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Exploiting and Securing GitLab: Lessons from a TryHackMe Lab

Perimeter security isn’t enough—because sometimes the threat is already inside.

In this blog post, I’m sharing what I learned from a hands-on TryHackMe lab on GitLab security. It revealed how a simple internal misconfiguration—like open registration or overly permissive repo access—can lead to major data exposure inside an organization. I’ll walk you through the red team perspective on exploiting a misconfigured GitLab instance, and then flip the script to explain how you can secure your own internal build systems.

Scenario: Inside the Walls of a Large Organization

Think of a large organization—like a bank—with thousands of employees and multiple teams handling development, IT operations, and security. To keep intellectual property (IP) secure, these organizations often host self-managed GitLab instances on their internal network.

But here’s where things can go wrong:

  • GitLab is hosted internally
  • Allows anyone on the internal network to register
  • Has some projects set to public visibility

At first glance, this doesn’t sound too dangerous. But imagine:

  • 10,000 employees on the internal network
  • 2,000 developers needing GitLab access
  • If any one of those users is compromised, the attacker can register a GitLab account and view exposed projects

Suddenly, your attack surface grows exponentially—and your private codebase is at risk.

Exploiting a Vulnerable GitLab Instance

As a red teamer in this simulated environment, the first step was registering a GitLab account on the internal server (http://gitlab.tryhackme.loc/). Once logged in, I had visibility into all public repositories.

Instead of browsing them manually, I used a Python script that leverages the GitLab API to automatically enumerate and download every accessible repo.

Here’s the key part of the script:

  • Connects to the GitLab API using a personal access token
  • Lists all visible projects
  • Tries to download each project archive
  • Saves them locally with a unique ID

This script is not stealthy—it attempts to pull everything it can, making it obvious in logs. But in a red team or CTF context, it’s great for proof of concept.

GitLab API Token Setup

To use the script, I generated a GitLab Personal Access Token with read_repository scope:

  1. Go to your GitLab profile → Preferences
  2. Navigate to Access Tokens
  3. Set a name, expiry, and select scopes (e.g., read_repository)
  4. Generate and copy the token (it won’t be shown again)

Security Misconfigurations Exploited

  1. Open User Registration
    Anyone on the internal network could create a GitLab account. No restriction = expanded attack surface.
  2. Public Repositories Within the Internal Network
    Developers assumed “internal” meant “safe to share publicly.” Public visibility on GitLab meant any valid user could view the code.

Final Flag

How to Secure Your GitLab Environment

Group-Based Access Control

Organize projects into GitLab groups and set permissions at the group level. This simplifies permission management across multiple repositories and ensures consistent security policies.

Access Levels

GitLab roles include:

  • Guest – View limited info
  • Reporter – Read-only access to code and CI logs
  • Developer – Push code and trigger pipelines
  • Maintainer – Merge changes and manage pipelines
  • Owner – Full administrative access

Always follow the principle of least privilege.

Sensitive Information Protection

  • .gitignore: Prevent sensitive files like .env or config.yml from being tracked in version control.
  • Environment Variables: Store secrets like API keys securely during CI/CD runs without exposing them in source code.
  • Branch Protection: Lock down critical branches (like main) so that changes must pass code review and automated testing.

Additional Best Practices

  • Enable 2FA for all GitLab users.
  • Review access permissions regularly, especially when users change roles or leave.
  • Monitor audit logs to detect unauthorized access or suspicious activity.
  • Use secret scanning tools to detect accidental commits of credentials or API keys.
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