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attack-surface.md

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Attack Surface Analysis for lemmynet/lemmy

Attack Surface: Malicious Instance Interaction (Federation)

  • Description: A rogue or compromised Lemmy instance sends crafted data (posts, comments, user profiles, votes, etc.) to exploit vulnerabilities in other instances.
  • How Lemmy Contributes: Lemmy's core functionality relies on trusting data from other, independently operated instances. This trust model is inherent to federation.
  • Example: A malicious instance sends a specially crafted ActivityPub Create activity containing a comment with an extremely long string in a rarely-used field, triggering a buffer overflow in the receiving instance's parsing logic, leading to remote code execution.
  • Impact: Complete compromise of the receiving instance, including data theft, data modification, and potential further propagation of the attack.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Developers:
      • Comprehensive Input Validation: Rigorously validate all data received from federated instances, not just user input within the local instance. This includes length checks, type checks, and format validation for every field in ActivityPub objects.
      • Fuzz Testing: Employ fuzz testing techniques specifically targeting the ActivityPub parsing and processing logic.
      • Sandboxing: Consider sandboxing or isolating the processing of federated data to limit the impact of potential exploits.
      • Defensive Programming: Assume all external input is malicious and code defensively.
      • Regular Security Audits: Conduct regular security audits and penetration tests focused on federation-related vulnerabilities.
    • Users/Admins:
      • Instance Selection: Be cautious about federating with unknown or untrusted instances. Research instances before connecting.
      • Monitoring: Monitor instance logs for suspicious activity from federated instances.
      • Defederation: Be prepared to quickly defederate from instances exhibiting malicious behavior. Have a clear process for this.

Attack Surface: ActivityPub Protocol Exploits

  • Description: Vulnerabilities in the implementation of the ActivityPub protocol itself are exploited.
  • How Lemmy Contributes: Lemmy's federation is built entirely on ActivityPub. Any weakness in its implementation is a direct vulnerability.
  • Example: An attacker sends a malformed ActivityPub Follow activity that bypasses authentication checks, allowing them to force an instance to follow a malicious actor.
  • Impact: Varies depending on the specific exploit, ranging from denial of service to unauthorized access and data manipulation.
  • Risk Severity: High
  • Mitigation Strategies:
    • Developers:
      • Strict Specification Adherence: Adhere meticulously to the ActivityPub specification. Avoid custom extensions or deviations unless absolutely necessary and thoroughly vetted.
      • Library Updates: Keep the ActivityPub library (and all dependencies) up-to-date to patch known vulnerabilities.
      • Security Reviews: Conduct regular security reviews of the ActivityPub implementation code.
      • Formal Verification: Consider using formal verification techniques (where feasible) to prove the correctness of critical parts of the ActivityPub implementation.
    • Users/Admins:
      • Software Updates: Keep Lemmy instances updated to the latest version to receive security patches.

Attack Surface: Server-Side Request Forgery (SSRF) via Federation

  • Description: Lemmy fetches data (images, avatars, etc.) from URLs provided by other instances. A malicious URL can trick the server into making requests to internal or sensitive external resources.
  • How Lemmy Contributes: Federation inherently involves fetching data from external sources based on information provided by other instances.
  • Example: A malicious instance sets its avatar URL to http://127.0.0.1:22 (or an internal service port). When another instance fetches this avatar, it attempts to connect to the local SSH server (or internal service), potentially revealing information or allowing further attacks.
  • Impact: Exposure of internal services, data leakage, potential for further attacks against internal infrastructure.
  • Risk Severity: High
  • Mitigation Strategies:
    • Developers:
      • URL Allowlisting: Implement a strict allowlist of permitted domains and protocols for fetching external resources.
      • Private IP Blocking: Explicitly block requests to private IP address ranges (e.g., 127.0.0.1, 192.168.x.x, 10.x.x.x, 172.16.x.x - 172.31.x.x).
      • Network Isolation: Use a separate, restricted network context (e.g., a container or virtual machine with limited network access) for fetching external resources.
      • DNS Resolution Control: Control DNS resolution to prevent resolving to internal or sensitive hostnames.
      • Redirect Handling: Carefully handle HTTP redirects, enforcing the same restrictions on redirected URLs.
      • Timeout and Resource Limits: Implement timeouts and resource limits on external requests to prevent denial-of-service attacks.
    • Users/Admins:
      • Network Segmentation: If possible, run Lemmy instances in a network segment that has limited access to internal resources.