An internationalized domain name (IDN) homograph attack (also homoglyph attack) is a method used by malicious parties to deceive computer users about the identity of the remote system they are communicating with. This is achieved by exploiting the fact that many different characters look alike and the user is unlikely to spot a subtle substitution. For example, the Cyrillic, Greek and Latin alphabets each have a letter ⟨o⟩ that has the same shape but have different code points. This kind of spoofing attack is also known as script spoofing. Unicode supports numerous scripts (writing systems), and, for a number of reasons, similar-looking characters (such as Greek Ο, Latin O, and Cyrillic О) each has its own code point despite being homoglyphs. Their incorrect or malicious usage is potentially an opportunity for security attacks. Thus, for example, a regular user of exаmple.com (exаmple.com) may be lured to click on it unquestioningly as an apparently familiar link, unaware that the third letter is not the Latin character ⟨a⟩ but rather the Cyrillic character ⟨а⟩ and is thus an entirely different domain from the intended one. The registration of homographic domain names is akin to typosquatting, in that both forms of attacks use a similar-looking name to a more established domain to fool a user. The major difference is that in typosquatting the perpetrator attracts victims by relying on natural transposition errors commonly made when a URL is entered manually, while in homograph spoofing the perpetrator deceives the victims by presenting visually indistinguishable hyperlinks. Indeed, it would be a rare accident for a web user to type, for example, a Cyrillic letter within an otherwise English word, turning, say, "bank" into "bаnk". There are cases in which an abusive registration can use both typosquatting and homograph spoofing; the pairs of l/I, i/j, and 0/O are all both close together on keyboards and, depending on the typeface (computer font), in some cases can be difficult or impossible to distinguish visually.
History
An early nuisance of this kind, pre-dating the Internet and even text terminals, was the confusion between l (lowercase letter "L") / 1 (the number "one") and O (capital letter for vowel "o") / 0 (the number "zero"). Some typewriters even omitted digits 0 and 1, as users could type a lowercase L or uppercase O instead. On computers, where the zero/O distinction often matters, slashed zeros became widely used so they could be clearly distinguished from Os when reading or typing. Even earlier, handwriting provided rich opportunities for confusion. A notable example is the etymology of the word "zenith". The translation from the Arabic "samt" included the scribe's confusing of "m" into "ni". This was common in medieval blackletter, which did not connect the vertical columns on the letters i, m, n, or u, making them difficult to distinguish when several were in a row. The latter, as well as "rn"/"m"/"rri" ("RN"/"M"/"RRI") confusion, is still possible for a human eye even with modern advanced computer technology. Unicode contributes to homograph attacks due to its combining characters, accents, several types of hyphen, etc., often due to inadequate rendering support, especially with smaller font sizes and the wide variety of fonts. Intentional look-alike character substitution with different alphabets has been known in various contexts. For example, Faux Cyrillic has been used as an amusement or attention-grabber and "Volapuk encoding", in which Cyrillic script is represented by similar Latin characters, was used in early days of the Internet as a way to overcome the lack of support for the Cyrillic alphabet. Another example is that vehicle registration plates can have both Cyrillic (for domestic usage in Cyrillic script countries) and Latin (for international driving) with the same letters. Registration plates that are issued in Greece are limited to using letters of the Greek alphabet that have homoglyphs in the Latin alphabet, as European Union regulations require the use of Latin letters.
Homographs in ASCII ASCII has several characters or pairs of characters that look alike and are known as homographs (or homoglyphs). Spoofing attacks based on these similarities are known as homograph spoofing attacks. For example, 0 (the number zero) and O (the letter), l (lowercase "L"), and I (uppercase "i"). In a typical example of a hypothetical attack, someone could register a domain name that appears almost identical to an existing domain but goes somewhere else. For example, the domain "rnicrosoft.com" begins with "r" and "n", not "m". Other examples are G00GLE.COM which looks much like GOOGLE.COM in some fonts. Using a mix of uppercase and lowercase characters, googIe.com (capital i, not lowercase L) looks much like google.com in some fonts. PayPal was a target of a phishing scam exploiting this, using the domain PayPaI.com. In certain narrow-spaced fonts such as Tahoma (the default in the address bar in Windows XP), placing a c in front of a j, l or i will produce homoglyphs such as cl cj ci (d g a).
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