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 urlparse, and joining it to the base URL. urlparse 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.

>>> parse.urlparse("//example.com/test/path")
ParseResult(scheme='', netloc='example.com', path='/test/path', params='', query='', fragment='')

WebOb uses urljoin to take the request URI and joining the redirect location, so assuming the request URI is: https://example.org//example.com/some/path, and the URL to redirect to (for example by adding a slash automatically) is //example.com/some/path/ that gets turned by urljoin into:

>>> parse.urljoin("https://example.org//attacker.com/some/path", "//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 is patched in WebOb 1.8.8

Older versions of WebOb continue to be vulnerable to this issue, and should be avoided.

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.

Thanks

• Sara Gao

This issue was reported via the Pylons Project Security List