Userspace-only mode is an Open vSwitch deployment model where packet processing stays in user space instead of using the Linux kernel datapath. In this setup, the switch handles forwarding, filtering, and flow logic through a normal process rather than kernel-resident code.
This matters in cyber security because the kernel is part of the host trust boundary. Keeping traffic handling out of the kernel can reduce exposure to kernel bugs and make some privilege-escalation paths harder to reach. Operators may choose it to shrink attack surface in sensitive environments, especially where networking code does not need maximum throughput. The tradeoff is performance: userspace processing usually carries more latency and lower packet rates than the kernel fast path. In defense, it can be a deliberate hardening choice; in attacks, systems that rely on the kernel datapath are often more attractive because flaws there can lead to host-level compromise.