CVE-2025-61770: Rack's unbounded multipart preamble buffering enables DoS (memory exhaustion)

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% 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` buffers the entire multipart preamble (bytes before the first boundary) in memory without any size limit. A client can send a large preamble followed by a valid boundary, causing significant memory use and potential process termination due to out-of-memory (OOM) conditions. Remote attackers can trigger large transient memory spikes by including a long preamble in multipart/form-data requests. The impact scales with allowed request sizes and concurrency, potentially causing worker crashes or severe slowdown due to garbage collection. Versions 2.2.19, 3.1.17, and 3.2.2 enforce a preamble size limit (e.g., 16 KiB) or discard preamble data entirely. Workarounds include limiting total request body size at the proxy or web server level and monitoring memory and set per-process limits to prevent OOM conditions.

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

Get alerted when CVE-2025-61770 or the next vulnerability affects your stack