Rack provides an interface for developing web applications in Ruby. Prior to versions 2.2.13, 3.0.14, and 3.1.12, Rack::Static can serve files under the specified root: even if urls: are provided, which may expose other files under the specified root: unexpectedly. The vulnerability occurs because Rack::Static does not properly sanitize user-supplied paths before serving files.

Specifically, encoded path traversal sequences are not correctly validated, allowing attackers to access files outside the designated static file directory. By exploiting this vulnerability, an attacker can gain access to all files under the specified root: directory, provided they are able to determine then path of the file. Versions 2.2.13, 3.0.14, and 3.1.12 contain a patch for the issue.

Other mitigations include removing usage of Rack::Static, or ensuring that root: points at a directory path which only contains files which should be accessed publicly. It is likely that a CDN or similar static file server would also mitigate the issue.