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How to harden a Kong container (and is kong:3.8 safe for untrusted workloads?)

An API gateway sits at the edge, in front of everything, which is exactly why its container should be one of the tightest. A stock docker run kong:3.8 is closer than most, it already runs as a non-root user, but graded on IronClaw's seven-dimension containment scale it still scores only 63 of 100, grade C (partial). Higher is safer. A couple of runtime flags take the same image to 89 of 100, grade B, one point off an A, and the one dimension it cannot reach is the one a gateway needs by definition (clients and upstreams must connect through it). Here are the exact gaps and fixes from the scan data.

Graded from a read-only inspect of a running container started from kong:3.8 with plain docker run defaults, its entrypoint overridden with sleep purely to keep it alive. The scan itself executes nothing inside the container. It is the same data behind its isolation scorecard. How scoring works →

Where the default configuration leaks

ironctl scan grades seven independent containment boundaries. On a default docker run kong:3.8, three fail or warn (and, notably, non-root already passes):

Dimension Verdict Score What the scan found
Non-root user (uid != 0) ✅ PASS 15/15 runs as kong (uid != 0)
Dropped capabilities ❌ FAIL 4/20 default capability set retained (CAP_NET_RAW, CAP_MKNOD, and more)
Seccomp profile ✅ PASS 15/15 seccomp profile active
Network isolation / egress ⚠️ WARN 4/15 network=bridge: outbound egress is possible
Read-only root filesystem ❌ FAIL 0/10 root filesystem is writable
No docker.sock exposure ✅ PASS 15/15 no control socket mounted
No shared host namespaces ✅ PASS 10/10 no host PID/IPC/network sharing

Kong's image already did the hardest part, it drops root. The sharpest edges that remain are the retained capabilities and the writable rootfs: a plugin or Lua-runtime CVE that lands code execution lands it with CAP_NET_RAW and friends, on the process every request in your stack passes through, and with a filesystem it can rewrite to persist.

Harden it: the exact --fix remediation

ironctl scan my-kong --fix prints one remediation per failed dimension, then one hardened run. For kong:3.8:

  • --cap-drop=ALL (Dropped capabilities, +16): drop every Linux capability; add back only what the workload provably needs. Kong needs none of the defaults for a standard listen on an unprivileged port.
  • --read-only --tmpfs /tmp (Read-only rootfs, +10): make the root filesystem read-only and mount Kong's prefix path (/usr/local/kong) as an explicit writable volume. Removes the persistence surface.
  • Scoped network (Network isolation): --network=none scores the full 15 but is wrong for a gateway, clients and upstreams must reach it. Any named or bridge network scores 4 of 15 (a WARN, not a fail): a connection path exists. This is the one dimension a gateway cannot max out. Contain it anyway: attach a user-defined network scoped to just its upstreams, with no default route out, so a compromised gateway cannot call arbitrary internet addresses.

No --user flag is needed: the image already runs as kong.

Before and after

# Before: 63/100, grade C
docker run -d --name kong kong:3.8

# After: 89/100, grade B (scoped private network for clients and upstreams)
docker run -d --name kong-hardened \
  --cap-drop=ALL \
  --security-opt=no-new-privileges \
  --read-only --tmpfs /tmp \
  -v kong-prefix:/usr/local/kong \
  --network=kong-internal \
  kong:3.8

Rescan: ironctl scan kong-hardened reports 89/100 grade B. A 26-point swing with no custom image build, just the right flags. The only dimension still short of full marks is the network (4 of 15), because a gateway exists to be connected through; network=none would score the last points but leave nothing able to reach the upstreams. That is the honest ceiling for a gateway, and it is a clear step up from the default C.

Verify it on your own gateway

# install (Homebrew)
brew install ironsecco/ironclaw/ironclaw

# grade your running container, then print the fixes
ironctl scan my-kong
ironctl scan my-kong --fix

ironctl scan also reads a docker-compose.yml service or a Kubernetes manifest, so you can grade the Kong in your stack, not just a bare docker run.

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