CVE-2025-61771

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 << 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.

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.

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.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2025-61771 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.2.19, 3.1.17, 3.2.2); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-61771? CVE-2025-61771 is a high-severity uncontrolled resource consumption vulnerability in rack (rubygems), affecting versions < 2.2.19. It is fixed in 2.2.19, 3.1.17, 3.2.2. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2025-61771? CVE-2025-61771 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of rack are affected by CVE-2025-61771? rack (rubygems) versions < 2.2.19 is affected.
4. Is there a fix for CVE-2025-61771? Yes. CVE-2025-61771 is fixed in 2.2.19, 3.1.17, 3.2.2. Upgrade to this version or later.
5. Is CVE-2025-61771 exploitable, and should I be worried? Whether CVE-2025-61771 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-61771 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-61771?
   • Upgrade rack to 2.2.19 or later
   • Upgrade rack to 3.1.17 or later
   • Upgrade rack to 3.2.2 or later