Affected: Pylons webob

WebOb provides objects for HTTP requests and responses. Prior to 1.8.10, the normalization of the HTTP Location header during a redirect is vulnerable to an open redirect: WebOb joins the redirect target to the request URI using Python's urljoin, and since Python 3.10 the underlying urlsplit strips ASCII tab, carriage return, and newline characters before parsing, so a redirect target containing such characters can be reinterpreted as a protocol-relative URL whose authority is an attacker-controlled host. This bypasses the CVE-2024-42353 fix that escaped a leading double slash, allowing an attacker who influences the redirect location to send users to an arbitrary external site instead of the intended one. This vulnerability is fixed in 1.8.10.

Affected: PyPI: webob

### 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.c…