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threat-modeling.md

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Threat Model Analysis for schollz/croc

Threat: Relay Server Modification Attack

  • Description: A malicious relay server operator (or an attacker who has compromised the relay) actively modifies the data in transit. They attempt to inject malicious data or alter file contents, bypassing croc's integrity checks. This implies a flaw in croc's integrity mechanisms or a vulnerability that allows the attacker to circumvent them.
    • Impact: Integrity of transferred files is compromised. The receiver receives a modified or corrupted file, potentially containing malicious code or altered data.
    • Affected Component: croc relay server; croc client (specifically, the integrity verification logic); github.com/schollz/croc/v9/pkg/croc.Send(), github.com/schollz/croc/v9/pkg/croc.Receive(), and the underlying hashing/encryption functions.
    • Risk Severity: High.
    • Mitigation Strategies:
      • Robust Integrity Verification: Ensure croc's integrity checks (hashing, encryption) are cryptographically strong and cannot be bypassed. This requires rigorous code review and potentially formal verification.
      • Independent Verification (Application-Level): As a defense-in-depth measure, the receiving application could independently verify the file's integrity using a pre-shared hash (out-of-band). This mitigates the risk even if croc's internal checks fail.
      • Auditing of Cryptographic Libraries: Regularly audit the cryptographic libraries used by croc (e.g., for hashing and encryption) for known vulnerabilities.

Threat: croc Library Vulnerability Exploitation (Zero-Day)

  • Description: An attacker exploits a previously unknown (zero-day) vulnerability in the croc library itself (or one of its direct dependencies, like spake2 or the cryptographic libraries) to gain control of the application, intercept data, or cause other harm. This is a broad category encompassing any undiscovered flaws.
    • Impact: Varies greatly depending on the specific vulnerability. Could range from data leakage (confidentiality breach) to complete system compromise (loss of control, integrity violation).
    • Affected Component: Potentially any part of the croc library (github.com/schollz/croc/v9/...) or its direct dependencies.
    • Risk Severity: High (until proven otherwise, all unknown vulnerabilities must be treated as high risk).
    • Mitigation Strategies:
      • Proactive Vulnerability Research: Encourage security researchers to analyze croc and its dependencies for vulnerabilities (e.g., through bug bounty programs).
      • Fuzzing: Employ fuzzing techniques to test croc's code for unexpected inputs that might trigger vulnerabilities.
      • Static Analysis: Use static analysis tools to identify potential security flaws in croc's codebase.
      • Dependency Auditing: Continuously monitor croc's dependencies for known vulnerabilities and update them promptly.
      • Rapid Patching: Establish a process for quickly patching and deploying updates to croc in response to discovered vulnerabilities.
      • Defense in Depth (Application Level): Implement security measures at the application level that can mitigate the impact of a croc vulnerability (e.g., sandboxing, input validation).

Threat: PAKE Bypass (Highly Unlikely, but Critical if Possible)

  • Description: An attacker finds a way to bypass or break the Password-Authenticated Key Exchange (PAKE) protocol (spake2) used by croc. This would allow them to intercept the file transfer without knowing the code phrase. This is extremely unlikely if spake2 is implemented correctly and croc integrates it properly, but the impact is so severe that it warrants inclusion.
    • Impact: Complete compromise of confidentiality. The attacker can decrypt the transferred files.
    • Affected Component: github.com/schollz/croc/v9/pkg/croc.Send(), github.com/schollz/croc/v9/pkg/croc.Receive(), the spake2 library implementation, and croc's integration of spake2.
    • Risk Severity: Critical (if a bypass is found).
    • Mitigation Strategies:
      • Formal Verification of spake2: If feasible, formally verify the correctness of the spake2 implementation used by croc.
      • Expert Cryptographic Review: Have experienced cryptographers review the spake2 implementation and its integration into croc.
      • Stay Updated: Keep the spake2 library and croc itself updated to the latest versions, as any security fixes will be included in updates.
      • Monitor for Research: Stay informed about any published research on spake2 or related cryptographic protocols that might reveal weaknesses.