CVE-2025-61921: 
Ruby vulnerability analysis and mitigation

A denial of service vulnerability (CVE-2025-61921) was discovered in Sinatra's ETag generation functionality, specifically in the If-Match and If-None-Match header parsing component. The vulnerability affects Sinatra versions below 4.2.0 when running on Ruby versions below 3.2. The issue was disclosed and patched in October 2025 (Sinatra Advisory).

The vulnerability is a Regular Expression Denial of Service (ReDoS) issue that occurs when parsing If-Match and If-None-Match headers during ETag generation. The vulnerable code used a regex pattern /\s*,\s*/ for splitting header values, which could be exploited with carefully crafted input to cause exponential processing time. The issue specifically affects applications that use the etag method when generating responses (Sinatra PR).

When exploited, the vulnerability can cause the application to take an unexpected amount of time to process certain requests, potentially resulting in a denial of service condition. This affects the application's availability and performance when processing requests with specially crafted If-Match or If-None-Match headers (Sinatra Advisory).

The vulnerability has been fixed in Sinatra version 4.2.0 and later. Users are recommended to upgrade to this version or later. Alternatively, upgrading to Ruby 3.2 or later also mitigates this issue as it includes built-in protections against ReDoS attacks through cache-based optimization for regular expression matching (Ruby Issue).

The vulnerability was responsibly disclosed through GitHub's security advisory system. The Sinatra maintainers acknowledged the issue and quickly released a patch. The community response highlighted the importance of proper input validation and the benefits of Ruby 3.2's new regex optimization features in preventing such vulnerabilities (Sinatra Advisory).