Escaping Docker Containers: Techniques for Gaining Host System Access and Expanding Your Attack Surface

Introduction link

Docker containers provide isolation and resource management benefits, but security misconfigurations or vulnerabilities within containers can potentially lead to container escape scenarios. In such scenarios, an attacker who has gained initial access to a container might be able to break out of the container’s isolated environment and gain access to the underlying host system. This can have severe consequences, as the attacker can then potentially escalate privileges, access sensitive data, or compromise other containers running on the host.

Understanding Docker Security link

Docker containers utilize various security mechanisms to isolate containers from the host and from each other, including:

• Namespaces: Isolate containers in terms of process IDs, network interfaces, users, and mount points.
• Control Groups (cgroups): Limit resource usage (CPU, memory, I/O) by containers.
• Seccomp profiles: Restrict system calls available to containers.
• Capabilities: Fine-grained control over specific privileges granted to containers.

However, vulnerabilities or misconfigurations in these mechanisms can create opportunities for container escape.

Common Container Escape Techniques link

1. Exploiting Kernel Vulnerabilities:

   • If a container shares the host’s kernel and a kernel vulnerability exists, an attacker within the container might be able to exploit the vulnerability to gain root privileges on the host.
   • Example: A vulnerability in a system call used by the container could allow an attacker to overwrite kernel memory and gain control.

2. Mounting Sensitive Host Directories:

   • If a container has access to sensitive host directories (e.g., /, /etc, /var) through mounts, an attacker might be able to modify host configuration files, install malware, or access sensitive data.
   • Example: Mounting the Docker socket (/var/run/docker.sock) inside a container can give the container control over the Docker daemon, allowing it to create new containers with elevated privileges.

3. Leveraging Weak Seccomp Profiles:

   • If a container’s Seccomp profile is not restrictive enough, an attacker might be able to execute system calls that are not intended to be allowed within the container.
   • Example: An attacker might be able to use the chroot system call to change the container’s root directory, effectively escaping the container’s filesystem isolation.

4. Exploiting Docker Daemon Vulnerabilities:

   • Vulnerabilities in the Docker daemon itself can potentially be exploited by an attacker within a container to gain control of the host.
   • Example: A vulnerability in the Docker API could allow an attacker to escalate privileges or execute commands on the host.

5. Abusing Capabilities:

   • If a container is granted excessive capabilities (e.g., CAP_SYS_ADMIN), an attacker might be able to perform actions that should be restricted, such as mounting filesystems or modifying network settings.
   • Example: The CAP_SYS_ADMIN capability grants a wide range of privileges, including the ability to modify kernel parameters, which could be abused to escalate privileges.

Practical Example: Escaping Through Docker Socket Mounting link

Let’s imagine a scenario where a container has the Docker socket mounted inside it:

1. Gaining Initial Access: An attacker exploits a vulnerability in the application running inside the container to gain command execution.

2. Accessing the Docker Socket: The attacker identifies that the Docker socket is mounted at /var/run/docker.sock inside the container.

3. Creating a Privileged Container: The attacker uses the Docker CLI within the compromised container to create a new container with privileged access:

     docker run -it -v /:/mnt --privileged ubuntu bash 
     

   This command creates a new container with root privileges that has access to the entire host filesystem (/) mounted at /mnt.

4. Escaping to the Host: The attacker can now access the host’s filesystem through /mnt and potentially escalate privileges further, install malware, or access sensitive data.

Remediation and Mitigation link

• Minimize Host Directory Mounts: Only mount necessary directories inside containers and avoid mounting sensitive directories like /.
• Utilize User Namespaces: Isolate container users from host users to prevent privilege escalation in case of container escape.
• Implement Robust Seccomp Profiles: Restrict system calls available to containers to the bare minimum required for their functionality.
• Avoid Granting Excessive Capabilities: Only grant necessary capabilities to containers and avoid using --privileged mode.
• Keep Docker Updated: Regularly update Docker to patch known vulnerabilities.
• Implement Intrusion Detection Systems: Monitor container activity for suspicious behavior that might indicate a container escape attempt.

Conclusion link

Container escape vulnerabilities can pose a serious threat to the security of Docker environments. By understanding the common techniques used for container escape and implementing appropriate security measures, you can significantly reduce the risk of such attacks and protect your systems. Remember that container security is a multi-layered approach that involves securing the host system, the Docker daemon, and the containers themselves.