CVE-2025-61770: 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 pr...

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

0.85%

53.4th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-400

Affected Software

Vendor	Product	Versions
Rack	Rack	< 2.2.19
Rack	Rack	>= 3.1.0, < 3.1.17
Rack	Rack	>= 3.2.0, < 3.2.2

References

Timeline

Published: Oct 7, 2025
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2025-61770?

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.

How severe is CVE-2025-61770?

CVE-2025-61770 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 0.85% probability of exploitation in the next 30 days.

How do I fix CVE-2025-61770?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.