Firewall is everywhere in the Internet now. Most of the exploits released in the public have little concern over firewall rules because they are just proof of concept. In real world, we would encounter targets with firewall that will make exploitation harder. We need to overcome these obstacles for a successful penetration testing job. The research of this paper started when we need to take over (own) a machine which is heavily protected with rigid firewall rules. Although we can reach the vulnerable service but the strong firewall rules between us and the server hinder all standard exploits useless.
The objective of the research is to find alternative ways which allow penetration tester to take control of a machine after a successful buffer overflow. A successful buffer overflow in a sense that it will eventually leads to arbitrary code execution. These alternative mechanisms should succeed where others fail even in the most rigid firewall rules.
In our research to find a way to by pass these troublesome firewall rules, we looked into various existing techniques used by exploits in the public and why they fail. Then, we found several mechanisms that will work, but dependence to the vulnerable service. Although we can take over the server using these techniques, we take one step further to develop a more generic technique which is not dependence to any service and can be reuse in most other buffer overflows.
This paper will start with dissection on a standard Win32 shellcode as an introduction. We will then explore the techniques being used by proof of concept codes to allow attacker to control the target and their limitations. Then, we will introduce a few alternatives techniques which we call "One-way shellcode" and how they may by pass firewall rules. Finally, we also discussed on a possible way to transfer file from command line without breaking the firewall rule.
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