ARG to handle secrets can lead to sensitive information being disclosed to an inappropriate sphere.
ENV instructions in a Dockerfile are used to configure the image build and the container environment
respectively. Both can be used at image build time, during the execution of commands in the container, and
ENV can also be used at
In most cases, build-time and environment variables are used to propagate configuration items from the host to the image or container. A typical
example for an environmental variable is the
PATH variable, used to configure where system executables are searched for.
ENV to propagate configuration entries that contain secrets causes a security risk. Indeed, in most cases,
artifacts of those values are kept in the final image. The secret information leak can happen either in the container environment itself, the image
metadata or the build environment logs.
The concrete impact of such an issue highly depends on the secret’s purpose and the exposure sphere:
- Financial impact if a paid service API key is disclosed and used.
- Application compromise if an application’s secret, like a session signing key, is disclosed.
- Infrastructure component takeover, if a system secret, like a remote access key, is leaked.
Ask Yourself Whether
- The variable contains a value that should be kept confidential.
- The container image or Dockerfile will be distributed to users who do not need to know the secret value.
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
- Use Buildkit’s secret mount options when secrets have to be used at build time.
- For run time secret variables, best practices would recommend only setting them at runtime, for example with the
--env option of
docker run command.
Note that, in both cases, the files exposing the secrets should be securely stored and not exposed to a large sphere. In most cases, using a secret
vault or another similar component should be preferred. For example, Docker Swarm provides a secrets service that
can be used to handle most confidential data.
Sensitive Code Example
For build time secrets, use Buildkit’s secret mount type
RUN --mount=type=secret,id=build_secret ./installer.sh
For runtime secrets, leave the environment variables empty until runtime:
Store the runtime secrets in an environment file (such as
.env) and then start
the container with the
docker run --env-file .env myImage