threat-modeling.md

Threat Model Analysis for coil-kt/coil

Threat: Malicious Image Substitution (Coil-Specific Aspects)

• Description: An attacker exploits a vulnerability within Coil's URL parsing or request handling logic to bypass application-level URL validation. Even if the application attempts to validate the URL, a flaw in Coil could allow a malicious URL to be processed. This is distinct from the application simply failing to validate the URL. The attacker's server returns a malicious image.
• Impact:
  • Display of inappropriate or misleading content.
  • Potential execution of malicious code if the image contains an exploit targeting a vulnerability in the image decoder (see Decoder Vulnerability Exploitation).
  • Phishing attacks.
• Coil Component Affected:
  • ImageLoader: Specifically, the internal logic for handling ImageRequest and initiating the network request.
  • ImageRequest: The data property (URL handling) and any custom Fetcher used.
  • Any custom Fetcher or Decoder implementations if they are used and have vulnerabilities that bypass application-level checks.
• Risk Severity: High
• Mitigation Strategies:
  • Defense in Depth: Even though this threat targets Coil directly, strong application-level URL validation is still crucial. This provides a layer of defense even if Coil has a vulnerability. Use allow-lists, not block-lists.
  • Coil Updates: Keep Coil updated to the latest version. Vulnerabilities in Coil's URL handling would likely be patched in updates.
  • Review Custom Components: If using custom Fetcher or Decoder implementations, thoroughly audit them for security vulnerabilities, especially related to URL handling and input validation.
  • Report Vulnerabilities: If you suspect a vulnerability in Coil's URL handling, report it responsibly to the Coil maintainers.

Threat: Decoder Vulnerability Exploitation (Coil's Role)

• Description: An attacker crafts a malicious image file designed to exploit a vulnerability in the image decoding library used by Coil. While the vulnerability exists in the decoder (e.g., BitmapFactory on Android), Coil is the component that uses the decoder, making it a direct factor in the exploit chain. The attacker successfully triggers the vulnerability through Coil.
• Impact:
  • Application crash.
  • Potential for arbitrary code execution, leading to complete compromise of the application and potentially the device.
• Coil Component Affected:
  • Decoder: The component responsible for decoding the image data. This often relies on platform-provided decoders (like BitmapFactory on Android).
  • ImageLoader: The component that uses the Decoder.
• Risk Severity: Critical
• Mitigation Strategies:
  • Keep Dependencies Updated: This is the most critical mitigation. Keep Coil and its dependencies (especially the underlying image decoding libraries, which are often part of the OS) up-to-date. Security patches for decoders are frequently released.
  • Coil's Role in Updates: Monitor Coil releases for updates that may include changes to how it interacts with decoders or that bundle updated decoder libraries (if applicable).
  • Input Validation (Limited Effectiveness): While basic input validation (e.g., checking file extensions) is good practice, it's not a reliable defense against sophisticated decoder exploits. However, it can help prevent some simpler attacks.
  • Sandboxing (Difficult): Ideally, image decoding would happen in a sandboxed process, limiting the impact of a vulnerability. This is often very difficult to achieve in practice, especially on Android.
  • Alternative Decoders (If Available and Secure): In some very specific, high-security scenarios, and if technically feasible, you might consider using a different, potentially more secure, image decoding library with Coil. This requires extreme care and expertise to ensure the alternative decoder is actually more secure and doesn't introduce new vulnerabilities. This is generally not recommended unless absolutely necessary.

Threat: Cached Image Tampering (If Coil's Caching is Misconfigured)

• Description: While primarily a file system permission issue, if Coil's DiskCache is misconfigured (e.g., using a world-writable directory) by the application developer, an attacker could replace legitimate cached images with malicious ones. This highlights Coil's direct involvement because the misconfiguration is within Coil's setup.
• Impact:
  • Display of malicious images, even offline.
  • Potential code execution (via decoder vulnerabilities).
• Coil Component Affected:
  • diskCache: Specifically, the configuration and location of the disk cache.
• Risk Severity: High (due to the potential for persistent compromise)
• Mitigation Strategies:
  • Correct DiskCache Configuration: The application developer must ensure that Coil's DiskCache is configured to use a secure, private directory with appropriate file permissions. This is not something Coil can automatically enforce; it's the developer's responsibility. Follow platform-specific best practices for secure file storage.
  • Review Documentation: Carefully review Coil's documentation regarding DiskCache configuration and security recommendations.
  • Least Privilege: The application should run with the least necessary privileges, limiting the potential damage from a compromised cache.