CVE-2026-34827

Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing.

An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing.

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

Details

Rack::Multipart::Parser#handle_mime_head parses quoted parameter values by repeatedly:

1. Searching for the next quote or backslash,
2. Copying the preceding substring into a new buffer, and
3. Removing the processed prefix from the original string with slice!.

An attacker can exploit this by sending a multipart request with many parts whose name parameters contain long escape-heavy values such as:

name="a\\a\\a\\a\\a\\..."

Under default Rack limits, a request can contain up to 4095 parts. If many of those parts use long quoted values with dense escape characters, the parser performs disproportionately expensive CPU work while remaining within normal request size and part-count limits.

Mitigation

• Update to a patched version of Rack that parses quoted multipart parameters without repeated rescanning and destructive prefix deletion.
• Apply request throttling or rate limiting to multipart upload endpoints.
• Where operationally feasible, restrict or isolate multipart parsing on untrusted high-volume endpoints.

Any Rack application that accepts multipart/form-data requests may be affected, including file upload endpoints and standard HTML form handlers.

An unauthenticated attacker can send crafted multipart requests that consume excessive CPU time during request parsing. Repeated requests can tie up application workers, reduce throughput, and degrade or deny service availability.

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

CVE-2026-34827 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 (3.1.21, 3.2.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34827? CVE-2026-34827 is a high-severity uncontrolled resource consumption vulnerability in rack (rubygems), affecting versions >= 3.0.0.beta1, < 3.1.21. It is fixed in 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-2026-34827? CVE-2026-34827 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-2026-34827? rack (rubygems) versions >= 3.0.0.beta1, < 3.1.21 is affected.
4. Is there a fix for CVE-2026-34827? Yes. CVE-2026-34827 is fixed in 3.1.21, 3.2.6. Upgrade to this version or later.
5. Is CVE-2026-34827 exploitable, and should I be worried? Whether CVE-2026-34827 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-2026-34827 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-2026-34827?
   • Upgrade rack to 3.1.21 or later
   • Upgrade rack to 3.2.6 or later