I can only reproduce the issue if the current directory (directory used by the HTTP server, see --directory command line option) contains a .ssh/ subdirectory.

The problem is that the HTTP Header Location starts with "//domain/" and such URL is interpreted as an absolute URL of a new domain name ("domain"), rather than a relative path of the same domain ("localhost").

Maybe we should simply strip all additional leading slashes to only keep one. Replace "//path" or  "/////path" with "/path" for example.

---

By the way, http.server uses urllib.parse.urlsplit() on the request URL without passing its own domain, and urllib.parse.urlsplit() interprets "//attacker.com/path" as if attacker.com is a host with no scheme:

>>> urllib.parse.urlsplit('//attacker.com/path')
SplitResult(scheme='', netloc='attacker.com', path='/path', query='', fragment='')

Maybe parse_qs() should be used instead? Or we should reinject the server domain and port number? I am not sure that it's an issue in practice.

SimpleHTTPRequestHandler.translate_path('//attacker.com/..%2f..%2f..%2f..%2f..%2f../.ssh') returns os.path.join(self.directory, ".ssh"). I don't think that it's an issue, it sounds like the expected behavior. We don't attempt to reject ".." in URL.

---

To reproduce the issue, I used two terminals.

Terminal 1:

$ python3.8 -V
Python 3.8.7
$ python3.8 -m http.server
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...
127.0.0.1 - - [15/Feb/2021 09:18:20] "GET
//attacker.com/..%2f..%2f..%2f..%2f..%2f../.ssh HTTP/1.1" 301 -

Terminal 2:

$ wget 'http://127.0.0.1:8000//attacker.com/..%2f..%2f..%2f..%2f..%2f../.ssh'
(...)
HTTP request sent, awaiting response... 301 Moved Permanently
Location: //attacker.com/..%2f..%2f..%2f..%2f..%2f../.ssh/ [following]

--2021-02-15 09:18:20--  http://attacker.com/..%2f..%2f..%2f..%2f..%2f../.ssh/
Resolving attacker.com (attacker.com)... 45.88.202.115
Connecting to attacker.com (attacker.com)|45.88.202.115|:80... connected.

(...)

wget is redirected and connects to attacker.com.

The HTTP redirection comes from Lib/http/server.py:

    def send_head(self):
        path = self.translate_path(self.path)
        f = None
        if os.path.isdir(path):
            parts = urllib.parse.urlsplit(self.path)
            if not parts.path.endswith('/'):
                # redirect browser - doing basically what apache does
                self.send_response(HTTPStatus.MOVED_PERMANENTLY)
                new_parts = (parts[0], parts[1], parts[2] + '/',
                             parts[3], parts[4])
                new_url = urllib.parse.urlunsplit(new_parts)
                self.send_header("Location", new_url)
                self.end_headers()
                return None
            ...
        ...

The problem is that the "Location" header starts with "//".