# Linux Post-Exploitation

{% hint style="success" %}
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</details>
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## Sniffing Logon Passwords with PAM

Let's configure a PAM module to log each password each user uses to login. If you don't know what is PAM check:

{% content-ref url="linux-post-exploitation/pam-pluggable-authentication-modules" %}
[pam-pluggable-authentication-modules](https://angelica.gitbook.io/hacktricks/linux-hardening/linux-post-exploitation/pam-pluggable-authentication-modules)
{% endcontent-ref %}

**For further details check the** [**original post**](https://embracethered.com/blog/posts/2022/post-exploit-pam-ssh-password-grabbing/). This is just a summary:

**Technique Overview:** Pluggable Authentication Modules (PAM) offer flexibility in managing authentication on Unix-based systems. They can enhance security by customizing login processes but also pose risks if misused. This summary outlines a technique to capture login credentials using PAM, alongside mitigation strategies.

**Capturing Credentials:**

* A bash script named `toomanysecrets.sh` is crafted to log login attempts, capturing the date, username (`$PAM_USER`), password (via stdin), and remote host IP (`$PAM_RHOST`) to `/var/log/toomanysecrets.log`.
* The script is made executable and integrated into the PAM configuration (`common-auth`) using the `pam_exec.so` module with options to run quietly and expose the authentication token to the script.
* The approach demonstrates how a compromised Linux host can be exploited to log credentials discreetly.

```bash
#!/bin/sh
echo " $(date) $PAM_USER, $(cat -), From: $PAM_RHOST" >> /var/log/toomanysecrets.log
sudo touch /var/log/toomanysecrets.sh
sudo chmod 770 /var/log/toomanysecrets.sh
sudo nano /etc/pam.d/common-auth
# Add: auth optional pam_exec.so quiet expose_authtok /usr/local/bin/toomanysecrets.sh
sudo chmod 700 /usr/local/bin/toomanysecrets.sh
```

### Backdooring PAM

**For further details check the** [**original post**](https://infosecwriteups.com/creating-a-backdoor-in-pam-in-5-line-of-code-e23e99579cd9). This is just a summary:

The Pluggable Authentication Module (PAM) is a system used under Linux for user authentication. It operates on three main concepts: **username**, **password**, and **service**. Configuration files for each service are located in the `/etc/pam.d/` directory, where shared libraries handle authentication.

**Objective**: Modify PAM to allow authentication with a specific password, bypassing the actual user password. This is particularly focused on the `pam_unix.so` shared library used by the `common-auth` file, which is included by almost all services for password verification.

### Steps for Modifying `pam_unix.so`:

1. **Locate the Authentication Directive** in the `common-auth` file:
   * The line responsible for checking a user's password calls `pam_unix.so`.
2. **Modify Source Code**:
   * Add a conditional statement in the `pam_unix_auth.c` source file that grants access if a predefined password is used, otherwise, it proceeds with the usual authentication process.
3. **Recompile and Replace** the modified `pam_unix.so` library in the appropriate directory.
4. **Testing**:
   * Access is granted across various services (login, ssh, sudo, su, screensaver) with the predefined password, while normal authentication processes remain unaffected.

{% hint style="info" %}
You can automate this process with <https://github.com/zephrax/linux-pam-backdoor>
{% endhint %}

{% hint style="success" %}
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<details>

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