Started off by complaining about an enterprise browser security platform blocking my browsing... and ended up discovering something unexpected: its debug console could be brute-forced to bypass certain security controls.

That curiosity pulled me into browser extension security best practices and made me realize beginners need a simple, hands-on way to learn this space.

So I built Damn Vulnerable Chrome Extension (DVCE) in a few hours, shoutout to Codex, as a deliberately vulnerable extension to practice identifying and fixing real-world security issues.

Sure, tools can generate code and even spot bugs, but understanding the flaws is where the real learning happens.

Project repository

Open Damn Vulnerable Chrome Extension on GitHub

What DVCE is

DVCE is a local-only training lab for teaching common Chrome extension vulnerability patterns. It is intentionally insecure so learners can understand how extension vulnerabilities work and how to fix them.

What it teaches

  • Unsafe window.postMessage bridges from web pages into extension logic.
  • Cross-origin iframe abuse when event.source and event.origin are not validated.
  • DOM injection from content scripts using unsafe sinks like innerHTML.
  • How web_accessible_resources can expose packaged extension data.
  • Why excessive permissions increase blast radius when another bug exists.
  • Insecure storage of secrets in extension storage.
  • Remote configuration becoming a command channel.
  • HTML injection in privileged extension pages.
  • CSP, supply-chain, update, and privacy risks around browser extensions.

How the lab works

The lab models a pattern that appears in real extensions: a local web page sends messages, a content script bridges them into the extension, and a service worker performs privileged actions such as tabs, scripting, storage, or configuration behavior.

DVCE has two modes: vulnerable mode, where unsafe behavior is intentionally allowed, and fixed mode, where basic checks demonstrate mitigation patterns such as sender validation and command allowlisting.

Why I built it

Browser extensions sit in a strange and powerful place: they touch web pages, extension pages, storage, permissions, and sometimes enterprise policy. For a beginner, that can feel abstract until they can see one small flaw become a privileged action.

DVCE makes that learning loop concrete: trigger the bug, observe the impact, switch to fixed mode, and understand which control changed the outcome.