CVE-2025-61919: Rack is vulnerable to a memory-exhaustion DoS through unbounded URL-encoded body parsing

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-10-10 · updated 2026-10-19
Weakness: CWE-400

Description

Rack is a modular Ruby web server interface. Prior to versions 2.2.20, 3.1.18, and 3.2.3, `Rack::Request#POST` reads the entire request body into memory for `Content-Type: application/x-www-form-urlencoded`, calling `rack.input.read(nil)` without enforcing a length or cap. Large request bodies can therefore be buffered completely into process memory before parsing, leading to denial of service (DoS) through memory exhaustion. Users should upgrade to Rack version 2.2.20, 3.1.18, or 3.2.3, anu of which enforces form parameter limits using `query_parser.bytesize_limit`, preventing unbounded reads of `application/x-www-form-urlencoded` bodies. Additionally, enforce strict maximum body size at the proxy or web server layer (e.g., Nginx `client_max_body_size`, Apache `LimitRequestBody`).

Affected products

rack: before 2.2.20; from 3.0.0 before 3.1.18; from 3.2.0 before 3.2.3 — fixed in 2.2.20, 3.1.18, 3.2.3
Rubygems Rack: before 2.2.20; from 3.0 before 3.1.18; from 3.2 before 3.2.3; 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.9.0, 0.9.1 (+158) — fixed in 2.2.20, 3.1.18, 3.2.3

How to fix

Upgrade to a fixed version: rack 2.2.20, 3.1.18, 3.2.3; Rubygems Rack 2.2.20, 3.1.18, 3.2.3. Check the vendor advisory for mitigations.

References

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