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

2026-09-29T06:35:04.899896+00:00

Affected: rack

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.

GHSA-w9pc-fmgc-vxvw — Rack: Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)

2026-09-29T06:35:04.900011+00:00

Affected: RubyGems: rack

## 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:

```ruby body = String.new # non-file → in-RAM buffer @mime_parts[mime_index].body << 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.

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

GHSA-w9pc-fmgc-vxvw — Rack: Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)