![]() ![]() Make this change and run it through GDB to verify that this is true. In order for this to work, buffer must be allocated first, so that it's below filename on the stack. The initial setup initialized filename first and that worked, so what's happening here? As you should remember, the heap grows upward, so space is allocated for filename first, and then buffer is allocated above that. If you run this through GDB, you'll find that you cannot overwrite filename. Using the previous program, we can allocate data on the heap as follows: Overflow attacks against memory allocated with malloc and other memory management tools in the heap are also possible. They're BSS/Data overflows and often discussed in association with heap overflows. Since these variables aren't actually stored in the program's heap, they're not actually heap overflows. Strcpy(filename, "/tmp/heap-overflow.txt") Īfter discussing what you think should happen with each of these changes, run them through GDB and see what happens. These nuances are important to realize when looking for heap overflow vulnerabilities.ĭiscuss what would happen if the following changes were made: bss section of the compiled binary while filename is stored in the. ![]() This is because the uninitialized buffer is actually stored in the. Recompiling and running through GDB will lead to the same result, filename remains unchanged. What happens, then, if you change the order in which they are declared: If you run through the same GDB instructions above, you'll see that printing the filename actually prints out /tmp/heap-overflow.txt. This small change will greatly affect the result. Simply remove the initialization of buffer. To demonstrate some things to look for, let's make a small change to the program that we wrote initially. Starting program: /home/wbyoung/heap/a.outĪaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaįor programs that have special access, you can exploit these types of vulnerabilities to gain access to privileged files (like /etc/passwd and /etc/shadow). The following demonstrates gcc -g vuln.c -o gdb vulnīreakpoint 1 at 0x8048435: file test.c, line 9. Different compilers may put buffer above filename in the heap, but GCC on Ubuntu 7.10 puts it below, allowing us to overwrite filename. Like information on the stack, though, the compiler might put things in places that you might not expect. You cannot overwrite the return address of main like you could with a stack based buffer overflow. This looks pretty much exactly like a stack based buffer overflow, but there are a good number of differences. Static char filename = "/tmp/heap-overflow.txt" The following program initializes variables that are stored in the heap. There are a few other tactics that you can use, however. Since you're overwriting information in the heap, you can't simply overwrite the return address of a function to use shellcode. The main difference is that it it not as straightforward to execute custom code. Heap overflows are a type of buffer overflow and actually very similar to stack based buffer overflows. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |