Mozilla Foundation has announced changes to Firefox concerning Internationalized Domain Names (IDN) to deal with homograph spoofing attacks. According to the organization, “Mozilla Foundation products now only display IDNs in a whitelist of TLDs, which have policies stating what characters are permitted, and procedures for making sure that no homographic domains are registered to two different entities.”

Following is a statement explaining the current status of the Mozilla changes to Firefox regarding IDN:

“We have implemented a TLD whitelist system, which currently contains 21 TLDs for which we correctly display IDN domain names in the UI. Any IDN domain name in a non-whitelisted TLD displays as punycode. This is a security feature and so there is no user interface for adding or removing TLDs.

Any registry which wishes to be added to the whitelist should follow the instructions on that page. In terms of what constitutes a homograph, we are being guided by the Unicode Consortium’s confusables list and by common sense. Our policy in this area is still somewhat in flux -  in particular, we are not yet sure whether we should require that registries consider two characters which differ only in accent (sometimes by the shade of a single pixel at normal font sizes) as homographic. In the mean time, we strongly advise that registries do this.

We have implemented a character blacklist, which will soon contain ‘DIVISION SLASH’ (U+2215) and ‘FRACTION SLASH’ (U+2044). After that, we may extend it to forbid more characters which may be used to spoof URL punctuation. This is not meant to prejudice the outcome of the current IAB-IDN discussions on potentially reducing the number of characters permitted in IDN, but we feel the danger posed by the use of such characters in 3rd and 4th level domains is great enough to require an immediate ban. Any domain name which contains one or more of these characters displays as punycode.”

As a temporary response, Mozilla Foundation first considered disabling IDN support but later reconsidered and decided instead to display IDNs in Punycode, an ASCII representation of Unicode.