CVE-2026-34827

Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser

Published Apr 2, 2026 CVSS 3.1 · 7.5 High EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DSA Debian DSA DSA-6492-1 ruby-rack security update
Github GHSA Github GHSA GHSA-v6x5-cg8r-vv6x Rack's multipart header parsing allows Denial of Service via escape-heavy quoted parameters
Ubuntu USN Ubuntu USN USN-8182-1 Rack vulnerabilities
Ubuntu USN Ubuntu USN USN-8989-1 Rack vulnerabilities
History
Type Values Removed Values Added
CPEs cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Rack
Rack rack
Vendors & Products Rack
Rack rack
Type Values Removed Values Added
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.
Title Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser
Weaknesses CWE-400
CWE-407
CWE-770
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Jul 15, 2026, 1:03 AM
Reserved
Mar 30, 2026, 8:52 PM
Status
Modified
Published
Apr 2, 2026, 6:16 PM
Modified
Jul 24, 2026, 9:10 PM
Severity
Important
Public date
Apr 2, 2026, 5:07 PM