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attack-surface.md

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Attack Surface Analysis for path/fastimagecache

Attack Surface: Cache Poisoning (Source-Side)

  • Description: An attacker manipulates the image source (URL, filename, etc.) to inject a malicious image into the cache, replacing a legitimate image.
    • fastimagecache Contribution: The library's core function is to cache images based on a source identifier. If this identifier is user-controllable and the library doesn't perform sufficient validation itself, it directly enables this attack.
    • Example: An attacker provides a URL parameter ?image=http://attacker.com/evil.svg which, when cached by fastimagecache, replaces the legitimate logo.png. Subsequent users requesting logo.png receive the malicious SVG.
    • Impact: XSS, defacement, malware distribution, data exfiltration (via XSS).
    • Risk Severity: Critical (if XSS is possible) or High (if limited to defacement).
    • Mitigation Strategies:
      • fastimagecache Internal Validation: The library should internally validate and sanitize the image source before fetching or caching it. This includes checking for allowed URL schemes (e.g., http://, https://), validating domain names against a whitelist (if applicable), and rejecting suspicious characters or patterns. This is the primary mitigation that should be implemented within the library itself.
      • Image Type Verification (Within fastimagecache): The library must verify the Content-Type header and the file's magic bytes to ensure it's a valid image of an expected type before caching. This prevents caching malicious files disguised as images.
      • Application-Level Mitigations (Secondary): While the library should handle the core validation, the application using it should also implement:
        • Strict Source Whitelisting (at the application level).
        • Input Validation and Sanitization (at the application level).
        • Content Security Policy (CSP).
        • Subresource Integrity (SRI) - if applicable.

Attack Surface: Path Traversal (Storage-Side)

  • Description: An attacker manipulates the file path used to store cached images, allowing them to write files outside the intended cache directory.
    • fastimagecache Contribution: The library is directly responsible for writing cached images to the storage location. If it constructs file paths insecurely using user-provided data, it's directly vulnerable.
    • Example: An attacker provides an image name like ../../../etc/passwd. If fastimagecache doesn't sanitize this, it might overwrite a critical system file.
    • Impact: Arbitrary file overwrite, code execution (if a critical system file or application file is overwritten), denial of service.
    • Risk Severity: Critical (if code execution is possible) or High.
    • Mitigation Strategies:
      • fastimagecache Internal Path Handling: The library must use absolute, hardcoded paths for the cache directory or a strictly controlled, sanitized relative path. It must not directly incorporate user input into the path construction without thorough sanitization and validation. This is the primary mitigation.
      • Safe Filename Generation (Within fastimagecache): The library should generate safe filenames for cached images, regardless of the original filename. This could involve using a hash of the image content or a unique identifier. This prevents attackers from controlling the filename at all.
      • Application-Level Mitigations (Secondary):
        • Least Privilege (for the application process).
        • Chroot Jail (advanced, for the application process).
        • Strict File System Permissions (for the cache directory).

Attack Surface: Server-Side Request Forgery (SSRF)

  • Description: An attacker provides a URL that causes the server (via fastimagecache) to make requests to internal or sensitive resources.
    • fastimagecache Contribution: If the library fetches images from remote URLs based on user input, and it doesn't have built-in SSRF protections, it acts as a vulnerable proxy.
    • Example: An attacker provides ?image=http://169.254.169.254/latest/meta-data/. fastimagecache fetches this, exposing AWS metadata.
    • Impact: Information disclosure, access to internal services.
    • Risk Severity: High or Critical.
    • Mitigation Strategies:
      • fastimagecache Internal URL Validation: The library must internally validate URLs before making any requests. This should include:
        • Strict Scheme Validation: Only allow specific schemes (e.g., https://).
        • Whitelist of Allowed Domains (Ideally): If possible, the library should have a configurable whitelist of allowed domains.
        • IP Address Restrictions: The library should not allow requests to private IP address ranges (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.1/8, 169.254.0.0/16). This is crucial.
        • DNS Resolution Control (Advanced): The library could resolve hostnames to IP addresses internally and then check the IP against allowed/blocked lists before making the request. This prevents DNS rebinding.
      • Application-Level Mitigations (Secondary):
        • Strict URL Whitelisting (at the application level).
        • Network Segmentation.
        • Disable Internal Network Access.

Attack Surface: Denial of Service (DoS) - Disk Exhaustion

  • Description: An attacker floods the cache with images, consuming all disk space.
    • fastimagecache Contribution: The library's core function is to store images. Without internal limits, it's directly responsible for this vulnerability.
    • Example: Repeated requests for many different, non-existent images.
    • Impact: Denial of service.
    • Risk Severity: High.
    • Mitigation Strategies:
      • fastimagecache Internal Limits: The library must implement:
        • Maximum Cache Size: A configurable limit on the total size of the cache.
        • Maximum Image Size: A configurable limit on the size of individual images that can be cached.
        • Cache Eviction Policy: An automatic eviction policy (e.g., LRU) to remove old images when the cache is full.
      • Application-Level Mitigations (Secondary):
        • Rate Limiting.
        • Monitoring (disk space usage).
        • Input Validation (Size) - if source image size is known beforehand.

Attack Surface: Image Processing Vulnerabilities (e.g., ImageTragick)

  • Description: Exploits in underlying image processing libraries used by fastimagecache.
    • fastimagecache Contribution: If the library performs any image processing (resizing, format conversion), it directly exposes vulnerabilities in those libraries.
    • Example: A crafted image triggers a buffer overflow in libjpeg (used by fastimagecache).
    • Impact: Remote code execution, denial of service.
    • Risk Severity: Critical or High.
    • Mitigation Strategies:
      • fastimagecache Dependency Management: The library must use up-to-date and patched versions of all image processing libraries. This is the most important mitigation.
      • fastimagecache Input Validation (Format): The library should perform strict validation of the image format and structure before passing it to any processing library.
      • fastimagecache Sandboxing (Ideally): If possible, the library should perform image processing in a sandboxed or isolated environment. This is a significant architectural change but provides strong protection.
      • Application-Level Mitigations (Secondary):
        • Disable Unnecessary Processing (if possible, at the application level).
        • WAF (Web Application Firewall).
        • Least Privilege (for the process doing image processing).