Docker engine does the heavy lifting of running and managing Containers. Docker engine uses Linux kernel features like **Namespaces** and **Cgroups** to provide basic **isolation** across Containers. Advanced isolation can be achieved using Linux kernel features like **Capabilities**, **Seccomp**, **SELinux/AppArmor**. Docker exposes these Linux kernel capabilities either at Docker daemon level or at each Container level.
Finally, an **auth plugin** can be used to **limit the actions** users can perform.\
Docker client can access Docker engine **locally using Unix socket or remotely using http** mechanism. To use it remotely, it is needed to use https and **TLS** so that confidentiality, integrity and authentication can be ensured.
By default listens on the Unix socket `unix:///var/`\
`run/docker.sock` and in Ubuntu distributions, Docker start options are specified in `/etc/default/docker`. To allow Docker API and client to access Docker engine remotely, we need to **expose Docker daemon using http socket**. This can be done by:
```bash
DOCKER_OPTS="-D -H unix:///var/run/docker.sock -H
tcp://192.168.56.101:2376" -> add this to /etc/default/docker
Sudo service docker restart -> Restart Docker daemon
```
Exposing Docker daemon using http is not a good practice and it is needed to secure the connection using https. There are two options: first option is for **client to verify server identity** and in second option **both client and server verify each other’s identity**. Certificates establish the identity of a server. For an example of both options [**check this page**](https://sreeninet.wordpress.com/2016/03/06/docker-security-part-3engine-access/).
### **Container image security**
Container images are stored either in private repository or public repository. Following are the options that Docker provides for storing Container images:
* [Docker hub](https://hub.docker.com) – This is a public registry service provided by Docker
* [Docker registry](https://github.com/%20docker/distribution) – This is an open source project that users can use to host their own registry.
* [Docker trusted registry](https://www.docker.com/docker-trusted-registry) – This is Docker’s commercial implementation of Docker registry and it provides role based user authentication along with LDAP directory service integration.
Containers can have **security vulnerabilities** either because of the base image or because of the software installed on top of the base image. Docker is working on a project called **Nautilus** that does security scan of Containers and lists the vulnerabilities. Nautilus works by comparing the each Container image layer with vulnerability repository to identify security holes.
The `docker scan` command allows you to scan existing Docker images using the image name or ID. For example, run the following command to scan the hello-world image:
```
docker scan hello-world
Testing hello-world...
Organization: docker-desktop-test
Package manager: linux
Project name: docker-image|hello-world
Docker image: hello-world
Licenses: enabled
✓ Tested 0 dependencies for known issues, no vulnerable paths found.
Note that we do not currently have vulnerability data for your image.
```
### Docker Image Signing
Docker Container images can be stored either in public or private registry. It is needed to **sign****Container** images to be able to confirm images haven't being tampered. Content **publisher** takes care of **signing** Container image and pushing it into the registry.\
Following are some details on Docker content trust:
* The Docker content trust is an implementation of the [Notary open source project](https://github.com/docker/notary). The Notary open source project is based on [The Update Framework (TUF) project](https://theupdateframework.github.io).
* Docker content **trust is enabled** with `export DOCKER_CONTENT_TRUST=1`. As of Docker version 1.10, content trust is **not enabled by default**.
* **When** content trust is **enabled**, we can **pull only signed images**. When image is pushed, we need to enter tagging key.
* When the publisher **pushes** the image for the **first****time** using docker push, there is a need to enter a **passphrase** for the **root key and tagging key**. Other keys are generated automatically.
* Docker has also added support for hardware keys using Yubikey and details are available [here](https://blog.docker.com/2015/11/docker-content-trust-yubikey/).
Following is the **error** we get when **content trust is enabled and image is not signed**.
```
$ docker pull smakam/mybusybox
Using default tag: latest
No trust data for latest
```
Following output shows Container **image being pushed to Docker hub with signing** enabled. Since this is not the first time, user is requested to enter only the passphrase for repository key.
```
$ docker push smakam/mybusybox:v2
The push refers to a repository [docker.io/smakam/mybusybox]
**Namespaces** are a feature of the Linux kernel that **partitions kernel resources** such that one set of **processes****sees** one set of **resources** while **another** set of **processes** sees a **different** set of resources. The feature works by having the same namespace for a set of resources and processes, but those namespaces refer to distinct resources. Resources may exist in multiple spaces. 
