Software Engineering Institute

The Domain Name System

The Domain Name System (DNS) provides name, address, and other information about Internet Protocol (IP) networks and devices. DNS was designed to support domain names that use a subset of the American Standard Code for Information Interchange (ASCII) character set.

Unicode

The Unicode character set contains more than 96,000 characters. Because of this, Unicode can be used to represent a wide range of languages.

Internationalized Domain Names

Internationalizing Domain Names in Applications (IDNA) is a mechanism for translating Unicode domain names into an ASCII representation that is supported by the existing DNS infrastructure. The encoding syntax used by IDNA is called Punycode (RFC 3492). A web browser that supports Internationalized Domain Names (IDN) can visit web sites that contain Unicode characters in the domain name. The request that is sent to the DNS server is encoded as Punycode, but the domain name displayed to the user is in Unicode format. Most modern web browsers support IDN. Microsoft Internet Explorer can support IDN through use of the VeriSign i-Nav plug-in.

The Problem

Many Unicode characters have a similar appearance to ASCII characters. By using a domain name that contains Unicode characters, a web site operator could make it appear that the content from his or her web site actually originated from another site. The text displayed in the browser's address bar or status bar could be deceptive if the domain name contains Unicode characters. Other programs where the user is making a trust decision based on the appearance of a domain name may also be affected. IDNA is not limited to web browsers.

By making a malicious web site appear to be a site that the user trusts, an attacker could convince the user to provide sensitive information.

Upgrade or Patch
For vendor-specific information regarding vulnerable status and patch availability, please see the Systems Affected section of this document.

Do not follow unsolicited links

Do not click on unsolicited links received in email, instant messages, web forums, or internet relay chat (IRC) channels. Type URLs directly into the browser to avoid these misleading links. While these are generally good security practices, following these behaviors will not prevent exploitation of this vulnerability in all cases, particularly if a trusted site has been compromised or allows cross-site scripting.

Check Certificates

US-CERT recommends that prior to providing any sensitive information over a secure (HTTPS) connection, you check the name recorded in the certificate to be sure that it matches the name of the site to which you think you are connecting.