Malicious actors can create legitimate-looking phishing domains by leveraging the fact that some popular web browsers fail to properly protect their users against homograph attacks.
Web developer Xudong Zheng has demonstrated how an attacker could have registered the domain name “xn--80ak6aa92e.com,” which is displayed by web browsers such as Chrome, Opera and Firefox as “apple.com.”
Unicode is a standard for encoding and representing all characters and glyphs from all languages. Unicode characters can be used in Internet hostnames through Punycode. For instance, the Chinese word “短” is equivalent to “xn--s7y.”
Characters such as the Cyrillic “а” and the Latin “a” may look the same, but they are represented differently in Punycode, allowing malicious actors to create domains where Latin letters are replaced with similar-looking Greek or Cyrillic characters. This is known as an internationalized domain name (IDN) homograph attack.
Modern web browsers are designed to prevent these types of attacks – for example, "xn--pple-43d.com" will be displayed as "xn--pple-43d.com" instead of “apple.com.” However, Zheng discovered that this filter can be bypassed in Chrome, Firefox and Opera by creating the entire domain name using Cyrillic characters, leading to "xn--80ak6aa92e.com” being displayed as “apple.com.”
For a proof-of-concept (PoC), the expert registered the domain “xn--80ak6aa92e.com” and obtained a free digital certificate for it from Let’s Encrypt. When the domain is accessed via Opera, Chrome or Firefox, the user sees the domain name “apple.com” with a certificate issued for “apple.com.”
Zheng reported his findings to Google and Mozilla on January 20, and while the upcoming Chrome 58 will resolve the issue, Mozilla is still trying to figure out how to address the problem.
Mozilla initially classified the vulnerability report as “WONTFIX,” but later reopened it and assigned it a low severity rating. Until the organization comes up with a fix, Firefox users can protect themselves against potential attacks by typing “about:config” in the address bar to access advanced settings, and changing the “network.IDN_show_punycode” preference to “true.”
Edge, Internet Explorer and Safari are not affected. However, it’s worth noting that researchers did report recently that cybercriminals had been targeting Office 365 business email users by exploiting a weakness in how Office 365 handles Punycode.