Skip to content

Latest commit

 

History

History
114 lines (95 loc) · 185 KB

sec-design-deep-analysis.md

File metadata and controls

114 lines (95 loc) · 185 KB

Okay, let's perform a deep security analysis of Square's Picasso library based on the provided design review.

1. Objective, Scope, and Methodology

  • Objective: To conduct a thorough security analysis of the key components of the Picasso library, identify potential vulnerabilities, and propose actionable mitigation strategies. The analysis will focus on identifying vulnerabilities that could lead to arbitrary code execution, denial of service, information disclosure, or other security breaches. We will pay particular attention to image processing, network interactions, and caching mechanisms.

  • Scope: The analysis will cover the core components of Picasso as described in the C4 diagrams and the provided documentation. This includes:

    • RequestHandler
    • Downloader (OkHttp)
    • Cache
    • Transformation
    • Dispatcher
    • The interaction of these components with the Mobile App and Remote Server.
    • The build and deployment process. We will not analyze:
    • The security of the Remote Server itself (this is outside Picasso's control).
    • The security of the Mobile App using Picasso, except where the app's interaction with Picasso creates vulnerabilities.
    • Vulnerabilities in the Android OS itself.

  • Methodology:

    1. Component Breakdown: Analyze each key component identified in the C4 Container diagram, focusing on its security responsibilities and potential attack vectors.
    2. Data Flow Analysis: Trace the flow of data (primarily image data) through the system, identifying points where vulnerabilities might exist.
    3. Threat Modeling: Identify potential threats based on the component breakdown and data flow analysis, considering the business risks outlined in the design review. We'll use a STRIDE-based approach (Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, Elevation of Privilege).
    4. Mitigation Strategies: Propose specific, actionable mitigation strategies for each identified threat, tailored to the Picasso library and its context.
    5. Review of Assumptions and Questions: Address the outstanding questions and assumptions from the design review, incorporating any new insights gained during the analysis.

2. Security Implications of Key Components

Let's break down each component and analyze its security implications:

  • Picasso (Main Entry Point):

    • Threats:
      • Input Validation (T, DoS): Malicious URLs (e.g., excessively long, pointing to unexpected resources) could lead to crashes or resource exhaustion. Lack of size limits on image requests could lead to DoS.
      • Configuration Errors (ID, EOP): If Picasso allows developers to disable security features (e.g., HTTPS), this could lead to information disclosure or man-in-the-middle attacks.
    • Mitigation:
      • Strict URL Validation: Implement robust URL parsing and validation, potentially using a whitelist of allowed schemes (e.g., https://, content://, file://). Reject URLs that don't conform to expected patterns.
      • Enforce Size Limits: Implement configurable, but strict by default, limits on image dimensions and file sizes. These limits should be enforced before downloading the entire image.
      • Secure Defaults: Ensure that all security-relevant settings (e.g., HTTPS enforcement) are enabled by default. Make it difficult for developers to accidentally weaken security.
      • Documentation: Clearly document all security-related configuration options and their implications.

  • RequestHandler:

    • Threats:
      • Request Type Confusion (T, EOP): If a RequestHandler incorrectly handles a request type (e.g., treating a network request as a file request), it could lead to vulnerabilities. For example, a specially crafted content:// URI might be used to access arbitrary files on the device.
      • Path Traversal (ID, EOP): If handling file or content URIs, vulnerabilities in path handling could allow access to files outside the intended directory.
    • Mitigation:
      • Strict Type Checking: Ensure that each RequestHandler only handles the specific request types it's designed for. Use strong typing and validation to prevent type confusion.
      • Safe Path Handling: If handling file paths, use platform-provided APIs for safe path resolution and canonicalization. Avoid manual string manipulation of paths. Sanitize and validate all components of a file path before using it.
      • Principle of Least Privilege: Ensure that RequestHandler instances only have the minimum necessary permissions to access the resources they need.

  • Downloader (OkHttp):

    • Threats:
      • Man-in-the-Middle (ID, T): If HTTPS is not enforced or certificate validation is bypassed, attackers could intercept or modify image data.
      • Server-Side Request Forgery (SSRF) (EOP): While less likely with OkHttp, vulnerabilities in URL handling or redirects could potentially be exploited to make requests to internal servers.
      • Resource Exhaustion (DoS): Slow or malicious servers could cause the downloader to consume excessive resources (threads, memory).
    • Mitigation:
      • Enforce HTTPS: Ensure that OkHttp is configured to use HTTPS for all network requests. Reject any attempts to use plain HTTP.
      • Strict Certificate Validation: Enable strict certificate validation, including hostname verification and checking against trusted certificate authorities. Do not allow developers to easily disable these checks.
      • Connection Timeouts: Implement reasonable connection and read timeouts to prevent resource exhaustion caused by slow servers.
      • Redirect Handling: Carefully handle redirects, limiting the number of redirects and validating the target URL of each redirect.
      • HSTS (HTTP Strict Transport Security): Encourage (or enforce if possible) the use of HSTS headers on image servers to prevent downgrade attacks.

