Please compile and run the following code, and see whether a shell is invoked. The following program shows how to launch a shell by executing a shellcode stored in a buffer. The shellcode that we use is just the assembly version of the above program. Name = “/bin/sh” name = NULL execve(name, name, NULL) It has to be loaded into the memory so that we can force the vulnerable program to jump to it. A shellcode is the code to launch a shell. We use the following commands to link /bin/sh to zsh (there is no need to do these in Ubuntu 12.04):īefore starting the attack, let us get familiar with the shellcode. We have installed a shell program called zsh in our Ubuntu 16.04 VM. Therefore, we will link /bin/sh to another shell that does not have such a countermeasure (in later tasks, we will show that with a little bit more effort, the countermeasure in /bin/dash can be easily defeated). Since our victim program is a Set-UID program, and our attack relies on running /bin/sh, the countermeasure in /bin/dash makes our attack more difficult. ![]() The dash program in Ubuntu 12.04 does not have this behavior. Basically, if dash detects that it is executed in a Set-UID process, it immediately changes the effective user ID to the process’s real user ID, essentially dropping the privilege. The dash shell in Ubuntu 16.04 has a countermeasure that prevents itself from being executed in a Set-UID process. However, the dash program in these two VMs have an important difference. In both Ubuntu 12.04 and Ubuntu 16.04 VMs, the /bin/sh symbolic link points to the /bin/dash shell. To change that, use the following option when compiling programs:Ĭonfiguring /bin/sh (Ubuntu 16.04 VM only). This marking is done automatically by the recent versions of gcc, and by default, stacks are set to be non-executable. Kernel or dynamic linker uses this marking to decide whether to make the stack of this running program executable or non-executable. Ubuntu used to allow executable stacks, but this has now changed: the binary images of programs (and shared libraries) must declare whether they require executable stacks or not, i.e., they need to mark a field in the program header. For example, to compile a program example.c with StackGuard disabled, we can do the following: We can disable this protection during the compilation using the -fno-stack-protector option. In the presence of this protection, buffer overflow attacks will not work. The GCC compiler implements a security mechanism called StackGuard to prevent buffer overflows. $ sudo sysctl -w kernel.randomize_va_space=0 In this lab, we disable this feature using the following command: This makes guessing the exact addresses difficult guessing addresses is one of the critical steps of buffer-overflow attacks. ![]() Ubuntu and several other Linux-based systems uses address space randomization to randomize the starting address of heap and stack. Later on, we will enable them one by one, and see whether our attack can still be successful.Īddress Space Randomization. ![]() To simplify our attacks, we need to disable them first. Ubuntu and other Linux distributions have implemented several security mechanisms to make the buffer-overflow attack difficult. You can execute the lab tasks using our pre-built Ubuntu virtual machines.
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