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The redirect after login only checked that the value starts with '/'. A referer like https://crackmes.one//evil.com passes the same-host check, its path '//evil.com' gets stored in the session, and after a successful login the browser receives Location: //evil.com, which resolves to https://evil.com. So someone can be shipped off to another site right after logging in on the real domain. The '\evil.com' variant works the same way in browsers since they treat backslashes as slashes.
Fix is one check at the only place the stored redirect gets used (login_post): reject values starting with '//' or containing a backslash. That covers both places that write the value (login_get and register_get).
Added a test for both cases, it fails without the fix. test_controller_edge_cases.py and test_routes.py pass, 44 total.