source: https://www.securityfocus.com/bid/650/info

Lack of user input validation in ProFTPD can lead to a remote root vulnerability.

On systems that support it ProFTPD will attempt to modify the name of the program being executed (argv[0]) to display the command being executed by the logged on user. It does this by using snprintf to copy the input of the user into a buffer.

The call to snprintf is in the 'set_proc_title' function in the main.c source file. It is only compiled in if the define PF_ARGV_TYPE equals the PF_ARGV_WRITABLE define.

ProFTPD passes the user input to snprintf as the format argument string of the function call. This allows remote users to supply possible dangerous format arguments to snprintf. 

Tymm Twillman gives the following example:

- ftp to host
- login (anonymous or no)

(this should be all on one line, no spaces)

ftp> ls aaaXXXX%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%u%653300u%n

(replace the X's with the characters with ascii values 0xdc,0x4f,0x07,0x08
consecutively)


Since proftpd will pass on user input data to snprintf, argument attacks are easy. The a's at the beginning are just for alignment, the %u's to skip bytes in the stack, the %653300u is to increment the # of bytes that have been "output", and the %n stores that value (whose LSBs have now flipped over to 0) to the location pointed to by the current "argument" -- which just happens to point right after the a's in this string. The bytes that replace the X's are the address where proftpd keeps the current user ID...

Logging in as an anonymous user, you are still restricted as to some of the things you can do. But with a local login, root compromise at this point is trivial. And it is possible to modify this exploit for other systems, and for remote attacks.