Rack's multipart header parsing allows Denial of Service via escape-heavy quoted parameters
Date: 2026-04
Affected: rubygems/rack, rubygems/rack
Attack vector: dos
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-34827
CWE: CWE-400
CWE-407
CWE-770
CVSS: 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)

## Summary `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: ```text 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. ## Impact Any Rack application that accepts `multipart/form-data` requests may be affected, including file upload endpoints and standard HTML form handlers. An unauth


Mitigations:

Architectural layers (MAESTRO):