mitigations.md

Mitigation Strategies Analysis for mozilla/pdf.js

Mitigation Strategy: Keep pdf.js Updated

• Description:

  1. Identify Current Version: Determine the currently used version of pdf.js in your project. This is usually found in your project's package.json file (if using npm/yarn) or a similar dependency management file.
  2. Establish Update Mechanism: Implement an automated update process. For npm/yarn projects, this typically involves:
     • Using npm update pdfjs-dist (or the equivalent yarn command) regularly.
     • Integrating a dependency management tool like Dependabot or Snyk. These tools automatically scan your dependencies for updates and known vulnerabilities, creating pull requests to update them.
  3. Monitor for Releases: Subscribe to the pdf.js GitHub repository's release notifications (or use the dependency management tools mentioned above). This ensures you're immediately aware of new versions, especially security releases.
  4. Testing After Update: After updating, thoroughly test your application's PDF rendering functionality to ensure no regressions were introduced by the update. Automated testing is highly recommended.
  5. Rollback Plan: Have a clear plan to revert to the previous version of pdf.js if a critical issue arises after an update.

• Threats Mitigated:

  • Remote Code Execution (RCE) (Critical): Exploits targeting known vulnerabilities in older pdf.js versions can allow attackers to execute arbitrary code on the user's machine.
  • Cross-Site Scripting (XSS) (High): Vulnerabilities can allow attackers to inject malicious scripts into the context of your web application, potentially stealing user data or performing actions on their behalf.
  • Denial of Service (DoS) (Medium): Some vulnerabilities can cause pdf.js to crash or consume excessive resources, making the application unavailable.
  • Information Disclosure (Low-Medium): Vulnerabilities might allow attackers to extract sensitive information from the PDF or the user's environment.

• Impact:

  • RCE: Risk reduction: Very High (mitigates the most severe threat).
  • XSS: Risk reduction: High.
  • DoS: Risk reduction: Medium.
  • Information Disclosure: Risk reduction: Medium.

• Currently Implemented:

  • Example: Partially Implemented. package.json specifies pdfjs-dist, but automated updates (Dependabot/Snyk) are not yet configured. Manual updates are performed sporadically.

• Missing Implementation:

  • Example: Automated update mechanism (Dependabot/Snyk) is missing. Regular, scheduled testing after updates is not formalized. A rollback plan is not documented.

Mitigation Strategy: Disable Potentially Dangerous Features

• Description:

  1. Review Configuration Options: Examine the pdf.js API documentation (specifically the getDocument and related options) to understand all available configuration settings.
  2. Identify Unnecessary Features: Determine which features are not essential for your application's core functionality. Prioritize disabling:
     • enableXfa: Set to false unless absolutely required for rendering specific PDF forms.
     • disableAutoFetch: Set to true to prevent automatic fetching of external resources.
     • disableFontFace: Set to true if custom font rendering is not needed.
     • isEvalSupported: Set to false if possible (requires thorough testing).
     • disableRange: Set to true to prevent partial content requests.
     • disableStream: Set to true to disable streaming.
  3. Implement Configuration Changes: Modify your application's code to explicitly set these options when initializing pdf.js. For example:

     pdfjsLib.getDocument({
         url: pdfUrl,
         enableXfa: false,
         disableAutoFetch: true,
         disableFontFace: true,
         isEvalSupported: false, // If possible
         disableRange: true,
         disableStream: true
     }).promise.then(function(pdf) {
         // ... your rendering code ...
     });

  4. Thorough Testing: After making configuration changes, rigorously test your application with a variety of PDF files to ensure that the necessary functionality is still working correctly and that no unexpected rendering issues occur.

• Threats Mitigated:

  • RCE (Critical): Disabling features like XFA and eval() reduces the attack surface for code execution vulnerabilities.
  • XSS (High): Limiting external resource fetching reduces the risk of injecting malicious scripts.
  • DoS (Medium): Disabling features like range requests and streaming can mitigate some DoS attacks.
  • Information Disclosure (Low-Medium): Restricting external resource access reduces the potential for leaking information.

• Impact:

  • RCE: Risk reduction: Medium-High.
  • XSS: Risk reduction: Medium.
  • DoS: Risk reduction: Low-Medium.
  • Information Disclosure: Risk reduction: Low.

• Currently Implemented:

  • Example: Partially Implemented. disableAutoFetch is set to true. Other options are at their default values.

• Missing Implementation:

  • Example: enableXfa, disableFontFace, isEvalSupported, disableRange, and disableStream are not explicitly configured. A comprehensive review of configuration options and their security implications has not been performed.

Mitigation Strategy: Sandboxing (Web Workers)

• Description:

  1. Create a Web Worker: Create a separate JavaScript file (e.g., pdf-worker.js) that will contain the pdf.js code.
  2. Load pdf.js in the Worker: Inside pdf-worker.js, import the pdf.js library.
  3. Message Passing: Establish a communication channel between your main application thread and the Web Worker using postMessage and onmessage event listeners. The main thread sends the PDF data (or URL) to the worker, and the worker sends back the rendered data (e.g., page images, text content).
  4. Isolate Rendering: Ensure that all PDF parsing and rendering logic is executed within the Web Worker. The main thread should only handle displaying the results received from the worker.
  5. Error Handling: Implement robust error handling to catch and handle any errors that occur within the worker. Communicate these errors back to the main thread appropriately.

  Example:

  Main Thread (main.js):

  const worker = new Worker('pdf-worker.js');
  
  worker.onmessage = function(event) {
      if (event.data.error) {
          console.error("Worker Error:", event.data.error);
      } else {
          // Process rendered data (e.g., display image)
          console.log("Rendered Data:", event.data);
      }
  };
  
  // Load PDF data (e.g., from a file input)
  const pdfData = ...;
  
  worker.postMessage({ pdfData: pdfData });

  Worker Thread (pdf-worker.js):

  importScripts('pdf.js'); // Or the appropriate path
  
  onmessage = function(event) {
      try {
          pdfjsLib.getDocument(event.data.pdfData).promise.then(function(pdf) {
              // ... Render PDF pages ...
              // Send rendered data back to main thread
              postMessage({ pageData: ... });
          });
      } catch (error) {
          postMessage({ error: error.message });
      }
  };

• Threats Mitigated:

  • RCE (Critical): Contains the impact of RCE to the worker's context, preventing direct access to the main thread's DOM and resources.
  • XSS (High): Significantly reduces the risk of XSS by isolating the vulnerable code.
  • DoS (Medium): Can help mitigate DoS by allowing the main thread to remain responsive even if the worker crashes.
  • Information Disclosure (Low-Medium): Limits the scope of potential information leaks.

• Impact:

  • RCE: Risk reduction: Very High.
  • XSS: Risk reduction: High.
  • DoS: Risk reduction: Medium.
  • Information Disclosure: Risk reduction: Medium.

• Currently Implemented:

  • Example: Not Implemented. pdf.js is loaded and executed directly in the main thread.

• Missing Implementation:

  • Example: The entire Web Worker implementation is missing. The application needs to be refactored to move PDF processing to a separate worker thread.