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Threat Model Analysis for signalapp/signal-server

Threat: Denial of Service (DoS) via Message Flooding

  • Description: An attacker sends a massive number of messages (or registration requests, or other API calls) to the Signal Server, overwhelming its resources and preventing legitimate users from communicating. The attacker might use a botnet or exploit a vulnerability that allows them to bypass rate limits.
  • Impact: Service unavailability for legitimate users. Potential financial losses if the service is tied to business operations. Reputational damage.
  • Signal-Server Component Affected:
    • MessageServlet (and related servlets for different message types)
    • AccountServlet (for registration-related DoS)
    • Rate limiting mechanisms (RateLimiter class and related configuration)
    • Websocket connection handling
  • Risk Severity: High
  • Mitigation Strategies:
    • Robust Rate Limiting: Configure and fine-tune rate limiting based on IP address, user ID, and other factors. Use adaptive rate limiting that adjusts based on server load.
    • DDoS Protection: Employ a DDoS mitigation service (e.g., Cloudflare, AWS Shield) to absorb and filter malicious traffic.
    • Resource Monitoring: Continuously monitor server resources (CPU, memory, network bandwidth) and set alerts for unusual activity.
    • CAPTCHA/Proof-of-Work: Implement CAPTCHA or proof-of-work challenges for registration and potentially for other high-volume operations.
    • Connection Limits: Limit the number of concurrent connections per IP address or user.
    • Request Validation: Strictly validate all incoming requests to prevent malformed or oversized data from consuming excessive resources.

Threat: Public Key Impersonation (Man-in-the-Middle)

  • Description: An attacker compromises the server's key storage or exploits a vulnerability in the key exchange process to replace a user's legitimate public key with their own. This allows the attacker to intercept and decrypt messages intended for the victim, and potentially send forged messages.
  • Impact: Complete compromise of confidentiality and integrity for affected communications. Loss of trust in the system. Potential for significant reputational damage.
  • Signal-Server Component Affected:
    • AccountManager (specifically, methods related to key storage and retrieval)
    • Database interactions related to key storage (e.g., PreKeyStore, SignedPreKeyStore, IdentityKeyStore)
    • Key exchange protocol implementation (within MessageServlet and related components)
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Key Verification (Safety Numbers): Implement and enforce out-of-band key verification using safety numbers (fingerprints). Provide clear UI guidance for users to compare safety numbers.
    • Trust on First Use (TOFU) with Key Pinning: Implement TOFU, but also allow users to "pin" trusted keys, preventing changes without explicit user approval.
    • Key Change Notifications: Alert users whenever a contact's keys change, highlighting the potential risk.
    • Database Security: Secure the database storing public keys with strong access controls, encryption at rest, and regular security audits.
    • Code Audits: Regularly audit the code responsible for key management and exchange to identify and fix vulnerabilities.

Threat: Exploitation of Server-Side Vulnerabilities

  • Description: An attacker exploits a vulnerability in the Signal Server code (e.g., a buffer overflow, injection flaw, or logic error) to gain unauthorized access to the server, potentially leading to code execution, data exfiltration, or denial of service.
  • Impact: Potentially complete server compromise. Loss of confidentiality, integrity, and availability. Access to all stored data (encrypted messages, public keys, etc.).
  • Signal-Server Component Affected: Potentially any component, depending on the vulnerability. High-risk areas include:
    • Input validation logic in all servlets
    • Database interaction code
    • Authentication and authorization mechanisms
    • External library integrations
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Secure Coding Practices: Follow secure coding guidelines (e.g., OWASP) to prevent common vulnerabilities.
    • Regular Code Reviews: Conduct thorough code reviews, focusing on security-critical areas.
    • Static Analysis: Use static analysis tools to automatically detect potential vulnerabilities.
    • Dynamic Analysis (Fuzzing): Use fuzzing techniques to test the server with unexpected inputs and identify crashes or unexpected behavior.
    • Dependency Management: Keep all dependencies (libraries, frameworks) up-to-date and regularly check for security vulnerabilities.
    • Vulnerability Scanning: Regularly scan the server for known vulnerabilities using vulnerability scanners.
    • Bug Bounty Program: Consider establishing a bug bounty program to incentivize security researchers to find and report vulnerabilities.

Threat: Compromise of Registration Lock

  • Description: An attacker bypasses or disables the registration lock mechanism, allowing them to register a phone number already associated with an existing account, effectively hijacking the account.
  • Impact: Account takeover. Loss of access to the account for the legitimate user. Potential for impersonation and reputational damage.
  • Signal-Server Component Affected:
    • AccountServlet (specifically, methods related to registration and verification)
    • RegistrationLockManager
    • Database interactions related to registration lock data
  • Risk Severity: High
  • Mitigation Strategies:
    • Strong Registration Lock Implementation: Ensure the registration lock mechanism is robust and resistant to brute-force attacks or bypass attempts.
    • Multi-Factor Authentication (MFA): Implement MFA for account recovery and registration, requiring a second factor (e.g., a one-time code sent to a trusted device) in addition to the phone number.
    • Rate Limiting: Rate limit registration attempts to prevent brute-force attacks on the registration lock.
    • Account Recovery Procedures: Implement secure account recovery procedures that are resistant to social engineering and other attacks.

Threat: Group Messaging Vulnerabilities (Sender Keys)

  • Description: Vulnerabilities in the implementation of Signal's group messaging protocol (Sender Keys) could allow an attacker to:
    • Add unauthorized members to a group.
    • Remove legitimate members from a group.
    • Decrypt group messages without being a member.
    • Forge messages that appear to come from a group member.
  • Impact: Compromise of confidentiality, integrity, and availability of group communications. Loss of trust in the group messaging feature.
  • Signal-Server Component Affected:
    • GroupManager
    • SenderKeyStore
    • GroupCipher
    • Methods related to group creation, membership management, and message processing within MessageServlet and related components.
  • Risk Severity: High
  • Mitigation Strategies:
    • Thorough Code Review: Conduct rigorous code reviews of the group messaging implementation, focusing on security-critical areas.
    • Formal Verification (Ideal): If feasible, use formal verification techniques to prove the correctness and security of the group messaging protocol implementation.
    • Regular Security Audits: Include group messaging in regular security audits.
    • Testing: Extensive testing, including fuzzing and penetration testing, specifically targeting the group messaging functionality.
    • Stay Updated: Monitor for any security advisories or updates related to Signal's group messaging protocol.