Skip to content

How to harden a Jenkins container (and is jenkins/jenkins:lts safe to run builds?)

Jenkins is one of the most attacked services in a stack: it holds deploy credentials, cloud keys, and source, and it runs arbitrary build steps by design. A stock docker run jenkins/jenkins:lts is not the boundary that role deserves. Graded on IronClaw's seven-dimension containment scale, the default configuration scores 63 of 100, grade C (partial). Higher is safer. A few 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 CI server needs by definition: agents and browsers must reach it. Here are the exact gaps and fixes from the scan data.

Graded from a read-only inspect of a running container started from jenkins/jenkins:lts 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 jenkins/jenkins:lts, two fail and one warns:

Dimension Verdict Score What the scan found
Non-root user (uid != 0) ✅ PASS 15/15 runs as jenkins (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

Jenkins already gets the hardest dimension right by dropping to the jenkins uid, which is why it starts a full grade above most CI images. The two that remain are the full capability set and the writable rootfs. A plugin CVE, a malicious pipeline, or a poisoned dependency that lands code execution keeps CAP_NET_RAW to craft raw packets and a writable rootfs to persist, next to the credentials store Jenkins guards. A very common Jenkins pattern is mounting docker.sock so pipelines can build images; do not, and note that this scan would fail the docker.sock dimension if you did. Mounting the host Docker socket hands a build step full control of the host whatever uid it started as. Use a rootless builder or a scoped socket-proxy instead.

Harden it: the exact --fix remediation

ironctl scan my-jenkins --fix prints one remediation per failed dimension, then one hardened run. For jenkins/jenkins:lts:

  • --user 65532:65532 (Non-root user, already +15): the image already drops to the jenkins uid, so this dimension is closed by default. Keep a non-root uid pinned if you override the entrypoint; --fix emits 65532, the distroless nonroot uid. Point JENKINS_HOME at a volume that uid owns.
  • --cap-drop=ALL (Dropped capabilities, +16): drop every Linux capability; the Jenkins controller serves its UI and agent port on high ports and needs none of the default set.
  • --read-only --tmpfs /tmp (Read-only rootfs, +10): make the root filesystem read-only and mount JENKINS_HOME as an explicit writable volume. Removes the persistence surface.
  • Scoped network (Network isolation): --network=none scores the full 15 but is wrong for a CI server, agents and browsers must reach it. Any named or bridge network scores 4 of 15 (a WARN, not a fail): a connection path exists. Contain it anyway: put the controller on a user-defined network scoped to just its agents and reverse proxy, with no default route out, so a compromised build cannot call arbitrary internet addresses.

Before and after

# Before: 63/100, grade C
docker run -d --name jenkins jenkins/jenkins:lts

# After: 89/100, grade B (scoped private network for agents and proxy)
docker run -d --name jenkins-hardened \
  --user 65532:65532 \
  --cap-drop=ALL \
  --security-opt=no-new-privileges \
  --read-only --tmpfs /tmp \
  -v jenkins-home:/var/jenkins_home \
  --network=ci-internal \
  jenkins/jenkins:lts

Rescan: ironctl scan jenkins-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 CI server exists to be reached by its agents and users; network=none would score the last points but leave nothing able to connect. That is the honest ceiling for this role, and it is a long way from the default C.

Verify it on your own Jenkins

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

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

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

Keep going