Drop Linux capabilities in a bwrap sandbox with --cap-drop and --cap-add
domain: github.com/containers/bubblewrap · 8 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Requirements / auth: No credentials. Requires bubblewrap and unprivileged user namespaces; capabilities available to add are limited to those bwrap itself holds.
Start from a clean slate with `--cap-drop ALL`, which drops all capabilities; `ALL` is accepted as a special value by both `--cap-drop` and `--cap-add`.
Add back only what the workload provably needs, after the drop: `--cap-drop ALL --cap-add CAP_NET_BIND_SERVICE`.
Rely on documented ordering: `--cap-add` and `--cap-drop` are processed in the order they appear on the command line, so a later `--cap-add` wins over an earlier `--cap-drop ALL` and vice versa.
Pair capability dropping with `--unshare-user`, since capabilities inside a user namespace apply only to resources owned by that namespace — a sandbox with CAP_SYS_ADMIN in its own userns is not the same as CAP_SYS_ADMIN on the host.
Confirm the resulting capability set from inside: `bwrap --ro-bind /usr /usr --symlink usr/bin /bin --proc /proc --unshare-all --cap-drop ALL /bin/sh -c 'grep ^Cap /proc/self/status'` and check CapEff is 0000000000000000.
Combine with `--disable-userns` so a process holding capabilities inside the namespace cannot mint a fresh user namespace to regain more.
Official documentation: https://manpages.debian.org/unstable/bubblewrap/bwrap.1.en.html
Known gotchas
`--cap-add ALL` only adds all permitted capabilities — it cannot grant capabilities bwrap did not have. It is still dangerous: use it for debugging, never in a deployed sandbox profile.
Ordering is significant and easy to get backwards. `--cap-add CAP_NET_ADMIN --cap-drop ALL` leaves you with nothing, because the drop is processed last.
Capabilities are not a substitute for a syscall filter. Dropping capabilities still leaves the full syscall surface reachable — add `--seccomp` if that surface is your concern.
In-namespace capabilities look alarming in /proc/self/status when you use `--uid 0`, but they are namespace-scoped. Judge the sandbox by what you mounted in, not by the CapEff bitmask alone.
Capability names must be given in the CAP_* form the kernel uses; a typo is rejected at startup rather than silently ignored, so test the exact spelling before deploying.
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