Docker makes use of the following Linux kernel Namespaces to achieve Container isolation:
* pid namespace
* mount namespace
* network namespace
* ipc namespace
* UTS namespace
For **more information about the namespaces** check the following page:
{% content-ref url="namespaces.md" %}
[namespaces.md](namespaces.md)
{% endcontent-ref %}
### cgroups
Linux kernel feature **cgroups** provides capability to **restrict resources like cpu, memory, io, network bandwidth among** a set of processes. Docker allows to create Containers using cgroup feature which allows for resource control for the specific Container.\
Following is a Container created with user space memory limited to 500m, kernel memory limited to 50m, cpu share to 512, blkioweight to 400. CPU share is a ratio that controls Container’s CPU usage. It has a default value of 1024 and range between 0 and 1024. If three Containers have the same CPU share of 1024, each Container can take upto 33% of CPU in case of CPU resource contention. blkio-weight is a ratio that controls Container’s IO. It has a default value of 500 and range between 10 and 1000.
Capabilities allow **finer control for the capabilities that can be allowed** for root user. Docker uses the Linux kernel capability feature to **limit the operations that can be done inside a Container** irrespective of the type of user.
[SELinux](https://www.redhat.com/en/blog/latest-container-exploit-runc-can-be-blocked-selinux) is a **labeling****system**. Every **process** and every **file** system object has a **label**. SELinux policies define rules about what a **process label is allowed to do with all of the other labels** on the system.
Container engines launch **container processes with a single confined SELinux label**, usually `container_t`, and then set the container inside of the container to be labeled `container_file_t`. The SELinux policy rules basically say that the **`container_t` processes can only read/write/execute files labeled `container_file_t`**.
An authorization plugin **approves** or **denies****requests** to the Docker **daemon** based on both the current **authentication** context and the **command****context**. The **authentication****context** contains all **user details** and the **authentication****method**. The **command context** contains all the **relevant****request** data.
If you are running a container where an attacker manages to get access as a low privilege user. If you have a **miss-configured suid binary**, the attacker may abuse it and **escalate privileges inside** the container. Which, may allow him to escape from it.
Running the container with the **`no-new-privileges`** option enabled will **prevent this kind of privilege escalation**.
```
docker run -it --security-opt=no-new-privileges:true nonewpriv
For more **`--security-opt`** options check: [https://docs.docker.com/engine/reference/run/#security-configuration](https://docs.docker.com/engine/reference/run/#security-configuration)
First of all, **do not put them inside your image!**
Also, **don’t use environment variables** for your sensitive info, either. Anyone w**ho can run `docker inspect` or `exec` into the container can find your secret**.
Docker volumes are better. They are the recommended way to access your sensitive info in the Docker docs. You can **use a volume as temporary file system held in memory**. Volumes remove the `docker inspect` and the logging risk. However, **root users could still see the secret, as could anyone who can `exec` into the container**.
Even **better than volumes, use Docker secrets**.
If you just need the **secret in your image**, you can use **BuildKit**. BuildKit cuts build time significantly and has other nice features, including **build-time secrets support**.
There are three ways to specify the BuildKit backend so you can use its features now.:
1. Set it as an environment variable with `export DOCKER_BUILDKIT=1`.
2. Start your `build` or `run` command with `DOCKER_BUILDKIT=1`.
3. Enable BuildKit by default. Set the configuration in /_etc/docker/daemon.json_ to _true_ with: `{ "features": { "buildkit": true } }`. Then restart Docker.
4. Then you can use secrets at build time with the `--secret` flag like this:
If you need your **secret in your running container**, and not just when building your image, use **Docker Compose or Kubernetes**.
With Docker Compose, add the secrets key-value pair to a service and specify the secret file. Hat tip to [Stack Exchange answer](https://serverfault.com/a/936262/535325) for the Docker Compose secrets tip that the example below is adapted from.
Example docker-compose.yml with secrets:
```yaml
version: "3.7"
services:
my_service:
image: centos:7
entrypoint: "cat /run/secrets/my_secret"
secrets:
- my_secret
secrets:
my_secret:
file: ./my_secret_file.txt
```
Then start Compose as usual with `docker-compose up --build my_service`.
