Secondary injection urldecode

Principle
Most web program will convert the input character, such as addslashes (), mysql_real_escape_string (),
mysql_escape_string (), it is simply the single quote ', double quotes ", NULL, backslash \ escape, but if the program there is
urldecode () or rawurldecode (), then there will be problems.
when the user incoming values, the program will automatically change the characters with a urldecode, if the program has conducted a urldecode (), there will be secondary injection, for example? id = 1% of the 2527, the first automatic decoding is? id = 1% 27, then once urldecode () become? id = 1 '
success bypass

vulnerability mining keyword
rawurldecode ()
urldecode ()