👉 Overview
👀 What ?
ld.so is the dynamic linker/loader in Unix-based systems that is responsible for loading shared libraries for an executable. The ld.so preloading technique can be exploited to escalate privileges in a Unix-based system by forcing the system to load a malicious shared library.
🧐 Why ?
Understanding ld.so exploit is crucial as it is a common technique used in privilege escalation attacks. It is important for system administrators and penetration testers to be aware of this vulnerability to secure their systems and to perform comprehensive vulnerability assessments.
⛏️ How ?
The exploit involves creating a malicious shared library that contains the payload for privilege escalation, and then setting the LD_PRELOAD environment variable to point to this library. When a setuid program is executed, it loads the malicious shared library before any other library, leading to privilege escalation. However, this exploit only works if certain security measures, such as SELinux, are not in place.
⏳ When ?
The ld.so exploit is typically used after gaining initial access to a Unix-based system, as a method to escalate privileges. It is a common technique in privilege escalation since the inception of Unix-based systems.
⚙️ Technical Explanations
The dynamic linker/loader, ld.so, plays a fundamental role in Unix-based systems. It's responsible for loading shared libraries into memory and linking them to executable programs during runtime. Shared libraries contain code that can be used by multiple programs and are loaded as needed at runtime, thus optimizing system resources.
The LD_PRELOAD environment variable can be manipulated to specify a shared library that should be loaded before any others when a program is run. This is a feature intended for debugging and testing, but it can be exploited for privilege escalation.
In a privilege escalation attack, a malicious shared library is created, which contains a payload designed to elevate the user's privileges. The LD_PRELOAD variable is then set to point to this malicious library. When a setuid program (a program that runs with the privileges of its owner) is executed next, the system loads the malicious shared library first, due to the LD_PRELOAD directive. This leads to the execution of the payload and subsequently, privilege escalation.
However, this exploit is not without its checks and balances. Security measures such as SELinux (Security-Enhanced Linux) can mitigate this exploit. SELinux is a security module that provides a mechanism for supporting access control security policies. It restricts the loading of shared libraries by setuid programs, thus limiting the potential for privilege escalation attacks using the LD_PRELOAD technique.
In conclusion, while the ld.so and LD_PRELOAD functionality can be crucial for system operations and testing, it's essential for system administrators and security professionals to understand the potential for misuse in privilege escalation attacks and to take measures to secure their systems accordingly.
Here's a detailed real-world example:
- Creation of the Malicious Shared Library: First, we need to create a simple C program that contains the payload for privilege escalation. Here's an example of such a program:
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
void _init() {
if (geteuid() == 0) {
printf("We have root privileges!\\n");
} else {
printf("Privilege escalation failed.\\n");
}
}
In this program, the _init()
function is automatically executed when the shared library is loaded. It checks whether we have root privileges (i.e., whether the effective user ID is 0). If we do, it prints a success message; otherwise, it indicates that the privilege escalation attempt failed.
- Compilation of the Shared Library: We can compile this program into a shared library using the
gcc
command:
gcc -shared -o mylibrary.so mylibrary.c
- Setting the LD_PRELOAD Environment Variable: Next, we set the LD_PRELOAD environment variable to point to our malicious shared library:
export LD_PRELOAD=/path/to/mylibrary.so
- Executing a Setuid Program: Finally, we execute a setuid program. Because of the LD_PRELOAD directive, the system loads our malicious shared library first. If the setuid program is owned by root, our
_init()
function is executed with root privileges, leading to privilege escalation.
Please note that this is a simplified example and may not work on systems with security measures in place, such as SELinux, which can restrict the loading of shared libraries by setuid programs.