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Mitigation Strategies Analysis for mozilla/pdf.js

Mitigation Strategy: Regularly Update pdf.js

1. Regularly Update pdf.js

  • Mitigation Strategy: Regularly Update pdf.js
  • Description:
    1. Monitor pdf.js Releases: Regularly check the mozilla/pdf.js releases page and security advisories for new versions and security updates. Subscribe to release notifications if available.
    2. Review Changelogs: When a new version is released, carefully review the changelog and release notes to understand the changes, especially security fixes relevant to pdf.js.
    3. Test Updates in a Staging Environment: Before deploying to production, update pdf.js in a staging or testing environment. Thoroughly test the application's PDF viewing functionality using pdf.js to ensure compatibility and no regressions are introduced specifically in the pdf.js integration.
    4. Deploy Updated pdf.js to Production: Once testing is successful, promptly deploy the updated pdf.js library to the production environment.
    5. Automate Updates (If Possible): Explore options for automating the update process of pdf.js, such as using dependency management tools that can notify you of updates or even automatically update pdf.js (with proper testing in place).
  • Threats Mitigated:
    • Exploitation of Known pdf.js Vulnerabilities (High Severity): Outdated versions of pdf.js may contain known security vulnerabilities that attackers can exploit specifically within the pdf.js context, such as Cross-Site Scripting (XSS) within the PDF viewer, Remote Code Execution (RCE) through PDF parsing flaws, or Denial of Service (DoS) by exploiting rendering inefficiencies in pdf.js.
  • Impact:
    • Exploitation of Known pdf.js Vulnerabilities (High Impact): Significantly reduces the risk of exploitation by patching known vulnerabilities within pdf.js.
  • Currently Implemented:
    • Yes, we have a process to check for updates monthly and update the pdf.js library in our frontend codebase. The current version is checked and updated in package.json and yarn.lock files in the frontend directory.
  • Missing Implementation:
    • Automation of the pdf.js update process is missing. Currently, it's a manual process. We could explore using dependency update tools or scripts to automate version checks and updates for pdf.js in our CI/CD pipeline.

Mitigation Strategy: Utilize Content Security Policy (CSP) - pdf.js Focused

2. Utilize Content Security Policy (CSP) - pdf.js Focused

  • Mitigation Strategy: Utilize Content Security Policy (CSP) - pdf.js Focused
  • Description:
    1. Define CSP Header: Configure your web server to send a Content-Security-Policy HTTP header with responses for pages that embed the pdf.js viewer.
    2. Restrict script-src for pdf.js Context: Set the script-src directive to strictly control the sources from which scripts can be loaded within the context of the pdf.js viewer.
      • Avoid unsafe-inline and unsafe-eval: These directives significantly weaken CSP and should be strictly avoided, especially when dealing with a complex library like pdf.js.
      • Whitelist Trusted Origins: Specify only trusted origins for script loading, such as your own domain or trusted CDNs serving pdf.js files. For example: script-src 'self' https://cdn.example.com; (if using CDN for pdf.js).
      • Use Nonces or Hashes (If Necessary for pdf.js): If inline scripts are absolutely unavoidable for pdf.js initialization or customization, use nonces or hashes to explicitly allow only those specific inline scripts.
    3. Restrict object-src for pdf.js Context: Set the object-src directive to control the sources from which objects (like plugins) can be loaded within the pdf.js viewer context.
      • Use object-src 'none' (Recommended for pdf.js): This is the most secure option and prevents loading of plugins within the pdf.js viewer, reducing potential attack vectors.
    4. Review Other Directives for pdf.js Pages: Configure other CSP directives like default-src, style-src, img-src, frame-ancestors, etc., to further restrict the capabilities of the web page hosting the pdf.js viewer and minimize the attack surface exposed to pdf.js.
    5. Test and Refine CSP for pdf.js Integration: Thoroughly test your CSP implementation specifically in pages using pdf.js. Use browser developer tools to identify and resolve CSP violations related to pdf.js. Start with a report-only policy (Content-Security-Policy-Report-Only) to test without blocking content, then switch to enforcing CSP once it's refined for pdf.js.
  • Threats Mitigated:
    • Cross-Site Scripting (XSS) in pdf.js (High Severity): CSP significantly reduces the risk and impact of XSS attacks originating from vulnerabilities within pdf.js or malicious PDFs processed by pdf.js, by limiting the ability of attackers to inject and execute malicious scripts within the pdf.js viewer.
  • Impact:
    • Cross-Site Scripting in pdf.js (High Impact): Substantially reduces the risk and impact of XSS vulnerabilities specifically related to pdf.js.
  • Currently Implemented:
    • Partially implemented. We have a basic CSP in place defined in our web server configuration (nginx.conf). It includes default-src 'self', script-src 'self' 'unsafe-inline', style-src 'self' 'unsafe-inline', img-src 'self' data:. This CSP applies to all pages, including those using pdf.js.
  • Missing Implementation:
    • CSP is not strict enough, especially for pages embedding pdf.js. unsafe-inline is used for both script-src and style-src, which weakens the CSP and increases XSS risk in the context of pdf.js. We need to remove unsafe-inline and implement nonces or hashes for inline scripts and styles if absolutely necessary for pdf.js initialization or customization. object-src is not defined and should be set to 'none' or a very restrictive list for pages using pdf.js. We also need to review and strengthen other directives like frame-ancestors and potentially add report-uri for monitoring CSP violations specifically on pages with pdf.js.

