Which of the following is an extremely common IDS evasion technique in the web world?

Unicode attacks can be effective against applications that understand it. Unicode is the international standard whose goal is to represent every character needed by every written human language as a single integer number. What is known as Unicode evasion should more correctly be referenced as UTF-8 evasion. Unicode characters are normally represented with two bytes, but this is impractical in real life.

One aspect of UTF-8 encoding causes problems: non-Unicode characters can be represented encoded. What is worse is multiple representations of each character can exist. Non-Unicode character encodings are known as overlong characters, and may be signs of attempted attack.

References: http://books.gigatux.nl/mirror/apachesecurity/0596007248/apachesc-chp-10-sect-8.html