Impact Analysis

This vulnerability can cause a denial of service condition in Rack applications that accept multipart form data, such as file upload endpoints or standard HTML form handlers.

• Excessive CPU usage during multipart parsing.
• Tying up application workers, which reduces throughput.
• Degradation or complete denial of service availability.

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Executive Summary

CVE-2026-34827 is a denial of service vulnerability in the Rack RubyGem package affecting multipart header parsing in certain versions. The vulnerability exists in the method that parses quoted multipart parameters, which uses repeated string searches and destructive prefix deletions. This parsing approach causes super-linear CPU usage when handling escape-heavy quoted values with many backslash-escaped characters.

An unauthenticated attacker can exploit this by sending a specially crafted multipart/form-data request containing many parts with long, escape-heavy quoted values. This triggers excessive CPU consumption during parsing, leading to denial of service by tying up application workers and reducing service availability.

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Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Detection Guidance

This vulnerability can be detected by monitoring for unusually high CPU usage on Rack applications that accept multipart/form-data requests, especially during file uploads or form submissions.

Detection involves identifying crafted multipart/form-data requests containing many parts with long backslash-escaped quoted parameter values that cause excessive CPU consumption during parsing.

While no specific commands are provided in the resources, general approaches include using network traffic analysis tools (e.g., tcpdump, Wireshark) to capture multipart/form-data requests and system monitoring tools (e.g., top, htop) to observe CPU spikes.

Additionally, inspecting application logs for repeated multipart parsing errors or slow request processing times may help identify exploitation attempts.

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Mitigation Strategies

The primary mitigation step is to update the Rack RubyGem package to a patched version: either 3.1.21 or 3.2.6 or later.

These patched versions fix the multipart parameter parsing to avoid repeated rescanning and destructive prefix deletion, eliminating the super-linear CPU usage issue.

• Apply request throttling or rate limiting on multipart upload endpoints to reduce the risk of denial of service.
• Where feasible, restrict or isolate multipart parsing on untrusted or high-volume endpoints to limit exposure.

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