A very common technique for a Unicode attack involves traversing directories looking for interesting files. An example of this idea applied to the Web is http://target.server/some_directory/../../../winnt In this case, the attacker is attempting to traverse to a directory that is not supposed to be part of standard Web services. The trick is fairly obvious, so many Web servers and scripts prevent it. However, using alternate encoding tricks, an attacker may be able to get around badly implemented request filters. In October 2000, an adversary publicly revealed that Microsoft's IIS server suffered from a variation of this problem. In the case of IIS, all the attacker had to do was provide alternate encodings for the dots and/or slashes found in a classic attack. The Unicode translations are . yields C0 AE / yields C0 AF \ yields C1 9C Using this conversion, the previously displayed URL can be encoded as http://target.server/some_directory/%C0AE/%C0AE/%C0AE%C0AE/%C0AE%C0AE/winnt See also: CVE-2000-0884 |