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Proof of Concept - Docker Image Secret Extraction

This repository explores why "secrets" should not be stored in Docker images. This is a feature of Docker, rather than an exploit or vulnerability, but a regularly overlooked security risk that can be used to bypass other authentication that may have been configured in your application.

Alongside this theoretical exploration, this repository contains proof of concept code that can be used to extract "secret" files from a Docker image in varying ways.

Introduction

After reading that "best practice" suggests that one should “Never put any secret or credentials in the Dockerfile instructions (environment variables, args, or hard coded into any command)” (Álvaro Iradier, 2021)1 and seeing how often this practice is ignored, I questioned how easy it would be to extract secrets from a Docker image.

Example Secret Extraction using Example Image

The Problem

One way that secrets tend to be stored into a Docker image is through copying sensitive files into the image, whether by mistake, due to naivety, or for "ease of use". Fundamentally, this issue is described by CWE-798: Use of Hard-coded Credentials in that “Hard-coded credentials typically create a significant hole that allows an attacker to bypass the authentication that has been configured” (Mitre.org, 2020)2.

Secrets could be stored into a Docker image by copying your .env file into the image, or by copying a private key, or other similar credentials, into the image - but the steps to reverse this process and extract the secrets copied into the image are simple.

The Impact

Research by RWTH Aachen University in Germany, analysing 337,171 images and 1,647,300 layers, found that 8.5% images analysed contained valid secrets. (Dahlmanns et al., 2023)3.

Some high-profile security events that can be attributed to the exploitation of secrets stored in Docker images include the Codecov breach in January 2021 and the case study of Google Cloud's Apigee.

In the case of Codecov, credentials exported from a Docker image were used to launch a man-in-the-middle attack on Codecov's infrastructure and further exfiltrate the environment variables of some of Codecov's 23,000 customers (Jackson, 2021)4 - and, consequentially, “Large organizations such as Twilio, Hashicorp, Rapid7, Confluent have released their own statements about how this has affected them.” (Jackson, 2021)4

Similarly, in the case of Apigee it was found that “Aside from a number of other bugs found by analysing the source code (SSRFs, path traversals, authorization bypasses and more), the image included several hard-coded passwords, which turned out to be re-used in production.” (August, 2023)‌‌‌5 As a result of these findings, the researcher was able to gain access to Google's own Apigee instance.

Why It Matters

The security concept of Least Privilege asserts that “a user or entity should only have access to the specific data, resources and applications needed to complete a required task.” (Palo Alto Networks, 2015)6 which, consequentially, helps to reduce attack surface and attack spread. By storing secrets in Docker images, it becomes possible to bypass other security procedures without the need for them to be compromised, nor for the attacker to escalate their privileges.

Alongside this, a pillar of zero-trust security is the assumption that “there are attackers both within and outside of the network, so no users or machines should be automatically trusted.” (Cloudflare.com, 2021)7

By leaving such such a security hole in your application, any RBAC or other procedures that you have in place to protect your application are bypassed and Docker repository access could be utilised to compromise production systems and infrastructure.

My Docker Repository is Private, So It's Safe, Right?

In the case of Apigee, this was accidentally not the case - "a Docker registry was available on docker.apigee.net, which allowed unauthenticated users to pull the Apigee Edge Docker image" (August, 2023)‌‌‌5. This is the kind of small oversight that can have large consequences. It is one issue to accidentally expose the source code of your application - whereas exposing the critical secrets of the application alongside it gives an attacker both the lock and the key and potentially full control for whatever intent they might have.

Regardless of the privacy of the Docker Repository, accidental public exposure of the Docker repository is not the only way that a Docker image, and thus the secrets within it, may be accessed or exposed: (August, 2023)‌‌‌5

  • Images are pulled to developer’s machines, stored and forgotten. These machines can be hacked, lost and stolen, kept after employee quits.
  • If a single application running an image from a private registry is compromised, all the images in the registry may leak as the system running the application often has pull privileges on the registry.

To apply the principles of zero-trust security, one should assume that the system, in this case the Docker Repository, has already been compromised and thus take steps to ensure that there is the least amount of impact possible if such a compromise were to occur.

What Can Be Done?

If you need to store secrets in your Docker image, it is recommended that you use a secret management system to store and retrieve these secrets. This allows you to keep your Docker image free of sensitive information and allows you to rotate secrets without needing to rebuild your Docker image.

“Ideally, secrets should be stored in a cryptographic secret vault and loaded when needed" (August, 2023)‌‌‌5 and such a solution can be implemented using tools such as HashiCorp Vault, AWS Secrets Manager, Google Cloud Secret Manager, or Azure Key Vault.

As an additional preventative step, “you can prevent files from accidentally winding up on your image by adding them to the .dockerignore file. For example, if you’re worried about accidentally adding SSH keys to your image, you can include: *id_rsa*” (Cotton, 2023)8

Related Issues

Best Practices and Resources

Note: Please feel free to submit a PR to add to this list of resources!

Usage

Alongside this exploration, this repository contains some simple scripts that demonstrate the extraction of secrets and values from a Docker image.

# Find "SECRET_KEY" in the image without running the image
$ ./extract.sh <image> "SECRET_KEY"
# ...
Found something_that_should_not_be_found in:
grep: images/<image>/blobs/sha256/...

Credits and Acknowledgements

CC-BY-4.0 © 2024 Dom Webber

References

Footnotes

  1. Álvaro Iradier (2021). Top 20 Dockerfile best practices. [online] Sysdig. Available at: https://sysdig.com/blog/dockerfile-best-practices/ [Accessed 28 Mar. 2024]. ↩

  2. Mitre.org. (2020). CWE - CWE-798: Use of Hard-coded Credentials (4.14). [online] Available at: https://cwe.mitre.org/data/definitions/798.html [Accessed 31 Mar. 2024]. ↩

  3. Dahlmanns, M., Sander, C., Decker, R. and Wehrle, K. (2023). Secrets Revealed in Container Images: An Internet-wide Study on Occurrence and Impact. Proceedings of the ACM Asia Conference on Computer and Communications Security. [online] doi:https://doi.org/10.1145/3579856.3590329. ↩

  4. Jackson, M. (2021). Codecov supply chain attack breakdown. [online] GitGuardian Blog - Code Security for the DevOps generation. Available at: https://blog.gitguardian.com/codecov-supply-chain-breach/ [Accessed 31 Mar. 2024]. ↩ ↩2

  5. August, C. (2023). Why you shouldn’t include secrets in Docker images, a Google Cloud case study. [online] Binary Security AS. Available at: https://www.binarysecurity.no/posts/2023/04/secrets-in-docker-images [Accessed 31 Mar. 2024]. ↩ ↩2 ↩3 ↩4

  6. Palo Alto Networks. (2015). What Is the Principle of Least Privilege? [online] Available at: https://www.paloaltonetworks.com/cyberpedia/what-is-the-principle-of-least-privilege [Accessed 31 Mar. 2024]. ↩

  7. Cloudflare.com. (2021). Zero Trust security | What is a Zero Trust network? [online] Available at: https://www.cloudflare.com/en-gb/learning/security/glossary/what-is-zero-trust/ [Accessed 31 Mar. 2024]. ↩

  8. Cotton, B. (2023). Protecting Secrets with Docker | Docker. [online] Docker. Available at: https://www.docker.com/blog/protecting-secrets-with-docker/ [Accessed 31 Mar. 2024]. ↩