Rack up to 3.1.20/3.2.5 Rack::Multipart algorithmic complexity

Summaryinfo

A vulnerability marked as problematic has been reported in Rack up to 3.1.20/3.2.5. Impacted is the function Rack::Multipart. The manipulation leads to algorithmic complexity. This vulnerability is listed as CVE-2026-34827. The attack may be initiated remotely. There is no available exploit. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Rack up to 3.1.20/3.2.5. This issue affects the function Rack::Multipart. The manipulation with an unknown input leads to a algorithmic complexity vulnerability. Using CWE to declare the problem leads to CWE-407. An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached. Impacted is availability. The summary by CVE is:

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, 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. This issue has been patched in versions 3.1.21 and 3.2.6.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-34827 since 03/30/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1499 for this issue.

The vulnerability scanner Nessus provides a plugin with the ID 304838 (Linux Distros Unpatched Vulnerability : CVE-2026-34827), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.1.21 or 3.2.6 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (304838). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Name

• Rack

Version

• 3.1.0
• 3.1.1
• 3.1.2
• 3.1.3
• 3.1.4
• 3.1.5
• 3.1.6
• 3.1.7
• 3.1.8
• 3.1.9
• 3.1.10
• 3.1.11
• 3.1.12
• 3.1.13
• 3.1.14
• 3.1.15
• 3.1.16
• 3.1.17
• 3.1.18
• 3.1.19
• 3.1.20
• 3.2.0
• 3.2.1
• 3.2.2
• 3.2.3
• 3.2.4
• 3.2.5

Website

• Product: https://github.com/rack/rack/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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