GHSA-fh3h-vg37-cc95

### Impact When WebOb normalizes the HTTP Location header to include the request hostname, it does so by parsing the URL that the user is to be redirected to with Python's `urllib.parse`, and joining it to the base URL. `urlsplit` (called internally by `urljoin`) however treats a `//` at the start of a string as a URI without a scheme, and then treats the next part as the hostname. `urljoin` will then use that hostname from the second part as the hostname replacing the original one from the request. In a previous advisory https://github.com/Pylons/webob/security/advisories/GHSA-mg3v-6m49-jhp3 an attempt to fix this was made by forcing the replacement of `//` with `/%2f`, however this did not take into account that since Python 3.10 `urlsplit` internally strips ASCII tab, carriage return, and newline characters from the string, so `/\t/attacker.com` gets turned into `//attacker.com` and the attacker is able to bypass the changes introduced in that previous advisory, thereby bringing back the problem that was attempted to be fixed. ``` >>> parse.urlparse("//attacker.com/some/path") ParseResult(scheme='', netloc='attacker.com', path='/some/path', params='', query='', fragment='') ``` WebOb uses `urljoin` to take the request URI and join the redirect location to it, so assuming the request URI is `https://example.org/` and the URL to redirect to is `/\t/attacker.com/some/path/`: ``` >>> parse.urljoin("https://example.org/", "/\t/attacker.com/some/path/") 'https://attacker.com/some/path/' ``` Which redirects from `example.org` where we want the user to stay to `attacker.com`. ### Patches This issue has been fixed in WebOb 1.8.10. ### Workarounds Any use of the `Response` class that includes a `location` can be rewritten to make sure to always pass a full URI that includes the hostname to redirect the user to, or to validate that the redirect target starts with a scheme (e.g. `http://` or `https://`) before assigning to `Response.location`. ### References - https://github.com/Pylons/webob/security/advisories/GHSA-mg3v-6m49-jhp3 - CVE-2024-42353 ### Thanks - Caleb Brown of Google