Classic builder cache poisoning

The classic builder cache system is prone to cache poisoning if the image is built FROM scratch. Also, changes to some instructions (most important being HEALTHCHECK and ONBUILD) would not cause a cache miss.

An attacker with the knowledge of the Dockerfile someone is using could poison their cache by making them pull a specially crafted image that would be considered as a valid cache candidate for some build steps.

For example, an attacker could create an image that is considered as a valid cache candidate for:

FROM scratch
MAINTAINER Pawel

when in fact the malicious image used as a cache would be an image built from a different Dockerfile.

In the second case, the attacker could for example substitute a different HEALTCHECK command.

Impact

23.0+ users are only affected if they explicitly opted out of Buildkit (DOCKER_BUILDKIT=0 environment variable) or are using the /build API endpoint (which uses the classic builder by default).

All users on versions older than 23.0 could be impacted. An example could be a CI with a shared cache, or just a regular Docker user pulling a malicious image due to misspelling/typosquatting.

Image build API endpoint (/build) and ImageBuild function from github.com/docker/docker/client is also affected as it the uses classic builder by default.

Patches

Patches are included in Moby releases:

• v25.0.2
• v24.0.9
• v23.0.10

Workarounds

• Use --no-cache or use Buildkit if possible (DOCKER_BUILDKIT=1, it's default on 23.0+ assuming that the buildx plugin is installed).
• Use Version = types.BuilderBuildKit or NoCache = true in ImageBuildOptions for ImageBuild call.