of service in Multipart Parser

Source: https://junglewise.ai/threats/cve-2026-34827-rack-algorithmic-complexity-denial-of-service-in-multipart-parser  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-02. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-34827 · Severity: high · CVSS 7.5 · Published 2026-04-02

Rack, a widely used interface for Ruby web applications, is vulnerable to a denial-of-service attack. An attacker can send a specially crafted web request that forces the server to spend an excessive amount of time processing data, effectively freezing the application. This can lead to service outages, preventing legitimate users from accessing the website or application.

Technologies: [rack (RubyGems)](https://junglewise.ai/threats/technologies/rack). Vendors: [RubyGems](https://junglewise.ai/threats/vendors/rubygems).

## Quick answers

**Is CVE-2026-34827 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-34827 being exploited in the wild.

**How severe is CVE-2026-34827?** High severity, with a CVSS score of 7.5, published 2 April 2026.

**Which products and versions are affected?** rack (RubyGems) ECOSYSTEM:introduced=3.0.0.beta1,fixed=3.1.21.

**Is there a fix for CVE-2026-34827?** Yes. rack (RubyGems): upgrade to 3.1.21 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-34827 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-34827?** Check whether you run rack (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- rack (RubyGems): upgrade to 3.1.21 or later

  ```sh
  bundle update rack
  ```


## Technical details

A vulnerability exists in Rack::Multipart::Parser#handle_mime_head due to inefficient algorithmic complexity (CWE-407). The parser handles quoted multipart parameters using repeated String#index searches combined with destructive String#slice! prefix deletion. For values containing many backslash-escaped characters, this results in super-linear (O(n^2)) processing time. An unauthenticated remote attacker can exploit this by sending a multipart/form-data request containing many parts with long, escape-heavy parameter values. This triggers excessive CPU consumption, leading to a denial of service. The issue is fixed in Rack versions 3.1.21 and 3.2.6.

## Affected products

- Rack Rack >= 3.0.0.beta1, < 3.1.21; >= 3.2.0, < 3.2.6
- Red Hat Logging Subsystem for Red Hat OpenShift 5
- Red Hat Red Hat 3scale API Management Platform 2 2

## Identifiers

- CVE-2026-34827
- GHSA-v6x5-cg8r-vv6x

## Timeline

- 2026-04-01: advisory: GitHub advisory published by maintainers
- 2026-04-02: disclosed: CVE published to NVD
- 2026-04-02: patched: Patched versions 3.1.21 and 3.2.6 released

## References

- [https://github.com/rack/rack/security/advisories/GHSA-v6x5-cg8r-vv6x](https://github.com/rack/rack/security/advisories/GHSA-v6x5-cg8r-vv6x)
- [https://access.redhat.com/security/cve/CVE-2026-34827](https://access.redhat.com/security/cve/CVE-2026-34827)
- [https://bugzilla.redhat.com/show_bug.cgi?id=2454501](https://bugzilla.redhat.com/show_bug.cgi?id=2454501)
- [https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-34827.json](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-34827.json)

## Related threats

- [Rack multipart parser denial of service via unbounded scan buffer growth](https://junglewise.ai/threats/cve-2026-125111-rack-multipart-parsing-allows-dos-via-unbounded-scan-buffer)
- [Rack::Static header_rules bypass via dot-segment normalization mismatch](https://junglewise.ai/threats/cve-2026-125110-rack-rack-static-header-rules-bypass-via-dot-segment)
- [Rack host validation bypass via HTTP headers](https://junglewise.ai/threats/cve-2026-125109-rack-is-a-modular-ruby-web-server-interface-from-3-0-until-3-1)
- [Rack is a modular Ruby web server interface. Prior to 2.2.25, 3.1.23, and 3.2.8…](https://junglewise.ai/threats/cve-2026-125108-rack-multipart-boundary-ambiguity-guard-in-rack-multipart-parser)
- [Rack Rack::Request host allowlist bypass via Host header poisoning](https://junglewise.ai/threats/cve-2026-34835-rack-rack-request-host-allowlist-bypass-via-host-header-poisoning)