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Path Traversal occurrences in Rack-cors
An issue was discovered in the rack-cors (aka Rack CORS Middleware) gem before 1.0.4 for Ruby. It allows ../ directory traversal to access private resources because resource matching does not ensure that pathnames are in a canonical format. (2019-11-14, CVE-2019-18978)
Why Path Traversal can be dangerous
Relative Path Confusion means that your web server is configured to serve responses to ambiguous URLs. This configuration can possibly cause confusion about the correct relative path for the URL. It is also an issue of resources, such as images, styles etc., which are specified in the response using relative path, not the absolute URL.
If the web browser permits to parse "cross-content" response, the attacker may be able to fool the web browser into interpreting HTML into other content types, which can then lead to a cross site scripting attack (link do XSS).