  • Cache:

    • Threats:
      • Cache Poisoning (T, ID): If the cache key is not properly generated or validated, attackers could potentially inject malicious image data into the cache, which would then be served to other users.
      • Information Disclosure (ID): If the cache is not properly secured, sensitive image data could be accessed by other applications or users on the device.
      • Denial of Service (DoS): The cache could be filled with excessively large or numerous images, leading to resource exhaustion.
    • Mitigation:
      • Secure Cache Key Generation: Generate cache keys based on a strong hash of the image URL and any transformation parameters. Ensure that the cache key cannot be easily predicted or manipulated by attackers.
      • File System Permissions: Use appropriate file system permissions to restrict access to the cache directory. Only the application using Picasso should have read/write access.
      • Cache Size Limits: Implement strict limits on the size of the cache to prevent DoS attacks. Use a least-recently-used (LRU) eviction policy.
      • Encryption (Optional): For applications dealing with highly sensitive image data, consider encrypting the cache contents at rest. This adds complexity but significantly increases security.

  • Transformation:

    • Threats:
      • Image Parsing Vulnerabilities (EOP, DoS): Vulnerabilities in image parsing libraries (e.g., libjpeg, libpng) could be exploited to execute arbitrary code or cause denial-of-service. This is a major concern.
      • Parameter Injection (T): If transformation parameters are not properly validated, attackers could inject malicious values that lead to unexpected behavior or vulnerabilities.
    • Mitigation:
      • Fuzz Testing: Implement extensive fuzz testing of the image decoding and transformation components. This is the most important mitigation for this component. Use a variety of image formats and malformed inputs.
      • Up-to-Date Libraries: Keep all image processing libraries (libjpeg, libpng, etc.) up-to-date with the latest security patches. Monitor for CVEs related to these libraries.
      • Sandboxing (Optional): Consider using sandboxing techniques (e.g., seccomp on Android) to isolate the image processing code and limit its access to system resources. This is a complex but highly effective mitigation.
      • Input Validation: Strictly validate all transformation parameters (e.g., width, height, crop dimensions) to ensure they are within expected ranges and do not contain malicious values.
      • Memory Safe Operations: Use memory safe operations when working with image buffers.

  • Dispatcher:

    • Threats:
      • Race Conditions (T, DoS): If the dispatcher is not properly synchronized, race conditions could lead to data corruption or crashes.
      • Thread Exhaustion (DoS): If the dispatcher creates too many threads, it could exhaust system resources.
    • Mitigation:
      • Thread Safety: Use appropriate synchronization primitives (e.g., locks, mutexes) to ensure thread safety and prevent race conditions.
      • Thread Pool Management: Use a well-configured thread pool to limit the number of concurrent threads and prevent resource exhaustion.

3. Data Flow Analysis

The primary data flow is:

  1. Mobile App requests an image from Picasso with a URL.
  2. Picasso creates a request and passes it to the Dispatcher.
  3. Dispatcher selects a RequestHandler based on the URL scheme.
  4. If the image is in the Cache, it's returned.
  5. If not, the RequestHandler uses the Downloader (OkHttp) to fetch the image from the Remote Server.
  6. The downloaded image is passed to the Transformation component (if transformations are requested).
  7. The transformed image is stored in the Cache.
  8. The image is returned to the Mobile App.

Key points of vulnerability in this flow are:

  • URL Input: The initial URL provided by the Mobile App is a critical input point.
  • Network Communication: The interaction with the Remote Server via OkHttp is vulnerable to network-based attacks.
  • Image Parsing: The decoding and transformation of the image data is a high-risk area.
  • Cache Storage: The storage and retrieval of images from the cache must be secure.

4. Threat Modeling (STRIDE)

| Threat | STRIDE Category | Component(s) | Description | Mitigation * Malicious Image URL | T, DoS | Picasso, RequestHandler | An attacker provides a crafted URL that, when processed, triggers a vulnerability in the image parsing library or causes excessive resource consumption.