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attack-surface.md

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Attack Surface Analysis for syncthing/syncthing

Attack Surface: Exposed Listening Ports

  • Description: Syncthing's core functionality relies on open network ports for device-to-device communication and discovery. Exposing these ports to untrusted networks allows direct network access to Syncthing services.
  • Syncthing Contribution: Syncthing requires listening on TCP port 22000 (default) for device connections and UDP port 21027 (default) for discovery. This is fundamental to its operation.
  • Example: A Syncthing instance running on a publicly accessible server with default port 22000 open is discovered by an attacker. The attacker exploits a hypothetical vulnerability in Syncthing's protocol handling on port 22000 to gain remote code execution.
  • Impact: Full system compromise, data breach, denial of service.
  • Risk Severity: Critical (if exposed to the internet or highly untrusted networks).
  • Mitigation Strategies:
    • Firewall Restriction: Strictly limit access to Syncthing ports (22000, 21027) using firewalls. Allow connections only from trusted IP addresses or networks.
    • Network Segmentation: Isolate Syncthing instances within a secure network segment, minimizing exposure to broader networks.
    • