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Modern operating systems include hardened security mechanisms to block exploit attempts. ASLR and...
Modern operating systems include hardened security mechanisms to block exploit attempts. ASLR and NX (DEP) are two examples of the mechanisms that are widely implemented for the sake of security. However, there exists ways to bypass such protections by leveraging advanced exploitation techniques. It becomes harder to achieve code execution when the exploitation originates from a remote location, such as when the attack originates from a client, targeting server daemons. In such cases it is harder to find out the context information of target systems and, therefore, harder to achieve code execution. Knowledge on the memory layout of the targeted process is a crucial piece of the puzzle in developing an exploit, but it is harder to figure out when the exploit attempt is performed remotely. Recently, there have been techniques to leverage information disclosure (memory leak) vulnerabilities to figure out where specific library modules are loaded in the memory layout space, and such classes of vulnerabilities have been proven to be useful to bypass ASLR. However, there is also a different way of figuring out the memory layout of a process running in a remote environment. This method involves probing for valid addresses in target remote process. In a Linux environment, forked child processes will inherit already randomized memory layout from the parent process. Thus every client connection made to server daemons will share the same memory layout. The memory layout randomization is only done during the startup of the parent service process, and not randomized again when it is forking a child process to handle client connections. Due to the inheritance of child processes, it is possible to figure out a small piece of different information from every connection, and these pieces can be assembled later to get the idea of a big picture of the target process’s remote memory layout. Probing to see if a given address is a valid memory address in context of the target remote process and assembling such information together, an attacker can figure out where the libc library is loaded on the memory, thus allowing exploits to succeed further in code execution. One might call it brute force, but with a smart brute forcing strategy, the number of minimal required attempts are significantly reduced to less than 10 in usual cases. In this talk, we will be talking about how it is possible to probe for memory layout space utilizing a piece of code to put the target in a blocked state, and to achieve stable code execution in remote exploit attempt scenarios using such information, as well as other tricks that are often used in remote exploit development in the Linux environment.