Write a Falco custom rule to detect suspicious exec inside a container
domain: falco.org · 6 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Create a custom rules file (e.g., custom_rules.yaml) alongside the default Falco rules directory
Define a list for trusted images and a macro for container context using the `list` and `macro` keywords
Write a rule with `condition: evt.type = execve and container and not proc.name in (trusted_procs)` and set `priority: WARNING`
Set `output` to include `%container.id`, `%proc.name`, `%proc.cmdline`, and `%user.name` for actionable alerts
Mount the custom rules file into the Falco pod or reference it in `falco.yaml` under `rules_file`
Restart Falco and verify rule loading with `falco --dry-run` or by checking startup logs for rule parse errors
Known gotchas
Falco evaluates rules in file order; a later `override` block is required to extend or replace a base rule rather than silently duplicating it
The `execve` event fires for every process launch including init containers and health-check scripts — keep the trusted list accurate to avoid alert fatigue
Field names differ between Falco versions; always verify field availability with `falco --list` for your installed version
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