ruby-advisory-db/gems/rack/CVE-2025-61771.yml at master · rubysec/ruby-advisory-db · GitHub

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---
gem: rack
cve: 2025-61771
ghsa: w9pc-fmgc-vxvw
url: https://github.com/rack/rack/security/advisories/GHSA-w9pc-fmgc-vxvw
title: Multipart parser buffers large non‑file fields
  entirely in memory, enabling DoS (memory exhaustion)
date: 2025-10-07
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:

  ```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.
cvss_v3: 7.5
patched_versions:
  - "~> 2.2.19"
  - "~> 3.1.17"
  - ">= 3.2.2"
related:
  url:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-61771
    - https://github.com/rack/rack/security/advisories/GHSA-w9pc-fmgc-vxvw
    - https://github.com/rack/rack/commit/589127f4ac8b5cf11cf88fb0cd116ffed4d2181e
    - https://github.com/rack/rack/commit/d869fed663b113b95a74ad53e1b5cae6ab31f29e
    - https://github.com/rack/rack/commit/e08f78c656c9394d6737c022bde087e0f33336fd
    - https://github.com/advisories/GHSA-w9pc-fmgc-vxvw