Jump Address Hacking to Execute Code Stored as a String

Operating systems typically give each process a virtual address space ranging from 0x0000000000000000 to 0xffffffffffffffff on a 64bit machine. This memory range is divded up in muliple sections. Usually there are at least two: .DATA and .TEXT.

Memory in .DATA is read/write, but cannot be executed. Similarly, memory in .TEXT is read/exec but cannot be written to. This security measure is called “W xor X” and prevents a malicious user from injecting new code into a running process. This does not prevent a malicious user from manipulating a program to run code that already exists when it shouldn’t.

This program is a demonstration of using jump address hacking to execute code stored in the string constant execString. This works because GCC puts string constants in the .TEXT section, the executable part of memory.

execString is approximately equivalent to the code below with modifications to avoid null bytes in the machine code.

long str = 0x0A47464544434241; // "ABCDEFG\n"
write(1, &str, 8);
exit(0);

If this program works properly you should see it print “ABCDEFG” and then exit with a return code of 0.

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This program was tested on a 2013 Macbook Air running 10.12.6 (16G29) and compiled with $ gcc jump_overwrite.c

$ gcc -v returns the following:

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 9.0.0 (clang-900.0.37)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin