Alpine

Where Secrets Really End Up in Docker Images: Testing 8 Common Methods

A detailed analysis of eight different approaches for handling secrets during Docker image builds reveals that most methods leave sensitive data exposed in layers, configuration files, or build history. Experiments conducted on Ubuntu 24.04.1 LTS with Docker Engine 29.1.3 and BuildKit v0.26.2 showed that simply using RUN rm after COPY or overwriting files fails to remove secrets from immutable layers. Environment variables via ENV and build arguments via ARG both persist in image metadata and history. Only BuildKit Secrets with --mount=type=secret and multi-stage builds that avoid copying secrets into the final stage successfully prevent leakage. The study demonstrates that secrets can be extracted from saved images without running any containers. Developers are advised to adopt these two secure patterns to protect credentials in container supply chains.