CVE-2025-61771 (rack): Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)

ADVISORIES

GEM

rack

SEVERITY

CVSS v3.x: 7.5 (High)

PATCHED VERSIONS

~> 2.2.19
~> 3.1.17
>= 3.2.2

DESCRIPTION

Summary

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).

Details

During multipart parsing, file parts are streamed to temporary files, but non-file parts are buffered into memory:

body = String.new  # non-file → in-RAM buffer
@mime_parts[mime_index].body &lt;&lt; content

There is no size limit on these in-memory buffers. As a result, any large text field—while technically valid—will be loaded fully into process memory before being added to params.

Impact

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.

Mitigation

Upgrade: Use a patched version of Rack that enforces a reasonable size cap for non-file fields (e.g., 2 MiB).
Workarounds:
- Restrict maximum request body size at the web-server or proxy layer (e.g., Nginx client_max_body_size).
- Validate and reject unusually large form fields at the application level.

RubySec