CVE-2025-61771: Rack's multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)

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: 2025-10-07 · updated 2026-10-19
Weakness: CWE-400

Description

Rack is a modular Ruby web server interface. In versions prior to 2.2.19, 3.1.17, and 3.2.2, ``Rack::Multipart::Parser` stores non-file form fields (parts without a `filename`) entirely in memory as Ruby `String` objects. A single large text field in a multipart/form-data request (hundreds of megabytes or more) can consume equivalent process memory, potentially leading to out-of-memory (OOM) conditions and denial of service (DoS). Attackers can send large non-file fields to trigger excessive memory usage. Impact scales with request size and concurrency, potentially leading to worker crashes or severe garbage-collection overhead. All Rack applications processing multipart form submissions are affected. Versions 2.2.19, 3.1.17, and 3.2.2 enforce a reasonable size cap for non-file fields (e.g., 2 MiB). Workarounds include restricting maximum request body size at the web-server or proxy layer (e.g., Nginx `client_max_body_size`) and validating and rejecting unusually large form fields at the application level.

Affected products

rack: before 2.2.19; from 3.1.0 before 3.1.17; from 3.2.0 before 3.2.2 — fixed in 2.2.19, 3.1.17, 3.2.2
Rubygems Rack: before 2.2.19; from 3.1 before 3.1.17; from 3.2 before 3.2.2; 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.9.0, 0.9.1 (+132) — fixed in 2.2.19, 3.1.17, 3.2.2

How to fix

Upgrade to a fixed version: rack 2.2.19, 3.1.17, 3.2.2; Rubygems Rack 2.2.19, 3.1.17, 3.2.2. Check the vendor advisory for mitigations.

References

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