CVE-2026-34829: Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% 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. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser only wraps the request body in a BoundedIO when CONTENT_LENGTH is present. When a multipart/form-data request is sent without a Content-Length header, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit. For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space. This results in a denial of service condition for Rack applications that accept multipart form data. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

Affected products

rack: before 2.2.23; from 3.0.0 before 3.1.21; from 3.2.0 before 3.2.6 — fixed in 2.2.23, 3.1.21, 3.2.6
Rubygems Rack: before 2.2.23; from 3.0.0.beta1 before 3.1.21; from 3.2.0 before 3.2.6; 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.9.0, 0.9.1 (+169) — fixed in 2.2.23, 3.1.21, 3.2.6

How to fix

Upgrade to a fixed version: rack 2.2.23, 3.1.21, 3.2.6; Rubygems Rack 2.2.23, 3.1.21, 3.2.6. Check the vendor advisory for mitigations.

References

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