Researcher Tells How Web Proxy Autodiscovery Protocol Could Be Abused

A bad proxy could potentially enable a man-in-the-middle attack, but there is a fix, security researcher Maxim Goncharov explains.

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man-in-the-middle attack

There are a number of different ways users can get an internet address when they connect into a network, and some of them involve the use of a proxy approach that could well enable user exploitation. In a session at the Black Hat USA conference, security researcher Maxim Goncharov detailed how the WPAD, or Web Proxy Autodiscovery Protocol, could be abused by an attacker to redirect and potentially intercept user traffic.

In an interview with eWEEK, Goncharov explained what the risks are and how in a test case he could have redirected internet users in Japan and elsewhere using his BadWPAD attack. Although Goncharov noted that he is now a researcher with Shape Security, he conducted most of his WPAD analysis while he was working at Trend Micro.

WPAD is an autodiscovery protocol that helps users get an address from a defined proxy. Goncharov noted that while WPAD has been in use since the Netscape browser era of the late 1990s, the actual Internet Engineering Task Force (IETF) standard was never finalized.

The way WPAD works today is, for example, a user brings their laptop into the office and then tries to connect to the network. There are typically two ways that network information is requested; the most common is by way of Dynamic Host Control Protocol (DHCP), he explained. Another option is the operating system or even an application looking for a WPAD domain name inside of the network to find a proxy auto-config (PAC) file. If WPAD cannot find information within the local network, it will look outside to the public internet to get the information. A PAC file can have JavaScript instructions inside that are then executed on a client machine.

"If the WPAD domain is registered in a special way outside of a network, it can be used to attack users," Goncharov said.

Goncharov registered multiple top-level domains (TLDs), including, and to test his theory that local network traffic would request WPAD information from the public internet.

"I received a lot of traffic from different organizations," Goncharov said. "The traffic was requests for the proxy information, and of course, I gave people back the [PAC] file, but if I was a bad guy, I could have provided proxy settings that would have made me a man in the middle [MitM]."

A MitM attack enables a hacker to intercept all of a user's traffic and can potentially enable an attacker to inject malicious attributes or redirect traffic.

With the WPAD.Tokyo domain, Goncharov had the most success, demonstrating how powerful the BadWPAD attack potentially could be in the hands of a malicious hacker.

"After just one week, I received several million requests to my test environment for WPAD.Tokyo," Goncharov said. "Most probably, there are some official organizations in Japan that use the dot-Tokyo domain, and because of that I was receiving the WPAD traffic."

Microsoft has issued patches to limit the risks of WPAD, though the issue still exists as the problem can't entirely be fixed since the protocol itself is bad and is still in use, he said.

However, the BadWPAD issue can be mitigated by blocking access at the network level to outbound WPAD requests. As well, Goncharov noted that if DHCP is configured correctly, the risks can also be reduced or eliminated.

"You can fix the problem if you configure your DHCP server in the proper way on your network," Goncharov said.

Sean Michael Kerner is a senior editor at eWEEK and Follow him on Twitter @TechJournalist.

Sean Michael Kerner

Sean Michael Kerner

Sean Michael Kerner is an Internet consultant, strategist, and contributor to several leading IT business web sites.