CVE-2026-34829

Rack::Multipart::Parser only wraps the request body in a BoundedIO when CONTENT_LENGTH is present. When a multipart/form-data request is sent without a Content-Length header, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit.

For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space.

This results in a denial of service condition for Rack applications that accept multipart form data.

Details

Rack::Multipart::Parser.parse applies BoundedIO only when content_length is not nil:

io = BoundedIO.new(io, content_length) if content_length

When CONTENT_LENGTH is absent, the parser reads the multipart body until EOF without a global byte limit.

Although Rack enforces BUFFERED_UPLOAD_BYTESIZE_LIMIT for retained non-file parts, file uploads are handled differently. When a multipart part includes a filename, the body is streamed to a Tempfile, and the retained-size accounting is not applied to that file content. As a result, file parts are not subject to the same upload size bound.

An attacker can exploit this by sending a chunked multipart/form-data request containing a file part and continuously streaming data without declaring a Content-Length. Rack will continue writing the uploaded data to disk until the client stops or the server exhausts available storage.

Mitigation

• Update to a patched version of Rack that enforces a total multipart upload size limit even when CONTENT_LENGTH is absent.
• Enforce request body size limits at the reverse proxy or application server.
• Isolate temporary upload storage and monitor disk consumption for multipart endpoints.

Any Rack application that accepts multipart/form-data uploads may be affected if no upstream component enforces a request body size limit.

An unauthenticated attacker can send a large chunked file upload to consume disk space on the application host. This may cause request failures, application instability, or broader service disruption if the host runs out of available storage.

The practical impact depends on deployment architecture. Reverse proxies or application servers that enforce upload limits may reduce or eliminate exploitability, but Rack itself does not impose a total multipart upload limit in this code path when CONTENT_LENGTH is absent.

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

CVE-2026-34829 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 (2.2.23, 3.1.21, 3.2.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34829? CVE-2026-34829 is a high-severity uncontrolled resource consumption vulnerability in rack (rubygems), affecting versions < 2.2.23. It is fixed in 2.2.23, 3.1.21, 3.2.6. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-34829? CVE-2026-34829 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-2026-34829? rack (rubygems) versions < 2.2.23 is affected.
4. Is there a fix for CVE-2026-34829? Yes. CVE-2026-34829 is fixed in 2.2.23, 3.1.21, 3.2.6. Upgrade to this version or later.
5. Is CVE-2026-34829 exploitable, and should I be worried? Whether CVE-2026-34829 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-2026-34829 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-2026-34829?
   • Upgrade rack to 2.2.23 or later
   • Upgrade rack to 3.1.21 or later
   • Upgrade rack to 3.2.6 or later