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Assessing the threat of proxy auto-config malware

Expert Nick Lewis explains how attackers are taking advantage of proxy auto-config capabilities in browsers and what mitigations can be put in place.

Cybercriminals have apparently been taking advantage of proxy auto-configuration in Web browsers that I've read is similar to the DNSChanger malware. Could you explain how such an attack works? How can browsers be secured against such an attack?

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Proxy auto-config functionality is used by some networks to automatically configure a Web proxy for systems on the network. Some popular Web browsers have functionality that allows them to check a certain local website (e.g., wpad.domainname.com) to download a configuration file. These config files are used to set information about Web proxies so that Web browsers are able to connect to the Internet through a proxy. If a browser is auto-configured to use a malicious proxy, users could be redirected to malware-laden Web pages, or their Web traffic could be inspected by malicious actors. Some Web proxies are even capable of inspecting HTTPS traffic, which could even put encrypted Web traffic at risk.

When using a Web browser, it is difficult to know if a WPAD file is in use for auto-configuring your systems, so identifying a malicious proxy would be difficult. To identify if or what proxy is used, inspect the IP traffic to see if the HTTP connection goes directly to the legitimate website or through a different IP. You can also use netstat to look for open network connections.

The simplest defenses against proxy auto-config malware are to just disable the proxy auto-config settings or set up a legitimate WPAD file with direct access to the correct proxy. In the future, browser vendors could use signed proxy auto-config files much like application whitelisting uses signed files, though even signed configs could be compromised if an attacker was able to reconfigure a system to trust the new configuration file.

This was first published in December 2013

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