Mitigation Strategy: Implement Resource Limits and Error Handling for pdf.js

3. Implement Resource Limits and Error Handling for pdf.js

  • Mitigation Strategy: Implement Resource Limits and Error Handling for pdf.js
  • Description:
    1. Rendering Timeouts for pdf.js: Set timeouts specifically for pdf.js rendering operations. If pdf.js rendering takes longer than a defined threshold, terminate the operation to prevent excessive resource consumption by pdf.js. Configure this within your pdf.js integration code.
    2. Error Handling in pdf.js Integration: Implement robust error handling in your JavaScript code that interacts with pdf.js. Catch exceptions and errors specifically during PDF loading, parsing, and rendering performed by pdf.js.
    3. Graceful Error Display for pdf.js Issues: When errors occur during pdf.js operations, display user-friendly error messages specifically related to PDF viewing issues, instead of exposing technical details or crashing the application. Avoid displaying stack traces or sensitive information in error messages related to pdf.js errors.
    4. Logging Errors (Client-Side) for pdf.js: Log client-side errors that occur during pdf.js operation (using browser APIs like window.onerror or error tracking services) to monitor for issues and potential attacks targeting pdf.js.
  • Threats Mitigated:
    • Client-Side Denial of Service (DoS) via pdf.js (Medium Severity): Malicious PDFs designed to consume excessive CPU or memory during pdf.js processing can lead to DoS on the user's browser. Resource limits and error handling within the pdf.js integration can mitigate this.
    • Information Disclosure through pdf.js Error Messages (Low Severity): Verbose error messages from pdf.js or your integration code can sometimes reveal sensitive information. Proper error handling prevents this.
    • Application Instability due to pdf.js Errors (Medium Severity): Unhandled errors from pdf.js can lead to application crashes or unexpected behavior in the PDF viewing functionality. Robust error handling improves application stability when using pdf.js.
  • Impact:
    • Client-Side Denial of Service (Medium Impact): Reduces the risk of client-side DoS caused by malicious PDFs processed by pdf.js by preventing runaway resource consumption during pdf.js operations.
    • Information Disclosure through Error Messages (Low Impact): Prevents information leakage through error messages related to pdf.js errors.
    • Application Instability (Medium Impact): Improves application stability and user experience specifically in the PDF viewing functionality by handling pdf.js errors gracefully.
  • Currently Implemented:
    • Partially implemented. We have basic error handling in our pdf.js integration to catch loading errors and display a generic error message to the user when PDF loading fails in pdf.js.
  • Missing Implementation:
    • Rendering timeouts are not explicitly implemented for pdf.js operations. We should add timeouts to pdf.js rendering operations to prevent long-running rendering processes within pdf.js. Client-side error logging specifically focused on pdf.js errors could be improved. We should integrate a client-side error logging service to capture and monitor errors specifically during PDF viewing with pdf.js. We also need to review error messages displayed to users in case of pdf.js errors to ensure they are user-friendly and do not expose sensitive information.

