---
title: "Container Runtime Security Boundaries"
description: "An in-depth look at the key security boundaries and isolation mechanisms in modern container runtimes"
keywords: ["container", "runtime", "security", "isolation", "namespaces"]
category: "security"
---

Containers leverage a variety of namespace isolation mechanisms to restrict the visibility and access of processes running inside the container. The `pid`, `net`, `mnt`, and `user` namespaces are commonly used to isolate process IDs, network interfaces, mount points, and user/group IDs respectively. For example, the `docker run --pid=host` option allows a container to share the host's PID namespace, while `--user=1000:1000` sets the container's user and group IDs to 1000. These namespace-based isolation techniques help prevent container breakouts and limit the potential blast radius of a container compromise.

Seccomp (secure computing mode) profiles provide another critical security boundary by restricting the system calls that a container is allowed to make. Seccomp profiles are specified in JSON format, such as `{ "defaultAction": "SCMP_ACT_ERRNO", "syscalls": [...] }`, and can be applied to a container via the `docker run --security-opt seccomp=/path/to/profile.json` option. By default, Docker applies a restrictive seccomp profile that blocks over 40 potentially dangerous syscalls. Developers can also create custom seccomp profiles to further lock down their containers.

AppArmor and SELinux are Linux security modules that provide mandatory access control (MAC) policies to restrict the actions a container can perform. These policies are defined in text-based profiles, e.g. `docker run --security-opt apparmor=profile_name`. AppArmor and SELinux can prevent containers from accessing sensitive host resources, executing unexpected binaries, or modifying critical system files, adding an additional layer of defense-in-depth.

Rootless containers, enabled by user namespaces in runtimes like Podman v3.0+, allow containers to run as an unprivileged user within the host's user namespace. This isolates the container's processes from the host's root user, reducing the potential impact of a container escape. Rootless containers also support read-only root filesystems to further limit writeable paths and prevent malicious modifications.

Capability dropping is another container security technique that removes unnecessary Linux capabilities from a container's processes. For example, the `CAP_NET_RAW` capability can be dropped to prevent network sniffing attacks, while `CAP_MKNOD` can be removed to block device node creation. Capabilities are dropped via the `docker run --cap-drop=NET_RAW` option.

Runtime security monitoring tools like Falco can detect and alert on suspicious container behavior that may indicate a security breach, such as unexpected system calls, network connections, or file modifications. Falco rules are defined in YAML and can be deployed alongside a container runtime to provide real-time runtime threat detection.

Finally, gVisor is an open-source container runtime that provides an additional security layer by running containers in a user-space kernel. This isolates the container's system calls from the host kernel, effectively creating a hardware-backed sandbox. gVisor can be used as a drop-in replacement for runtimes like Docker or Containerd via the `docker run --runtime=runc` option.In addition to the namespace isolation, security profiles, and runtime monitoring capabilities, container runtimes also leverage other techniques to enhance security boundaries. One such method is the use of read-only root filesystems, which prevent containers from modifying critical system files and directories. This can be enabled in Docker via the `--read-only` flag, restricting the container's ability to write to the root partition and limiting the potential impact of a compromise.

Capability dropping is another important security feature, allowing container administrators to selectively remove Linux capabilities from a container's processes. For example, the `CAP_NET_RAW` capability can be dropped to prevent network sniffing attacks, while `CAP_MKNOD` can be removed to block the creation of device nodes. These capabilities are specified using the `--cap-drop` option when running a container, further reducing the attack surface.

Runtime security monitoring tools like Falco provide an additional layer of defense by detecting and alerting on suspicious container behavior. Falco is an open-source project that uses a rich set of rules, defined in YAML, to identify potential security incidents, such as unexpected system calls, network connections, or file modifications. These rules can be deployed alongside a container runtime to provide real-time threat detection and response capabilities.

Finally, gVisor is an open-source container runtime developed by Google that takes a fundamentally different approach to security. Instead of relying solely on Linux kernel-based isolation, gVisor runs containers in a user-space kernel, effectively creating a hardware-backed sandbox. This additional layer of isolation helps to mitigate vulnerabilities in the host kernel and further reduces the attack surface. gVisor can be used as a drop-in replacement for runtimes like Docker or Containerd by specifying the `--runtime=runc` option.

