Patric Patric 1, 15 15 silver badges 18 18 bronze badges. Yu Hao k 42 42 gold badges silver badges bronze badges. Ronny Brendel Ronny Brendel 4, 5 5 gold badges 34 34 silver badges 54 54 bronze badges. JanusTroelsen if the library you write doesn't implement a certain part, that part would be loaded from the original library. If the preloaded library does not export a symbol, it will be found elsehwere. JanusTroelsen: It turns out that malloc and free are specifically designed in glibc to allow this and the stock calloc manages to call your imported malloc.
Don't try this with any other functions. It won't work so good. Rajesh Rajesh 5 5 silver badges 13 13 bronze badges. To export mylib. JulienGenoud JulienGenoud 9 9 silver badges 21 21 bronze badges. Thanks for posting your answer! Please note that you should post the essential parts of the answer here, on this site, or your post risks being deleted See the FAQ where it mentions answers that are 'barely more than a link'.
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Sign up or log in Sign up using Google. Sign up using Facebook. This application will simply:. Back in the parent directory of the repo, there is a Makefile. This file will be used anytime you call the make command. There is also a help target that can be invoked to show you all of the build targets you can use with make. The inject. The default build will be with color support on the inject.
Below is a screenshot of the make help output:. That command should create the binary for the hello world program in the root of the repo directory as. The output from that make command and a follow-up ls -l should show the following output:. The screenshot below shows what a normal run of this program would look like for reference. The code for the inject. The screenshot below shows the complete code for the function. Most of the comments within the function explain the code, but there are a few things I want to point out.
The function declaration is important:. The final lines of code are all printf statements to display information to the user. Preloading a library means that its functions will be used before others of the same name in later libraries. This enables library functions to be intercepted and replaced overwritten.
As a result program behavior can be non-invasively modified, i. For example, you could write a library which implements alternative malloc and free functionality. Note the dlfcn. This function is used to keep track of the original implementation of strcmp. Then, I return the orignal result of strcmp using the pointer to libc. This method is based on the ptrace system call which is used to observe and control the execution of another process e.
GDB use ptrace. By implementing this syscall in a program, the call to ptrace will generate an error on error, all requests return -1 if the process is running with a debugger.
However, if this process is attached in a debugger, ptrace will return a error and exit. But, we could build a shared library to be loaded with GDB to reimplement the ptrace call and bypass the anti-debug trick. Now, we will apply our skills on a live target. SSH : ssh utumno0 utumno.
So, it seems that the program is using puts to print the message. Now, we could use printf to read data on the stack by employing the same method used by the Format String vulnerability.
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