Mitigation Strategy: Isolate pdf.js Execution (Iframes with Sandboxing)

4. Isolate pdf.js Execution (Iframes with Sandboxing)

  • Mitigation Strategy: Isolate pdf.js Execution (Iframes with Sandboxing)
  • Description:
    1. Create an Iframe for pdf.js Viewer: Embed the pdf.js viewer within an <iframe> element in your main application page to isolate its execution environment.
    2. Sandbox Attribute for pdf.js Iframe: Add the sandbox attribute to the <iframe> tag to restrict the capabilities of the content loaded within the iframe, specifically the pdf.js viewer.
    3. Configure Sandbox Attributes for pdf.js: Carefully configure the sandbox attributes to allow only the necessary permissions for pdf.js to function correctly within the iframe, while restricting potentially dangerous features that pdf.js does not require.
      • allow-scripts: Generally required for pdf.js to function within the iframe.
      • allow-same-origin: Often needed if pdf.js needs to access resources from the same origin as the main application. Carefully consider if this is necessary and the security implications for pdf.js. Try to avoid if possible.
      • Restrictive Attributes: Avoid or minimize the use of attributes like allow-forms, allow-popups, allow-top-navigation, allow-pointer-lock, allow-modals, allow-orientation-lock, allow-presentation, allow-same-origin (if possible), allow-storage-access-by-user-activation, allow-downloads-without-user-activation, allow-autoplay within the pdf.js iframe sandbox.
    4. Communication with Sandboxed pdf.js (If Needed): If the main application needs to communicate with the pdf.js viewer in the iframe, use secure cross-document messaging mechanisms like postMessage() and carefully validate messages received from the iframe hosting pdf.js.
  • Threats Mitigated:
    • Cross-Site Scripting (XSS) in pdf.js (High Severity): If pdf.js itself or a PDF processed by it has an XSS vulnerability, sandboxing can limit the impact by preventing the malicious script from accessing the main application's context, cookies, or local storage. The XSS exploit would be contained within the iframe sandbox isolating pdf.js.
    • Privilege Escalation via pdf.js (Medium Severity): Sandboxing restricts the privileges of the code running within the iframe (pdf.js), limiting the potential for privilege escalation attacks if a vulnerability is exploited in pdf.js.
    • Side-Channel Attacks originating from pdf.js (Low to Medium Severity): Sandboxing can provide some level of isolation, making it slightly harder for certain side-channel attacks to propagate from the pdf.js context to the main application.
  • Impact:
    • Cross-Site Scripting in pdf.js (High Impact): Significantly reduces the impact of XSS vulnerabilities specifically in pdf.js by containing the attack within the iframe sandbox.
    • Privilege Escalation via pdf.js (Medium Impact): Reduces the risk of privilege escalation originating from pdf.js vulnerabilities by limiting the capabilities of the sandboxed environment.
    • Side-Channel Attacks from pdf.js (Low to Medium Impact): Provides a degree of isolation, making certain attacks originating from pdf.js more difficult.
  • Currently Implemented:
    • No, pdf.js viewer is currently directly embedded in the main application page. It is not running within an iframe.
  • Missing Implementation:
    • We should refactor our pdf.js integration to load the viewer within an <iframe> with the sandbox attribute. We need to carefully analyze the required functionalities of pdf.js and configure the sandbox attributes to be as restrictive as possible while still allowing pdf.js to work correctly within the iframe. We also need to consider if any communication is needed between the main application and the iframe hosting pdf.js and implement secure cross-document messaging if required.

