Docker Compose Secrets Bug: UID/GID Defaults To Root

Oct 16, 2025 by ADMIN 53 views

Hey guys! Today, we're diving deep into a sneaky bug in Docker Compose that can cause some serious head-scratching. It's all about how secrets (and likely configs) are handled when it comes to user and group IDs (UID/GID). So, if you've been battling with permission issues in your Docker containers, this one's for you. Let's break it down and see what's going on.

The Issue: Secrets Defaulting to Root

So, here's the deal. When you create a secret in Docker Compose from an environment variable and then mount it into a container, you'd expect it to inherit the UID/GID from the USER instruction in your Dockerfile. At least, that's what the official Docker documentation implies. According to the docs, the UID and GID should default to the USER specified in your Dockerfile. However, in a twist that could ruin your day, these secrets often end up belonging to 0:0 – good ol' root.

This is a major bummer because it doesn't matter if you've explicitly set a user in your service definition or not. Unless you specify the UID/GID directly when defining the secret, it's going to be owned by root. This can lead to all sorts of permission denied errors, especially if your application is running as a non-root user, which, let's be honest, it should be! Imagine setting your secret file mode to 440 or even 400 for security, only to find out your application can't read it because it's owned by root. Frustrating, right?

To reiterate, the core issue here is that Docker Compose isn't correctly applying the expected UID/GID to secrets created from environment variables. This unexpected behavior can cause significant problems in production environments where security and least privilege principles are crucial. The default behavior should align with the USER instruction in the Dockerfile, or at least provide a clear and straightforward way to configure the correct ownership.

Why This Matters

This might seem like a small detail, but it has significant implications for the security and reliability of your applications. Running processes as non-root users is a fundamental security best practice. It limits the potential damage if a container is compromised. When secrets are owned by root, it can create a situation where your application needs elevated privileges just to access its configuration, which defeats the purpose of running as a non-root user in the first place.

Steps to Reproduce: Let's Get Our Hands Dirty

Okay, enough talk! Let's get our hands dirty and reproduce this bug ourselves. I'm going to walk you through the steps to see this issue in action. Trust me, seeing is believing, and once you've seen it, you'll know exactly what to look for in your own setups.

Create a Folder: First things first, let's create a new directory to keep our test files organized. Name it something descriptive, like compose-secret-test.
```
mkdir compose-secret-test
cd compose-secret-test
```
Add the Files: Now, we're going to add the necessary files to our test directory. This includes a compose.yml file, a Dockerfile, and a .env file. I'll paste the contents of each file below, so you can copy and paste them directly. This setup is designed to demonstrate the issue clearly, with two containers: one built from a Dockerfile and another using a base image.
Run docker compose up: This is where the magic happens (or, in this case, the bug manifests). Open your terminal, navigate to the compose-secret-test directory, and run the command docker compose up. This will build and start our containers.
```
docker compose up
```
Check the Output: Once the containers are up and running, we need to inspect their output. Look for the lines that show the file listing of the /run/secrets directory. This is where our secret is mounted, and we'll see who owns it.
Observe the Owner: This is the critical step. You should see that the secret file, test-secret, is owned by root:root in both containers. This confirms the bug – our secret isn't inheriting the UID/GID of the specified user.

Here are the contents of the files you'll need:

# compose.yml
services:
  test-build:
    build:
      context: .
      dockerfile: Dockerfile
    secrets:
      - source: test-secret
        mode: 0o440

  test-image:
    image: ubuntu:24.04
    command: ["ls", "-al", "/run/secrets"]
    user: "1000:1000"
    secrets:
      - source: test-secret
        mode: 0o440

secrets:
  test-secret:
    environment: TEST_SECRET

# Dockerfile
FROM ubuntu:24.04
USER 1000:1000
ENTRYPOINT [ "ls", "-al", "/run/secrets" ]

# .env
TEST_SECRET=VerySecureSecret12345

Note: In this example, we're using ubuntu:24.04 because it conveniently has a user with UID/GID 1000, which makes it easy to demonstrate the issue. However, the bug isn't specific to this image; it'll occur regardless of the base image you use.

Expected vs. Actual Behavior

What we expected was for the secret file to be owned by the user with UID 1000, as specified in both the Dockerfile and the user directive in the compose.yml file. This would ensure that our application, running as a non-root user, could access the secret without any permission issues.

However, the actual behavior is that the secret file is owned by root:root. This is a clear violation of the principle of least privilege and can lead to applications failing to start or function correctly.