Overall, container runtimes leverage a diverse set of security boundaries and isolation mechanisms to protect against a wide range of threats. From namespace-based isolation to security profiles, capability dropping, runtime monitoring, and user-space kernel sandboxing, these techniques work together to create a robust security architecture for containerized applications.Another key security feature in container runtimes is the use of read-only root filesystems. By default, containers have a writable root filesystem, which can pose a security risk if the container is compromised and an attacker gains the ability to modify critical system files. To mitigate this, container runtimes like Docker support the use of read-only root filesystems, where the container's root partition is mounted as read-only. This can be enabled with the `--read-only` flag when running a container, effectively preventing the container from making unauthorized changes to the root filesystem and limiting the potential impact of a security breach.

Capabilities are another important security boundary in container runtimes. Linux capabilities are a fine-grained access control mechanism that allow the kernel to grant or revoke specific privileges to processes, rather than relying solely on the all-or-nothing root user model. Container runtimes leverage this capability system to drop unnecessary privileges from the container's processes, reducing the attack surface. For example, the `CAP_NET_RAW` capability can be dropped to prevent a container from performing network sniffing attacks, while `CAP_MKNOD` can be removed to block the creation of device nodes. Capabilities are dropped using the `--cap-drop` option when running a container.

In addition to these security features, container runtimes can also integrate with runtime security monitoring tools like Falco to provide real-time threat detection and response capabilities. Falco is an open-source project that uses a rich set of rules, defined in YAML, to identify suspicious container behavior, such as unexpected system calls, network connections, or file modifications. These rules can be deployed alongside a container runtime to provide an additional layer of security, allowing administrators to quickly detect and respond to potential security incidents.

Finally, some container runtimes, such as gVisor, take a fundamentally different approach to security by running containers in a user-space kernel. Unlike traditional container runtimes that rely on Linux kernel-based isolation, gVisor creates a hardware-backed sandbox by running the container's system calls in a user-space kernel. This additional layer of isolation helps to mitigate vulnerabilities in the host kernel and further reduces the attack surface. gVisor can be used as a drop-in replacement for runtimes like Docker or Containerd by specifying the `--runtime=runc` option.Another important security feature in container runtimes is the use of user namespaces, which allow containers to run as an unprivileged user within the host's user namespace. This is particularly relevant for the emerging trend of "rootless" containers, where the container's processes are isolated from the host's root user, reducing the potential impact of a container escape. Rootless containers, enabled by user namespaces in runtimes like Podman v3.0+, help to further restrict the privileges of the container's processes and prevent them from accessing sensitive host resources. This added layer of user-level isolation complements the other security boundaries provided by namespaces, security profiles, and capability dropping.

Complementing the user namespace isolation, container runtimes also support the use of read-only root filesystems to limit the writable paths available to the container's processes. By mounting the container's root partition as read-only, this security feature effectively prevents the container from making unauthorized modifications to critical system files and directories, even if the container is compromised. The read-only root filesystem can be enabled using the `--read-only` flag when running a container, further hardening the security posture and reducing the attack surface.

In addition to the security features built into the container runtime itself, many organizations also leverage external runtime security monitoring tools, such as Falco, to provide real-time threat detection and response capabilities. Falco is an open-source project that uses a rich set of rules, defined in YAML, to identify suspicious container behavior, including unusual system calls, network connections, or file modifications. These Falco rules can be deployed alongside a container runtime to provide an additional layer of security, allowing administrators to quickly detect and respond to potential security incidents within the containerized environment.

Finally, some container runtimes, such as gVisor, take a fundamentally different approach to security by running containers in a user-space kernel. Unlike traditional container runtimes that rely on Linux kernel-based isolation, gVisor creates a hardware-backed sandbox by executing the container's system calls in a user-space kernel. This additional layer of isolation helps to mitigate vulnerabilities in the host kernel and further reduces the attack surface. gVisor can be used as a drop-in replacement for runtimes like Docker or Containerd by specifying the `--runtime=runc` option, providing an alternative security architecture for containerized applications.