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Clean build-time secrets, avoid secrets as args



One Paragraph Explainer

A Docker image isn't just a bunch of files but rather multiple layers revealing what happened during build-time. In a very common scenario, developers need the npm token during build time (mostly for private registries) - this is falsely achieved by passing the token as a build time args. It might seem innocent and safe, however this token can now be fetched from the developer's machine Docker history, from the Docker registry and the CI. An attacker who gets access to that token is now capable of writing into the organization private npm registry. There are two more secured alternatives: The flawless one is using Docker --secret feature (experimental as of July 2020) which allows mounting a file during build time only. The second approach is using multi-stage build with args, building and then copying only the necessary files to production. The last technique will not ship the secrets with the images but will appear in the local Docker history - This is typically considered as secured enough for most organizations.



Code Example – Using Docker mounted secrets (experimental but stable)

Dockerfile 
# syntax = docker/dockerfile:1.0-experimental

FROM node:12-slim

WORKDIR /usr/src/app
COPY package.json package-lock.json ./
RUN --mount=type=secret,id=npm,target=/root/.npmrc npm ci

# The rest comes here



Code Example – Building securely using multi-stage build

Dockerfile 
FROM node:12-slim AS build

ARG NPM_TOKEN

WORKDIR /usr/src/app
COPY . /dist

RUN echo "//registry.npmjs.org/:\_authToken=\$NPM_TOKEN" > .npmrc && \
 npm ci --production && \
 rm -f .npmrc


FROM build as prod

COPY --from=build /dist /dist
CMD ["node", "index.js"]

# The ARG and .npmrc won't appear in the final image but can be found in the Docker daemon un-tagged images list - make sure to delete those



Code Example Anti Pattern – Using build time args

Dockerfile 
FROM node:12-slim

ARG NPM_TOKEN

WORKDIR /usr/src/app
COPY . /dist

RUN echo "//registry.npmjs.org/:\_authToken=\$NPM_TOKEN" > .npmrc && \
 npm ci --production && \
 rm -f .npmrc

# Deleting the .npmrc within the same copy command will not save it inside the layer, however it can be found in image history

CMD ["node", "index.js"]



Blog Quote: "These secrets aren’t saved in the final Docker"

From the blog, Alexandra Ulsh

In November 2018 Docker 18.09 introduced a new --secret flag for docker build. This allows us to pass secrets from a file to our Docker builds. These secrets aren’t saved in the final Docker image, any intermediate images, or the image commit history. With build secrets, you can now securely build Docker images with private npm packages without build arguments and multi-stage builds.