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The learning objective of this part is for you to gain the first-hand experience on buffer-overflow vulnerability

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3 Buffer Overflow Vulnerability [30 Marks]

The learning objective of this part is for you to gain the first-hand experience on buffer-overflow vulnerability by putting what they have learned about the vulnerability from class into action. Buffer overflow is defined as the condition in which a program attempts to write data beyond the boundaries of pre-allocated fixed length buffers. This vulnerability can be utilised by an attacker to alter the flow control of the program, even execute arbitrary pieces of code to enable remote access attacks. This vulnerability arises due to the mixing of the storage for data (e.g. buffers) and the storage for controls (e.g. return addresses): an overflow in the data part can affect the control flow of the program, because an overflow can change the return address.

In this part, you will be given a program with a buffer-overflow vulnerability; the task is to develop a scheme to exploit the vulnerability and finally send a remote access to an attacker. In addition to the attacks,

 

you will be guided to walk through several protection schemes that have been implemented in the operating system to counter against the buffer overflow. You need to evaluate whether the schemes work or not and explain why.

 

3.1 Initial setup

You can execute the tasks using our pre-built Ubuntu virtual machines. Ubuntu and other Linux dis- tributions have implemented several security mechanisms to make the buffer-overflow attack difficult. To simplify our attacks, we need to disable them first.

 

Address Space Randomisation. Ubuntu and several other Linux-based systems uses address space ran- domisation to randomise the starting address of heap and stack. This makes guessing the exact addresses difficult; guessing addresses is one of the critical steps of buffer-overflow attacks. In this part, we disable these features using the following commands:

 

$ su root

Password: (enter root password "seedubuntu") # sysctl -w kernel.randomize_va_space=0

# exit

 

The StackGuard Protection Scheme. The GCC compiler implements a security mechanism called “Stack Guard” to prevent buffer overflows. In the presence of this protection, buffer overflow will not work. You can disable this protection if you compile the program using the -fno-stack-protector switch. For example, to compile a program example.c with Stack Guard disabled, you may use the following command:

$ gcc -fno-stack-protector example.c

 

Non-Executable Stack. 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. Kernel or dynamic linker uses this marking to decide whether to make the stack of this running program executable or non-executable. This marking is done automatically by the recent versions of gcc, and by default, the stack is set to be non-executable. To change that, use the following option when compiling programs:

For executable stack:

$ gcc -z execstack -o test test.c

 

For non-executable stack:

$ gcc -z noexecstack -o test test.c

 

3.2 Warm Up: Shellcode Practice

Before you start the attack, we want you to exercise with a shellcode example. A shellcode is the code to launch a shell. It is a list of carefully crafted instructions created by malicious users/attackers so that it can be executed once the code is injected into a vulnerable program. Therefore, it has to be loaded into the memory so that we can force the vulnerable program to jump to it. 

 

 

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