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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-02 · updated 2026-07-24
• Weakness: CWE-400

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.

Affected products

• rack: from 3.0.0 before 3.1.21; from 3.2.0 before 3.2.6 — fixed in 3.1.21, 3.2.6

How to fix

Upgrade to a fixed version: rack 3.1.21, 3.2.6. Check the vendor advisory for mitigations.

References

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

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