Sinatra up to 4.1.x Header If-Match/If-None-Match redos

Summaryinfo

A vulnerability has been found in Sinatra up to 4.1.x and classified as problematic. Affected is an unknown function of the component Header Handler. The manipulation of the argument If-Match/If-None-Match leads to redos. This vulnerability is listed as CVE-2025-61921. The attack may be initiated remotely. There is no available exploit. The affected component should be upgraded.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Sinatra up to 4.1.x. This issue affects an unknown part of the component Header Handler. The manipulation of the argument If-Match/If-None-Match with an unknown input leads to a redos vulnerability. Using CWE to declare the problem leads to CWE-1333. The product uses a regular expression with an inefficient, possibly exponential worst-case computational complexity that consumes excessive CPU cycles. Impacted is availability. The summary by CVE is:

Sinatra is a domain-specific language for creating web applications in Ruby. In versions prior to 4.2.0, there is a denial of service vulnerability in the `If-Match` and `If-None-Match` header parsing component of Sinatra, if the `etag` method is used when constructing the response. Carefully crafted input can cause `If-Match` and `If-None-Match` header parsing in Sinatra to take an unexpected amount of time, possibly resulting in a denial of service attack vector. This header is typically involved in generating the `ETag` header value. Any applications that use the `etag` method when generating a response are impacted. Version 4.2.0 fixes the issue.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2025-61921 since 10/04/2025. 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 T1449.003 for this issue.

The vulnerability scanner Nessus provides a plugin with the ID 270281 (Linux Distros Unpatched Vulnerability : CVE-2025-61921), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 4.2.0 eliminates this vulnerability. Applying a patch is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at Tenable (270281). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Name

• Sinatra

Version

• 4.0
• 4.1

Website

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

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

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