![]() This is no longer possible because the stack is now a so-called ‘non-executable’ memory segment if the CPU starts executing code on the stack (EIP is pointing somewhere on the stack), the CPU will throw an exception and the program will crash without the harmful code being executed.ĭEP can be worked around by, instead of loading your own code onto the stack, executing code that already exists in an executable section of the program, such as the program itself or a library (a trick called return-to-libc). Before DEP (or NX) came along, the common approach was injecting shellcode onto the stack and then overwriting the return value on the stack by the offset which is the start of the shellcode. When someone finds a bug in a program (for instance a buffer overflow), this can potentially be abused to take control of the flow of execution of the program by overwriting key values in the program, such as the return addresses on the stack. How does this make life difficult for Bad Guys™? What is being moved around?ĪSLR moves the following memory segments: After I completed my research and fully understood how ASLR works in Windows, I decided to write an article about. When I first wanted to figure out how ASLR works internally, I came across a lot of articles, but not one that tells the entire story. Before that, Linux had already been enabling ASLR by default since kernel 2.6.12, released in June 2005.ĪSLR tries to make life of bad guys™ more difficult by randomizing how an executable is laid out in memory. It was introduced in Windows with Vista, released to customers in early 2007. ASLR stands for Address Space Layout Randomisation. ![]()
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