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:
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.
References
vvoland Finder
rumpl Reporter
gabriellavengeo Remediation reviewer
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
HEALTHCHECKandONBUILD) 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:
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
HEALTCHECKcommand.Impact
23.0+ users are only affected if they explicitly opted out of Buildkit (
DOCKER_BUILDKIT=0environment variable) or are using the/buildAPI 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) andImageBuildfunction fromgithub.com/docker/docker/clientis also affected as it the uses classic builder by default.Patches
Patches are included in Moby releases:
Workarounds
--no-cacheor use Buildkit if possible (DOCKER_BUILDKIT=1, it's default on 23.0+ assuming that the buildx plugin is installed).Version = types.BuilderBuildKitorNoCache = trueinImageBuildOptionsforImageBuildcall.References