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attack-surface.md

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Attack Surface Analysis for matrix-org/synapse

Attack Surface: Federation with Malicious Homeservers

  • Description: The core principle of Matrix, allowing communication between different homeservers, also introduces the risk of interacting with malicious or compromised servers. This is a direct function of Synapse's federation implementation.
    • Synapse Contribution: Synapse implements the federation protocol, making it directly responsible for handling incoming and outgoing federated traffic, validating server identities, and processing federated events.
    • Example: A malicious homeserver joins a room and injects forged events that appear to come from a legitimate user, spreading misinformation or phishing links. Synapse processes and distributes these events.
    • Impact: Data breaches, impersonation, denial of service, room disruption, spread of malware.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Federation Allowlisting: Restrict federation to a pre-approved list of trusted homeservers.
      • Reputation Systems: Implement or integrate with systems that track homeserver reputation.
      • Strong Server Identity Verification: Ensure rigorous verification of TLS certificates and identities of federated servers within Synapse's configuration.
      • Event Validation: Implement strict event validation rules within Synapse to detect and reject malformed or suspicious events.
      • Rate Limiting (Federation): Implement rate limiting on incoming federation traffic within Synapse to prevent DoS.
      • Regular Security Audits: Audit Synapse's federation configuration and connected homeservers.
      • Monitoring and Alerting: Implement robust monitoring and alerting for unusual federation activity within Synapse.

Attack Surface: Compromised Application Service (AS)

  • Description: Application Services have elevated privileges within Synapse, making them high-value targets. Synapse directly manages and authorizes AS.
    • Synapse Contribution: Synapse provides the AS API, manages AS registration (including hs_token and as_token), and enforces the permissions defined in the registration YAML file. Synapse routes events to and from AS.
    • Example: An attacker compromises a bridge (an AS). The attacker uses the bridge's privileges, granted by Synapse, to send spam, eavesdrop, or impersonate users.
    • Impact: Complete control over bridged rooms/users, data breaches, impersonation, denial of service.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Secure AS Registration: Use strong, randomly generated hs_token and as_token values. Securely store these tokens, accessible only to Synapse and the AS.
      • Principle of Least Privilege: Grant AS within Synapse's configuration only the minimum necessary permissions. Carefully review namespaces.
      • Input Validation (AS via Synapse): While the AS itself should validate input, Synapse's handling of AS communication can be configured to add an extra layer of defense.
      • Code Auditing (AS): Thoroughly audit AS code, but also review how Synapse interacts with it.
      • Regular Updates (Synapse & AS): Keep both Synapse and the AS up-to-date.
      • Network Segmentation: Isolate AS, but ensure Synapse can still securely communicate with it.

Attack Surface: Client-Server API Abuse

  • Description: Attackers can target the Client-Server API to gain unauthorized access, perform denial-of-service, or enumerate users. This is a direct attack surface of Synapse.
    • Synapse Contribution: Synapse is the Client-Server API. It handles all client interactions, authentication, and authorization.
    • Example: An attacker uses a brute-force attack against Synapse's /login endpoint to guess user passwords.
    • Impact: Account compromise, denial of service, information disclosure (user enumeration).
    • Risk Severity: High
    • Mitigation Strategies:
      • Strong Password Policies: Enforce strong password policies within Synapse's configuration.
      • Rate Limiting (Client-Server API): Implement comprehensive rate limiting on all Client-Server API endpoints within Synapse.
      • Account Lockout: Implement account lockout policies within Synapse.
      • Multi-Factor Authentication (MFA): Encourage or require MFA, configured within Synapse.
      • CAPTCHA: Consider CAPTCHAs on registration/login, integrated with Synapse.
      • Web Application Firewall (WAF): A WAF can provide an additional layer of defense, but Synapse's internal protections are primary.

Attack Surface: Malicious Media Uploads

  • Description: Attackers can upload malicious files disguised as media, exploiting vulnerabilities in Synapse's media handling.
    • Synapse Contribution: Synapse's media repository directly handles the storage, retrieval, and (potentially) processing of media files.
    • Example: An attacker uploads a crafted image that exploits a vulnerability in an image processing library used by Synapse, leading to RCE on the Synapse server.
    • Impact: Server compromise, client-side attacks (if media is served to clients), data breaches, denial of service (storage exhaustion).
    • Risk Severity: High
    • Mitigation Strategies:
      • File Type Validation: Strictly validate file types within Synapse's media handling logic.
      • File Size Limits: Enforce file size limits within Synapse.
      • Media Processing Security: Use secure image/video processing libraries within Synapse. Keep these libraries updated. Consider sandboxing media processing as part of Synapse's deployment.
      • Virus Scanning: Integrate virus scanning into Synapse's media upload process.
      • Content Security Policy (CSP): While often client-side, Synapse can be configured to send appropriate CSP headers.
      • Regular Security Updates: Keep Synapse and all its dependencies up-to-date.