Original release date: May 23, 2016

Systems Affected

Windows, OS X, Linux systems, and web browsers with WPAD enabled

Overview

Web Proxy Auto-Discovery (WPAD) Domain Name System (DNS) queries that are intended for resolution on private or enterprise DNS servers have been observed reaching public DNS servers [1]. In combination with the New generic Top Level Domain (gTLD) program’s incorporation of previously undelegated gTLDs for public registration, leaked WPAD queries could result in domain name collisions with internal network naming schemes [2] [3]. Collisions could be abused by opportunistic domain registrants to configure an external proxy for network traffic, allowing the potential for man-in-the-middle (MitM) attacks across the Internet.

Description

WPAD is a protocol used to ensure all systems in an organization utilize the same web proxy configuration. Instead of individually modifying configurations on each device connected to a network, WPAD locates a proxy configuration file and applies the configuration automatically.

The use of WPAD is enabled by default on all Microsoft Windows operating systems and Internet Explorer browsers. WPAD is supported but not enabled by default on Mac and Linux-based operating systems, as well as, Safari, Chrome, and Firefox browsers.

With the New gTLD program, previously undelegated gTLD strings are now being delegated for public domain name registration [3]. These strings may be used by private or enterprise networks, and in certain circumstances, such as when a work computer is connected from a home or external network, WPAD DNS queries may be made in error to public DNS servers. Attackers may exploit such leaked WPAD queries by registering the leaked domain and setting up MitM proxy configuration files on the Internet.
 

Impact

Leaked WPAD queries could result in domain name collisions with internal network naming schemes. If an attacker registers a domain to answer leaked WPAD queries and configures a valid proxy, there is potential to conduct man-in-the-middle (MitM) attacks across the Internet.

The WPAD vulnerability is significant to corporate assets such as laptops. In some cases these assets are vulnerable even while at work but observations indicate that most assets become vulnerable when used outside an internal network (e.g. home networks, public Wi-Fi networks).

Solution

US-CERT encourages users and network administrators to implement the following recommendations to provide a more secure and efficient network infrastructure:

• Consider disabling automatic proxy discovery/configuration in browsers and operating systems during device setup if it will not be used for internal networks.
• Consider using a fully qualified domain name (FQDN) from global DNS as the root for enterprise and other internal namespace.
• Configure internal DNS servers to respond authoritatively to internal TLD queries.
• Configure firewalls and proxies to log and block outbound requests for wpad.dat files.
• Identify expected WPAD network traffic and monitor the public namespace or consider registering domains defensively to avoid future name collisions.
• File a report with ICANN if your system is suffering demonstrably severe harm as a consequence of name collision by visiting https://forms.icann.org/en/help/name-collision/report-problems.

References

• [1] Verisign – MitM Attack by Name Collision: Cause Analysis and Vulnerability Assessment in the New gTLD Era
• [2] ICANN – Name Collision Resources & Information
• [3] ICANN – New gTLDs
• [4] US-CERT – Controlling Outbound DNS Access

Revision History

• May 23, 2016: Initial Release

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