Diving Deeper: My Environment

To give you the full picture, here's a snapshot of the environment I used to reproduce this bug:

Docker Compose Version: v2.40.0
Docker Engine Version: 28.5.1
Operating System: Ubuntu 24.04.3 LTS
Kernel Version: 6.8.0-85-generic

This information is crucial because bugs can sometimes be specific to certain versions or configurations. If you're experiencing this issue, it's helpful to compare your environment to mine to see if there are any similarities.

A Peek into My Docker Environment

For the sake of transparency, here's the output of docker info from my environment. This gives you a detailed look at the Docker setup I was using when I encountered this bug. This information can be helpful for others who are trying to reproduce the issue or debug similar problems.

Client: Docker Engine - Community
 Version:    28.5.1
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.29.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.40.0
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 26
  Running: 24
  Paused: 0
  Stopped: 2
 Images: 28
 Server Version: 28.5.1
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: local
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: b98a3aace656320842a23f4a392a33f46af97866
 runc version: v1.3.0-0-g4ca628d1
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.8.0-85-generic
 Operating System: Ubuntu 24.04.3 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 15.5GiB
 Name: <redacted>
 ID: b30fcdfe-3673-432a-9a9b-2bba973e5bd0
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: false

Workarounds and Solutions: What Can We Do?

Okay, so we've identified the problem, we've reproduced it, and now we're asking the big question: what can we do about it? Fortunately, there are a few workarounds you can use to ensure your secrets have the correct UID/GID. Let's explore some of the options.

1. Explicitly Specify UID/GID

The most straightforward workaround is to explicitly specify the uid and gid when defining your secret in the compose.yml file. This bypasses the default behavior and ensures the secret is created with the correct ownership. It may be tedious if you have a lot of secrets to manage, but it gets the job done.

Here's how you can do it:

secrets:
  test-secret:
    environment: TEST_SECRET
    uid: "1000"
    gid: "1000"

By adding the uid and gid attributes, we're explicitly telling Docker Compose to create the secret with these values. This ensures that the secret file will be owned by the correct user and group, preventing permission issues.

2. Using `chown` in Your Dockerfile

Another approach is to use the chown command within your Dockerfile to change the ownership of the secret file after it's mounted. This is a more dynamic solution, as it allows you to set the ownership based on the user defined in your Dockerfile. However, it requires you to know the mount path of the secret, which is typically /run/secrets/<secret_name>. Furthermore, this method is more of a workaround than a true fix, and it adds extra steps to your build process.

FROM ubuntu:24.04
USER 1000:1000
COPY --chown=1000:1000 . /app
RUN chown 1000:1000 /run/secrets/test-secret
ENTRYPOINT ["/app/your-application"]

3. Init Containers

Init Containers can be used to set the correct permissions before the main container starts. This approach provides a clean and isolated way to manage file ownership. However, it adds complexity to your deployment configuration and is usually more suited for Kubernetes environments than simple Docker Compose setups. Init Containers run before the main container and can perform tasks such as setting file permissions. This ensures that the main container starts with the correct ownership in place.

Choosing the Right Workaround

The best workaround for you will depend on your specific needs and environment. Explicitly specifying the UID/GID is the most straightforward approach and works well for simple setups. Using chown in your Dockerfile provides more flexibility but adds complexity to your build process. Init Containers are best suited for more complex deployments, especially in Kubernetes environments.

Conclusion: Let's Get This Fixed!

Alright, guys, we've dug deep into this Docker Compose secret bug, and it's clear that this is a real issue that can cause headaches. The fact that secrets default to root ownership, despite documentation suggesting otherwise, is a problem that needs to be addressed. This bug can lead to significant security and operational challenges, especially in production environments where non-root users are crucial.

While we have workarounds, such as explicitly specifying UID/GID or using chown, these are just bandaids on a bigger problem. The ideal solution is for Docker Compose to correctly handle UID/GID inheritance for secrets (and configs) by default.

If you've encountered this issue, I encourage you to add your voice to the discussion! Reporting bugs and providing feedback to the Docker team helps them prioritize fixes and improve the overall experience. You can contribute by:

Commenting on existing bug reports: Search for similar issues on the Docker Compose GitHub repository and add your comments and experiences.
Creating a new bug report: If you can't find an existing report, create a new one with detailed steps to reproduce the issue.
Engaging in community forums: Discuss the issue with other Docker users and share your workarounds and solutions.

By working together, we can help make Docker Compose even better and ensure that secrets are handled securely and correctly. Let's keep the conversation going and push for a proper fix to this issue. Keep an eye on the Docker Compose release notes for updates, and in the meantime, use the workarounds we discussed to keep your applications running smoothly and securely. Stay tuned for more updates, and happy Dockering! 🐳