Ever wondered how to add custom code to an existing executable? ELF injection offers a fascinating way to do this. This technique allows you to insert relocatable code chunks into ELF (Executable and Linkable Format) files, effectively running your code before the original program even starts. This post will walk you through the basics of ELF injection and provide a practical understanding of how it works.
Understanding ELF Injection
ELF injection involves modifying an existing ELF executable to include a new code segment. This new segment contains the code you want to inject. The injection process alters the executable’s entry point, redirecting execution to your injected code first. After your code completes, control is then transferred back to the original entry point, allowing the original program to run as intended.
How ELF Injection Works
The process typically involves several key steps:
- Identifying the original entry point: You need to locate the original entry point of the ELF executable. This is the address where execution begins normally.
- Creating a new code segment: A new segment is created within the ELF file to house the injected code. This includes setting appropriate permissions (execute, read, etc.).
- Inserting the injected code: The compiled code you want to inject is placed into this new segment.
- Modifying the entry point: The ELF header is modified to point to the new code segment’s starting address. This makes the injected code the first to run.
- Transferring control back: At the end of the injected code, a jump instruction is added to transfer control back to the original entry point, ensuring the original program executes.
Practical Example: Injecting a ‘Hello, World!’ Chunk
Let’s illustrate this with a simple example. Imagine injecting a “Hello, World!” code chunk into a 32-bit ARM executable.
First, you’d write your “Hello, World!” code in C and compile it to ARM assembly. This assembly code would be your injected chunk.
Then, using an ELF manipulation tool or library, you’d follow the steps outlined earlier. You’d identify the original entry point, create a new code segment, insert the compiled “Hello, World!” assembly code, change the entry point to point to the “Hello, World!” code, and finally, add a jump instruction at the end of the “Hello, World!” code to return control to the original entry point.
When executed, the modified ELF file would first print “Hello, World!” and then proceed with the original program’s execution.
Porting to Other Architectures
While the example focuses on 32-bit ARM, the general principles apply to other architectures. The main difference lies in the specifics of the ELF file format for each architecture and the assembly language used for the injected code.
Tools and Libraries
Various tools and libraries can facilitate ELF manipulation. Researching architecture-specific resources is crucial for successful implementation. For learning purposes, creating a basic injector from scratch can be a valuable exercise in understanding the underlying mechanics.
Considerations and Potential Pitfalls
ELF injection requires careful attention to detail. Incorrect modifications can corrupt the executable, rendering it unusable. Thoroughly testing your injected code and understanding the target executable’s structure is essential.
