For that past several days, Symantec has observed an increase in spam messages containing hexadecimal obfuscated URLs. Hexadecimal character codes are simply the hexadecimal number to letter representation for the ASCII character set. To a computer, hexadecimal is just one out of the many systems for address expressions on the Internet.
The following samples are different hexadecimal representations for http://www.symantec.com.
Hexadecimal and ASCII characters:
(“http” and “com” are in ASCII characters and the rest of the URL is in hexadecimal)
(“http://www” is in hexadecimal and the rest of the URL is in ASCII characters)
Symantec has observed several hexadecimal URL obfuscation techniques used by spammers.
Figure 1. Spam email using hexadecimal URL obfuscation techniques
Figure 2. Source code of spam email (Figure 1.) using hexadecimal URL obfuscation techniques
Hexadecimal URL obfuscation is not a new spamming technique. The technique is used to evade anti-spam filters because anti-spam engines are sensitive to every single character in a message body. With the recent spike in hexadecimal spam volumes, Symantec will continue to monitor these attacks and will react accordingly.