sec-design-deep-analysis.md

Deep Security Analysis of pdf.js

1. Objective, Scope, and Methodology

Objective:

The objective of this deep security analysis is to conduct a thorough examination of the pdf.js library's key components, identify potential security vulnerabilities, assess their impact, and propose actionable mitigation strategies. The analysis will focus on the security implications of the library's design, implementation, and deployment, with a particular emphasis on preventing code execution, data breaches, and denial-of-service attacks. We aim to provide specific, actionable recommendations tailored to pdf.js, rather than generic security advice.

Scope:

This analysis covers the following aspects of pdf.js:

• PDF Parsing Engine: The core component responsible for interpreting the PDF file format (e.g., src/core/, src/shared/).
• Rendering Engine: The component that translates parsed PDF data into a visual representation (e.g., src/display/).
• JavaScript API: The interface exposed to web applications for interacting with pdf.js (e.g., web/, src/display/api.js).
• Data Handling: How pdf.js handles input data (PDF streams, fonts, images) and manages memory.
• Deployment Model (CDN): The security implications of using a CDN for distribution.
• Build Process: The security controls integrated into the build and deployment pipeline.
• Dependencies: Third-party libraries and their potential security impact.

This analysis does not cover:

• The security of the web browser itself (this is assumed to be a trusted environment, although its limitations are considered).
• The security of web applications that integrate pdf.js (this is the responsibility of the application developers).
• The security of the PDF source (e.g., the web server hosting the PDF files).

Methodology:

1. Code Review: Manual inspection of the pdf.js source code (primarily JavaScript) to identify potential vulnerabilities and insecure coding practices. This will focus on areas identified as high-risk in the Scope.
2. Architecture Review: Analysis of the C4 diagrams and documentation to understand the system's components, data flows, and trust boundaries.
3. Dependency Analysis: Examination of the project's dependencies (using package-lock.json and package.json) to identify known vulnerabilities in third-party libraries.
4. Threat Modeling: Identification of potential threats and attack vectors based on the system's architecture and functionality.
5. Review of Existing Security Controls: Assessment of the effectiveness of existing security measures, such as input sanitization, CSP, and the build process.
6. Vulnerability Research: Searching for publicly disclosed vulnerabilities related to pdf.js and PDF parsing in general.
7. Documentation Review: Examination of the project's documentation (including SECURITY.md, CONTRIBUTING.md, and test documentation) to understand security policies and procedures.

2. Security Implications of Key Components

This section breaks down the security implications of each key component, referencing specific files and directories in the pdf.js repository where possible.

2.1 PDF Parsing Engine (src/core/, src/shared/)

• Security Implications: This is the most critical component from a security perspective. PDF is a complex format with a large attack surface. Vulnerabilities in the parsing engine can lead to arbitrary code execution, information disclosure, or denial-of-service. The parser handles potentially untrusted data directly from the PDF file.
• Specific Concerns:
  • Buffer Overflows/Underflows: Incorrect handling of buffer sizes when parsing PDF objects (e.g., strings, arrays, streams) can lead to memory corruption. JavaScript's dynamic typing can make this harder to detect. (Example: src/core/primitives.js, src/core/parser.js)
  • Integer Overflows: Arithmetic operations on PDF object values (e.g., object numbers, lengths) could lead to integer overflows, potentially causing unexpected behavior or vulnerabilities. (Example: src/core/obj.js)
  • Type Confusion: Incorrectly interpreting the type of a PDF object could lead to unexpected code execution or data access. (Example: src/core/primitives.js)
  • Logic Errors: Flaws in the parsing logic for specific PDF features (e.g., XFA forms, JavaScript actions, embedded files) could be exploited. (Example: src/core/jpx.js, src/core/jbig2.js, src/core/pdf_manager.js)
  • Infinite Loops/Recursion: Malformed PDF structures could cause the parser to enter an infinite loop or exhaust the call stack, leading to a denial-of-service. (Example: src/core/parser.js)
  • External Entity (XXE) Attacks: If the parser processes XML data within the PDF (e.g., XFA forms), it could be vulnerable to XXE attacks, potentially allowing an attacker to read local files or access internal resources. (Example: src/core/xml_parser.js)
  • Resource Exhaustion: Parsing extremely large or complex PDF files could consume excessive memory or CPU, leading to a denial-of-service.

