Browser Unicode Domain Phishing Attack Resurfaces

Researchers are warning about a phishing attack that abuses the way some browsers handle unicode characters to display attack domains that are identical to legitimate ones.

The concept behind the attack is quite old, but it has resurfaced in the current versions of both Firefox and Chrome. The attack relies on the fact that the affected browsers will display unicode characters used in domain names as normal characters, making them virtually impossible to separate from legitimate domains.

“From a security perspective, Unicode domains can be problematic because many Unicode characters are difficult to distinguish from common ASCII characters. It is possible to register domains such as ‘’, which is equivalent to ‘а’. It may not be obvious at first glance, but ‘а’ uses the Cyrillic ‘а’ (U+0430) rather than the ASCII ‘a’ (U+0041). This is known as a homograph attack,” researcher Xudong Zheng wrote in a post on the attack.

This bug was reported to Chrome and Firefox on January 20, 2017 and was fixed in the Chrome trunk on March 24. The fix is included in Chrome 58 which is currently rolling out to users. The existence of the bug in Opera was brought to my attention only after the initial publication of this post. The problem remains in Firefox as they decided that it is a problem for domain registrars to deal with. You can find the detailed discussion in the Bugzilla issue.

Our IDN threat model specifically excludes whole-script homographs, because they can’t be detected programmatically and our “TLD whitelist” approach didn’t scale in the face of a large number of new TLDs. If you are buying a domain in a registry which does not have proper anti-spoofing protections (like .com), it is sadly the responsibility of domain owners to check for whole-script homographs and register them.

Firefox users can limit their exposure to this bug by going to about:config and setting network.IDN_show_punycode to true. This will force Firefox to always display IDN domains in its Punycode form, making it possible to identify malicious domains. Thanks to user MARKZILLA from reddit for this temporary solution. Chrome 58+ users and Firefox users who apply this fix will see the Punycode domain rather than “”.

Firefox Fix

A simple way to limit the damage from bugs such as this is to always use a password manager. In general, users must be very careful and pay attention to the URL when entering personal information. Until this is fixed, concerned users should manually type the URL or navigate to sites via a search engine when in doubt. This is a serious vulnerability because it can even fool those who are extremely mindful of phishing.

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