Code Caves in ELF Binaries

Introduction:

Code caves in Executable and Linkable Format (ELF) binaries are common vulnerabilities that can be exploited by attackers to inject malicious code into legitimate applications. However, the techniques used to locate and exploit code caves may vary depending on the CPU architecture of the target system. In this article, we will discuss how different code caves may appear in x86, x86_64, ARM, and ARM64 (AArch64) systems, and explore examples of vulnerabilities that can be exploited using these types of attacks. Additionally, we will demonstrate how to use the xxd tool to inject opcodes into the .fini section of an ELF binary.

CPU Architecture-Specific Code Caves:

1. x86: In 32-bit x86 systems, code caves are often found in unused areas within the executable file's memory map. Attackers may search for and exploit these vulnerabilities to inject their own malicious code and gain unauthorized access to sensitive information or resources.

2. x86_64: In 64-bit x86 systems, code caves can be found in unused areas within the executable file's memory map as well. However, the increased address space of these systems may make it more difficult for attackers to locate and exploit code caves.

3. ARM: In ARM (32-bit) systems, code caves are often found within unused instruction sequences or opcode areas within the executable file's memory map. Attackers can search for and manipulate these vulnerabilities to inject their own malicious code and gain unauthorized access to sensitive information or resources. However, discovering code caves in ARM binaries can be more challenging than in other CPU architectures because every assembly instruction is bound to a certain byte-length, making it harder for attackers to find suitable injection points for their payloads.

4. AArch64 (ARM64): In ARM64 systems, code caves may be found within unused instruction sequences or opcode areas as well. However, the increased complexity of ARM64's instruction set architecture can make it more difficult for attackers to locate and exploit these vulnerabilities.
   

Exploiting .fini Sections in ELF Binaries Across Different CPU Architectures with xxd Injection Techniques:

The xxd tool is a versatile command-line utility that can be used to manipulate binary files, including ELF binaries. By using the xxd -r -p options, you can inject specific opcodes into an ELF file's .fini section to redirect execution flow to your own malicious payload.

By using the built-in functionality of Vim and the power of xxd, you can easily manipulate binary files and exploit code caves in your target applications.

vim-based xxd Injection Techniques:

First, we need to find the offset and size of the .fini section in our target ELF file using tools like objdump or readelf:

readelf -S binary_file.o | grep '.fini ' | awk '{print $NF}'

1. Open the target binary file with Vim:

vim -b binary_file.o

1. Switch to hexadecimal mode using :%xxd command:

:%xxd

This will convert the entire binary file into a hexadecimal representation, allowing you to easily locate and manipulate specific sections of the file. For example prepend `cc cc` to the existing instructions in the `.fini` section, moving the existing instructions two bytes to the right, overwriting the two 00 00 bytes. Once complete, return the binary back to its normal state via:

:%xxd -r
:wq

If done correctly, this will cause the command to execute as normal w/ the exception of pausing during the execution instructions in the deconstructor phase of ELF execution (i.e. .fini).

Defense Strategies:

1. Ensure the permissions of binaries align with the principles of least privilege.

2. Implement regular security audits and vulnerability assessments on your software applications, operating systems, and firmware to identify potential code caves or other vulnerabilities that could be exploited by attackers.

3. Use binary protection tools such as code obfuscation or anti-tampering mechanisms to make it more difficult for attackers to locate and exploit code caves in your applications.

4. Implement digital signatures or checksums on your software applications, operating systems, or firmware to detect unauthorized modifications to the executable files.

5. Educate your users about the risks of downloading and installing untrusted software from the internet, as this is a common method used by attackers to distribute malicious code caves in ELF binaries.
   

In conclusion, while code caves in ELF binaries present a significant security risk, there are several effective defense strategies that can be implemented to protect your applications and data from these types of attacks. By staying informed about the latest threats and vulnerabilities and implementing strong security practices, you can minimize the impact of code caves on your organization's overall security posture.