ret2libc
The standard ROP exploit
A ret2libc is based off the system
function found within the C library. This function executes anything passed to it making it the best target. Another thing found within libc is the string /bin/sh
; if you pass this string to system
, it will pop a shell.
And that is the entire basis of it - passing /bin/sh
as a parameter to system
. Doesn't sound too bad, right?
Disabling ASLR
To start with, we are going to disable ASLR. ASLR randomises the location of libc in memory, meaning we cannot (without other steps) work out the location of system
and /bin/sh
. To understand the general theory, we will start with it disabled.
Manual Exploitation
Getting Libc and its base
Fortunately Linux has a command called ldd
for dynamic linking. If we run it on our compiled ELF file, it'll tell us the libraries it uses and their base addresses.
We need libc.so.6
, so the base address of libc is 0xf7dc2000
.
Libc base and the system and /bin/sh offsets may be different for you. This isn't a problem - it just means you have a different libc version. Make sure you use your values.
Getting the location of system()
To call system, we obviously need its location in memory. We can use the readelf
command for this.
The -s
flag tells readelf
to search for symbols, for example functions. Here we can find the offset of system from libc base is 0x44f00
.
Getting the location of /bin/sh
Since /bin/sh
is just a string, we can use strings
on the dynamic library we just found with ldd
. Note that when passing strings as parameters you need to pass a pointer to the string, not the hex representation of the string, because that's how C expects it.
-a
tells it to scan the entire file; -t x
tells it to output the offset in hex.
32-bit Exploit
64-bit Exploit
Repeat the process with the libc
linked to the 64-bit exploit (should be called something like /lib/x86_64-linux-gnu/libc.so.6
).
Note that instead of passing the parameter in after the return pointer, you will have to use a pop rdi; ret
gadget to put it into the RDI register.
Automating with Pwntools
Unsurprisingly, pwntools has a bunch of features that make this much simpler.
The 64-bit looks essentially the same.
Pwntools can simplify it even more with its ROP capabilities, but I won't showcase them here.
Last updated