Mitigation Strategy: Subresource Integrity (SRI) for CDN Hosted pdf.js Files

5. Subresource Integrity (SRI) for CDN Hosted pdf.js Files

  • Mitigation Strategy: Subresource Integrity (SRI) for CDN Hosted pdf.js Files
  • Description:
    1. Calculate SRI Hashes for pdf.js CDN Files: Generate SRI hashes (SHA-256, SHA-384, or SHA-512) for the pdf.js files (JavaScript and CSS) that are loaded from a CDN. Tools or online generators can be used to calculate these hashes specifically for the pdf.js files.
    2. Add integrity Attribute to pdf.js Tags: Add the integrity attribute to the <script> and <link> tags that load pdf.js files from the CDN. Set the value of the integrity attribute to the generated SRI hash, along with the crossorigin="anonymous" attribute. Ensure this is done for all pdf.js related files loaded from the CDN. For example: 
      <script src="https://cdn.example.com/pdf.js/pdf.min.js" integrity="sha384-HASH_VALUE" crossorigin="anonymous"></script>
<link rel="stylesheet" href="https://cdn.example.com/pdf.js/pdf_viewer.css" integrity="sha384-HASH_VALUE_CSS" crossorigin="anonymous">
    3. Update Hashes on pdf.js Version Updates: Whenever you update the pdf.js version from the CDN, regenerate the SRI hashes for the new pdf.js files and update the integrity attributes in your HTML to reflect the new hashes for the updated pdf.js files.
  • Threats Mitigated:
    • CDN Compromise of pdf.js Files (Medium to High Severity): If the CDN hosting pdf.js is compromised, attackers could inject malicious code into the pdf.js files. SRI prevents the browser from executing these tampered pdf.js files.
    • Man-in-the-Middle (MITM) Attacks on pdf.js Delivery (Medium Severity): In a MITM attack, an attacker could intercept the network traffic and inject malicious code into the pdf.js files being delivered over HTTP (less relevant for HTTPS, but still a defense-in-depth measure). SRI helps prevent execution of modified pdf.js files.
  • Impact:
    • CDN Compromise of pdf.js Files (High Impact): Effectively prevents execution of compromised pdf.js files from a CDN.
    • Man-in-the-Middle (MITM) Attacks on pdf.js Delivery (Medium Impact): Reduces the risk of MITM attacks injecting malicious code into pdf.js files.
  • Currently Implemented:
    • No, we are currently hosting pdf.js files from our own server and not using a CDN. Therefore, SRI is not currently implemented for pdf.js.
  • Missing Implementation:
    • If we decide to switch to using a CDN for hosting pdf.js to improve performance or scalability, we should implement SRI for the pdf.js files. We would need to generate SRI hashes for the CDN-hosted pdf.js files and add the integrity attributes to our HTML tags loading pdf.js from the CDN.

Mitigation Strategy: Minimize Exposed pdf.js Functionality

6. Minimize Exposed pdf.js Functionality

  • Mitigation Strategy: Minimize Exposed pdf.js Functionality
  • Description:
    1. Review Required pdf.js Features: Analyze the features of the pdf.js viewer that are actually needed for your application's specific use case. Identify features that are not essential.
    2. Disable Unnecessary pdf.js Controls: Configure pdf.js viewer to disable or hide UI controls and features that are not required. This directly reduces the interactive surface area of the pdf.js viewer. This might include:
      • Download button (if users should not be able to download PDFs via pdf.js viewer).
      • Print button (if printing via pdf.js viewer is not needed).
      • Text selection and copy functionality (if sensitive content is displayed and text operations via pdf.js are undesirable).
      • Annotations features (if not used and potentially complex features of pdf.js).
      • Search functionality (if not needed and potentially complex feature of pdf.js).
      • Zoom controls (if fixed zoom level is sufficient for your pdf.js integration).
    3. Customize pdf.js Viewer Configuration: Utilize pdf.js viewer configuration options to disable or customize features through the JavaScript API provided by pdf.js. Refer to the pdf.js documentation for available configuration options to tailor the viewer to your needs and minimize exposed functionality.
    4. Restrict pdf.js API Access (If Applicable): If you are directly using the pdf.js API in your code, only use the necessary API functions and avoid exposing or using potentially risky or less secure APIs provided by pdf.js if safer alternatives exist within the pdf.js API or your application logic.
  • Threats Mitigated:
    • Reduced Attack Surface in pdf.js Viewer (Low to Medium Severity): By disabling unnecessary features of the pdf.js viewer, you reduce the attack surface of the pdf.js viewer itself. Fewer features mean fewer potential vulnerabilities within the pdf.js viewer UI and functionality to exploit.
    • Complexity Reduction in pdf.js Integration (Low Severity): Simplifying the pdf.js viewer configuration and usage reduces complexity in your integration, which can indirectly improve security by making the application easier to understand, maintain, and audit for potential security flaws related to pdf.js.
  • Impact:
    • Reduced Attack Surface in pdf.js Viewer (Low to Medium Impact): Modestly reduces the attack surface of the pdf.js viewer. The impact depends on the specific features disabled and the potential vulnerabilities associated with those pdf.js viewer features.
    • Complexity Reduction in pdf.js Integration (Low Impact): Slightly improves maintainability of the pdf.js integration and potentially reduces the likelihood of introducing security flaws due to complexity in your pdf.js usage.
  • Currently Implemented:
    • Partially implemented. We have removed the download button from the default pdf.js viewer configuration as it was not required for our application. This is a specific customization of the pdf.js viewer.
  • Missing Implementation:
    • We need to conduct a more thorough review of the pdf.js viewer features and identify other unnecessary controls and functionalities that can be disabled or hidden through pdf.js configuration. We should further customize the viewer configuration to minimize the exposed functionality based on our application's specific requirements for PDF viewing using pdf.js. This includes reviewing print, text selection, annotation, and search features of the pdf.js viewer and disabling them if not essential for our application's use of pdf.js.