Containers offer process-level isolation but still rely on the shared kernel of the host system. This means that if a containerized process makes an unsafe or malicious system call, it can potentially destabilize or exploit the host environment. One of the most effective tools for tightening this vector is seccomp, short for secure computing mode.
Seccomp enables fine-grained control over which system calls a containerized process can make. By default, Docker includes a moderately restrictive seccomp profile that blocks over 40 potentially dangerous syscalls. However, for hardened environments, you can go further by customizing your own seccomp profile and tailoring it to the specific needs of your application.
A system call, or syscall, is a direct request from a running application to the operating system’s kernel. These are fundamental to how software interacts with hardware and system resources - opening files, accessing memory, launching processes, and more. While powerful, syscalls can also be abused by attackers to escalate privileges, inject code, or escape containers. Seccomp acts as a security filter, allowing only a safe subset of syscalls and denying the rest.
Docker’s built-in seccomp support lets you load a JSON file that defines allowed or denied syscalls. This file can specify not only which syscalls are permitted but also under what conditions—for example, based on arguments or context. It gives you the flexibility to build application-specific syscall policies that strike a balance between functionality and security.
To apply a custom seccomp profile, use the --security-opt flag when running a container.
Writing a good seccomp profile requires observation. One approach is to run your application under an unrestricted profile while logging the syscalls it makes, then iteratively block any that are unnecessary or risky. Tools like strace, auditd, and sysdig can help with syscall tracing. Once you identify the minimum set required for your workload, you can harden your profile.
Seccomp supports several actions for blocked syscalls: killing the process, logging the event, or returning a specific error. This allows flexibility in handling violations. For example, in testing, you might choose to log blocked syscalls before enforcing a deny policy. This way, you can refine your rules without disrupting functionality.
Seccomp does not operate in isolation. It is most effective when combined with other Docker security features like AppArmor, SELinux, capabilities dropping, and read-only filesystems. Together, they form a layered defense model that limits the blast radius of any compromise.
While customizing seccomp may seem daunting at first, it is a worthwhile investment for high-assurance systems. Services exposed to the internet, multitenant workloads, or applications processing sensitive data can all benefit from syscall filtering. Even if a vulnerability is exploited, seccomp ensures that the attacker has fewer tools available to pivot or escalate.
Docker’s default seccomp profile provides a good starting point, but for production workloads, it is advisable to audit and tailor it to your specific application requirements. Profiles should be tested rigorously before deployment, as overly restrictive rules can lead to unexpected failures or degraded performance.
For Kubernetes environments, seccomp can be enforced at the pod level using annotations or as part of PodSecurityPolicies. The ecosystem is maturing, and modern Kubernetes distributions now support native seccomp integration as part of their runtime class or security context configuration.
Ultimately, seccomp is about reducing trust. It assumes that even legitimate code may contain vulnerabilities and puts guardrails in place to prevent those flaws from becoming exploits. It is not a silver bullet, but it is a powerful tool in the container hardening toolbox.
If you are serious about deploying containers in production and want to secure your environment against modern threats, I encourage you to download my 20-page guide, Mastering Security & Isolation in Docker Like a Pro. This practical resource is filled with clear explanations, hardening techniques, Kubernetes security tips, and real-world strategies to help you build and operate resilient containerized systems with confidence.
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