If you’re using [Kubernetes](https://kubernetes.io/docs/concepts/configuration/secret/), it has support for secrets. [Helm-Secrets](https://github.com/futuresimple/helm-secrets) can help make secrets management in K8s easier. Additionally, K8s has Role Based Access Controls (RBAC) — as does Docker Enterprise. RBAC makes access Secrets management more manageable and more secure for teams.
### gVisor
**gVisor** is an application kernel, written in Go, that implements a substantial portion of the Linux system surface. It includes an [Open Container Initiative (OCI)](https://www.opencontainers.org) runtime called `runsc` that provides an **isolation boundary between the application and the host kernel**. The `runsc` runtime integrates with Docker and Kubernetes, making it simple to run sandboxed containers.
**Kata Containers** is an open source community working to build a secure container runtime with lightweight virtual machines that feel and perform like containers, but provide **stronger workload isolation using hardware virtualization** technology as a second layer of defense.
* **Do not use the `--privileged` flag or mount a** [**Docker socket inside the container**](https://raesene.github.io/blog/2016/03/06/The-Dangers-Of-Docker.sock/)**.** The docker socket allows for spawning containers, so it is an easy way to take full control of the host, for example, by running another container with the `--privileged` flag.
* Do **not run as root inside the container. Use a** [**different user**](https://docs.docker.com/develop/develop-images/dockerfile\_best-practices/#user) **and** [**user namespaces**](https://docs.docker.com/engine/security/userns-remap/)**.** The root in the container is the same as on host unless remapped with user namespaces. It is only lightly restricted by, primarily, Linux namespaces, capabilities, and cgroups.
* [**Drop all capabilities**](https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities) **(`--cap-drop=all`) and enable only those that are required** (`--cap-add=...`). Many of workloads don’t need any capabilities and adding them increases the scope of a potential attack.
* [**Use the “no-new-privileges” security option**](https://raesene.github.io/blog/2019/06/01/docker-capabilities-and-no-new-privs/) **** to prevent processes from gaining more privileges, for example through suid binaries.
* ****[**Limit resources available to the container**](https://docs.docker.com/engine/reference/run/#runtime-constraints-on-resources)**.** Resource limits can protect the machine from denial of service attacks.
* **Adjust** [**seccomp**](https://docs.docker.com/engine/security/seccomp/)**,** [**AppArmor**](https://docs.docker.com/engine/security/apparmor/) **(or SELinux)** profiles to restrict the actions and syscalls available for the container to the minimum required.
* **Use** [**official docker images**](https://docs.docker.com/docker-hub/official\_images/) **and require signatures** or build your own based on them. Don’t inherit or use [backdoored](https://arstechnica.com/information-technology/2018/06/backdoored-images-downloaded-5-million-times-finally-removed-from-docker-hub/) images. Also store root keys, passphrase in a safe place. Docker has plans to manage keys with UCP.
* **Regularly** **rebuild** your images to **apply security patches to the host an images.**
* Manage your **secrets wisely** so it's difficult to the attacker to access them.
* If you **exposes the docker daemon use HTTPS** with client & server authentication.
* In your Dockerfile, **favor COPY instead of ADD**. ADD automatically extracts zipped files and can copy files from URLs. COPY doesn’t have these capabilities. Whenever possible, avoid using ADD so you aren’t susceptible to attacks through remote URLs and Zip files.
* Have **separate containers for each micro-s**ervice
* **Don’t put ssh** inside container, “docker exec” can be used to ssh to Container.
* Have **smaller** container **images**
## Docker Breakout / Privilege Escalation
If you are **inside a docker container** or you have access to a user in the **docker group**, you could try to **escape and escalate privileges**:
If you have access to the docker socket or have access to a user in the **docker group but your actions are being limited by a docker auth plugin**, check if you can **bypass it:**
* The tool [**docker-bench-security**](https://github.com/docker/docker-bench-security) is a script that checks for dozens of common best-practices around deploying Docker containers in production. The tests are all automated, and are based on the [CIS Docker Benchmark v1.3.1](https://www.cisecurity.org/benchmark/docker/).\
You need to run the tool from the host running docker or from a container with enough privileges. Find out **how to run it in the README:** [**https://github.com/docker/docker-bench-security**](https://github.com/docker/docker-bench-security).