2.2 Rendering Engine (src/display/)

• Security Implications: While less critical than the parsing engine, the rendering engine still presents security risks. It handles potentially manipulated data from the parser and interacts with the browser's rendering APIs (Canvas, SVG).
• Specific Concerns:
  • Cross-Site Scripting (XSS): If the rendering engine doesn't properly sanitize text or other content extracted from the PDF, it could be vulnerable to XSS attacks, especially if the PDF contains embedded JavaScript or HTML. (Example: src/display/text_layer.js, web/viewer.js)
  • Image Decoding Vulnerabilities: Bugs in the image decoding logic (for formats like JPEG2000, JBIG2) could be exploited to trigger memory corruption or code execution. (Example: src/display/image_utils.js)
  • Font Handling Vulnerabilities: Similar to image decoding, vulnerabilities in font parsing and rendering could be exploited. (Example: src/display/font_loader.js)
  • Denial-of-Service: Rendering extremely complex graphics or animations could consume excessive resources, leading to a denial-of-service.
  • Information Disclosure: Careless handling of rendered content could potentially leak information from the PDF, especially if the rendering process involves temporary files or buffers.

2.3 JavaScript API (web/, src/display/api.js)

• Security Implications: The API is the primary interface for web applications to interact with pdf.js. It needs to be carefully designed to prevent misuse and protect the underlying parsing and rendering engines.
• Specific Concerns:
  • Improper Input Validation: The API must validate all input parameters (e.g., PDF URLs, data buffers, rendering options) to prevent attackers from passing malicious data to the parsing or rendering engines. (Example: src/display/api.js)
  • API Misuse: The API should be designed to prevent common security mistakes, such as allowing arbitrary code execution through event handlers or callbacks.
  • Cross-Origin Resource Sharing (CORS): If the API allows loading PDFs from different origins, it needs to handle CORS correctly to prevent unauthorized access to data. (Example: web/app.js)
  • Rate Limiting: To prevent denial-of-service attacks, the API might need to implement rate limiting or other mechanisms to restrict the number of requests from a single client. (Not currently implemented, but a potential future consideration).

2.4 Data Handling

• Security Implications: How pdf.js manages memory and handles input data (PDF streams, fonts, images) is crucial for preventing memory corruption vulnerabilities and data leaks.
• Specific Concerns:
  • Memory Management: JavaScript relies on garbage collection, but large PDF files or complex rendering operations can still lead to memory exhaustion. pdf.js needs to manage memory efficiently and release resources when they are no longer needed.
  • Input Sanitization: All input data from the PDF file should be treated as untrusted and carefully sanitized before being used. This includes validating data types, lengths, and encodings.
  • Data Buffers: The use of data buffers (e.g., ArrayBuffer, Uint8Array) needs to be carefully managed to prevent buffer overflows or underflows.
  • Temporary Files: If pdf.js uses temporary files (which it generally avoids), it needs to ensure that these files are created securely and deleted when no longer needed.

2.5 Deployment Model (CDN)

• Security Implications: Using a CDN (like Cloudflare or jsDelivr) is generally beneficial for security, as it provides DDoS protection and ensures that users receive the latest version of pdf.js. However, there are some considerations:
• Specific Concerns:
  • Compromised CDN: If the CDN itself is compromised, attackers could potentially inject malicious code into the pdf.js files served to users. This is a low-probability but high-impact risk.
  • Subresource Integrity (SRI): Using SRI tags can help mitigate the risk of a compromised CDN by ensuring that the browser only executes pdf.js files with a specific, known hash. This is highly recommended.
  • HTTPS: The CDN must use HTTPS to protect the integrity and confidentiality of the pdf.js files during transit.

2.6 Build Process

• Security Implications: A secure build process is essential for preventing the introduction of vulnerabilities during development and deployment.
• Specific Concerns:
  • Supply Chain Attacks: Compromised dependencies (e.g., NPM packages) could introduce vulnerabilities into pdf.js.
  • Build Tampering: Attackers could try to modify the build process itself to inject malicious code.
  • Insecure Build Environment: The build environment (e.g., CI server) needs to be secured to prevent unauthorized access.

2.7 Dependencies

• Security Implications: Third-party libraries used by pdf.js can introduce vulnerabilities.
• Specific Concerns:
  • Known Vulnerabilities: Dependencies should be regularly checked for known vulnerabilities (e.g., using npm audit or other vulnerability scanning tools).
  • Outdated Dependencies: Outdated dependencies are more likely to contain vulnerabilities.
  • Unmaintained Dependencies: Dependencies that are no longer actively maintained are a security risk.

3. Threat Modeling

This section identifies potential threats and attack vectors based on the architecture and functionality of pdf.js.

| Threat | Attack Vector

| Threat | Attack Vector