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WPAD spoofing

Theory

A proxy can be used to handle clients requests (for example to access the Internet). In a network in which the topology changes frequently, adaptive configurations are needed. A type of proxies called "Adaptive proxies" uses a configuration script.

The Web Proxy Automatic Discovery (WPAD) protocol helps clients in finding a proxy configuration script (wpad.dat). This script gives a set of proxies that can be used, and can be located with an hostname or an URL. Various mechanisms can be used to find its location (by order of resolution):

  1. WinHTTP/WinINET

  2. DHCP

  3. DNS

  4. Internet Explorer's LAN settings, or configuration file

Practice

WPAD spoofing can be combined with

Proxy auth NTLM authentication can either be

  • forced and captured with Responder with --wredir and --ProxyAuth

  • forced and relayed with ntlmrelayx by using the --http-port 3128 argument

through LLMNR, NBT-NS spoofing

On old Windows systems (i.e. lacking the MS16-077 security update), the WPAD location could be obtained through insecure name resolution protocols like LLMNR and NBT-NS when standard DNS queries were failing (i.e. no DNS record for WPAD). This allowed attackers to operate LLMNR and NBT-NS spoofing to answer those WPAD queries and redirect to a fake wpad.dat file, hence poisoning the web proxy configuration of the requesting clients, hence obtaining more traffic.

Responder (Python) and Inveigh (Powershell) are great tools for name poisoning. In addition to name poisoning, they also have the ability to start servers (listeners) that will capture authentications and echo the NTLM hashes to the attacker.

The following command will start LLMNR, NBTS and mDNS spoofing. Name resolution queries for the wpad server will be answered just like any other query. Fake authentication servers (HTTP/S, SMB, SQL, FTP, IMAP, POP3, DNS, LDAP, ...) will capture NTLM hashes.

  • The -w/--wpad option will make Responder start the WPAD rogue server so that fake wpad.dat file can be served to requesting clients.

  • The -P/--ProxyAuth option to force the Windows client to authenticate after the wpad.dat is accessed and when the client starts using the proxy

responder --interface "eth0" --wpad --ProxyAuth
responder -I "eth0" -wP

through ADIDNS spoofing

On up-to-date machines (i.e. with the MS16-077 security update applied), WPAD can still be abused through ADIDNS spoofing if the WPAD record does not exist. There is however a DNS block list mitigation called GQBL (Global Query Block List) preventing names like WPAD and ISATAP (default entries) to be resolved. This block list exists to reduce vulnerabilities associated with dynamic DNS updates but it can be edited when implementing WPAD.

Pre CVE-2018-8320

On machines that are not patched against CVE-2018-8320, there are two ways to bypass the GQBL: by registering a wildcard record or by registering a domain alias (DNAME) record, which can be conducted as follows with Powermad (Powershell).

New-ADIDNSNode -Node '*' -Data 'Pentest_IP_Address'
New-ADIDNSNode -Node wpad -Type DNAME -Data 'pentester01.TARGETDOMAIN.LOCAL'

Post CVE-2018-8320

On machines that are patched against that CVE, registering a name server (NS) record could still work.

New-ADIDNSNode -Node wpad -Type NS -Data 'pentester01.TARGETDOMAIN.LOCAL'

In order for the NS record technique to work, the tester has to have a DNS server running for DNS spoofing. This can easily be accomplished with dnschef (Python).

dnschef --fakeip 'Pentest_IP_Address' --interface 'Pentest_IP_Address' --port 53 --logfile dnschef.log

through DHCPv6 spoofing

On up-to-date machines (i.e. with the MS16-077 security update applied), WPAD can still be abused through ADIDNS spoofing, even if the WPAD record does exist. With DNS poisoning through DHCPv6 spoofing, an attacker can reply to DHCPv6 requests, and then reply to DNS queries.

This attack can be conducted with mitm6 (Python), see the DHCPv6 spoofing page for exploitation notes.

References

LogoADIDNS Revisited - WPAD, GQBL, and MoreNetSPI

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