Dearflip PDF Viewer: Secure Alternatives To 'unsafe-eval'

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Dearflip PDF Viewer: Secure Alternatives to 'unsafe-eval'

Hey guys, let's dive into a crucial security topic concerning the dearflip flip-book PDF viewer and the dreaded 'unsafe-eval' Content Security Policy (CSP) directive. In this article, we'll explore why 'unsafe-eval' is currently needed, the security implications, and most importantly, how we can ditch it for a more secure approach. This is super important because it directly impacts the security of our web applications. Let's get started!

The Problem: 'unsafe-eval' and the Dearflip Dilemma

The Current Situation: A Security Trade-off

Currently, the dearflip PDF viewer, specifically version 1.7.3.5 with the bundled PDF.js version 2.3.200, relies on the 'unsafe-eval' directive to function correctly. Without it, you'll be stuck staring at that frustrating "Loading PDF Worker..." spinner. But, and this is a BIG but, using 'unsafe-eval' weakens our CSP, which is designed to protect us from Cross-Site Scripting (XSS) attacks. Think of CSP as a gatekeeper that controls what scripts can run on your website. 'unsafe-eval' basically tells the gatekeeper, "Hey, let anything run!" Not ideal, right? This creates a trade-off: functional PDF viewer or robust security. And, as developers, we always strive for both!

With 'unsafe-eval': The dearflip PDF viewer works flawlessly, showing all the pages of your PDF ✅. But the CSP security is weakened ⚠️.

Without 'unsafe-eval': CSP is stricter and more secure ✅, but the dearflip viewer gets stuck on the loading screen, unable to display the PDF ❌. Not cool!

Technical Deep Dive: Why 'unsafe-eval' is Needed

So, why is 'unsafe-eval' necessary in the first place? The root of the problem lies in how PDF.js, the engine behind dearflip, handles font rendering optimizations. PDF.js uses eval() and the new Function() constructor internally for these optimizations. These methods dynamically execute JavaScript code, which is exactly what 'unsafe-eval' allows. When our CSP blocks eval(), PDF.js fails to initialize its web worker correctly. The web worker is crucial for handling the heavy lifting of PDF rendering, so without it, we get the "Loading PDF Worker..." error and a non-functional viewer. It's like trying to bake a cake without an oven – you're just not going to get the desired result.

Security Implications of 'unsafe-eval'

The 'unsafe-eval' directive is a security risk. By allowing eval(), new Function(), and even the use of string arguments in setTimeout() / setInterval(), we open the door for potential XSS vulnerabilities. If an attacker can inject malicious JavaScript code into your website, they can use these methods to execute that code. This could lead to anything from stealing user data to defacing your site. In a nutshell, using 'unsafe-eval' is like leaving the front door unlocked. It may be convenient, but it's definitely not secure. Attackers could potentially inject arbitrary JavaScript if they find an injection point. We must find another solution.

Current Decision and the Road Ahead

The Current State

For the initial integration of dearflip (PR #186), we had to keep 'unsafe-eval' to ensure the PDF viewer worked. This was a pragmatic choice to get the feature up and running. However, this is just a temporary solution. We created this issue to track the follow-up work to find a more secure solution and remove this potentially dangerous directive. We need to prioritize security, always!

Acceptance Criteria: The Goals

Our ultimate goals, our acceptance criteria, are clear:

  • [ ] Dearflip PDF viewer must work flawlessly, without the "Loading PDF Worker..." error, which means users can actually view their PDFs.
  • [ ] 'unsafe-eval' MUST be removed from the CSP header. No exceptions!
  • [ ] There must be NO console errors related to CSP violations. Everything must be squeaky clean in the console.
  • [ ] PDF rendering performance must remain acceptable, no matter what solution we apply. We don't want a slow PDF viewer.

Potential Solutions and Future Investigations

Now, let's explore possible solutions to get rid of 'unsafe-eval' and make our website more secure. Some approaches we could consider include:

1. Using eval() Alternatives

Instead of directly using eval(), we could explore alternative methods for achieving the same font rendering optimizations. Libraries and techniques that avoid dynamic code execution could be a great starting point. Several libraries provide similar functionalities, which might not need unsafe-eval.

2. CSP Nonce or Hash

Another approach is to use a CSP nonce or hash. This would involve generating a unique nonce (a random string) for each request and including it in the <script> tags that PDF.js uses. The CSP header would then include the script-src 'nonce-{nonce}' directive. While this is an improvement, it can be tricky to manage and might not be a perfect fit for the dynamic nature of PDF.js.

3. Web Workers and Inline Scripts (Carefully)

We could investigate how PDF.js uses web workers and explore options to load the web worker script using a more CSP-friendly approach. This might involve carefully managing inline scripts or using a separate, CSP-compliant script. This needs to be checked carefully because using inline scripts can be a huge security problem.

4. Updating PDF.js

We also need to see if we're using the latest PDF.js version and if any updates have addressed this issue. Newer versions may have improved their handling of eval() or offered alternative optimization techniques that don't require 'unsafe-eval'. It's possible that a newer version of PDF.js might have already addressed the issue, or at least provided better workarounds.

5. Code Review and Refactoring

A thorough code review of the dearflip integration and PDF.js usage is essential. Identifying exactly where eval() and new Function() are being used will help us understand the scope of the problem and explore alternative solutions. We should see if refactoring any parts of the code is needed. This will help with a good understanding of what part of the code causes unsafe-eval to be needed.

Conclusion: Prioritizing Security

Removing 'unsafe-eval' from our CSP is vital for the security of our web applications. It requires careful investigation and testing of alternative approaches, such as using eval() alternatives, nonce or hash, or even web worker changes. By removing it, we're taking a significant step towards mitigating XSS risks and creating a more secure user experience. Remember, security is an ongoing process, not a one-time fix. Let's keep working to make our web applications as safe as possible!

Related Resources

  • PR #186: Integrate dearflip flip-book PDF viewer (where this whole thing started!)
  • Issue #183: Original PDF viewer feature request (the inspiration!)

Labels

These labels are applied to this project: security, enhancement, technical-debt.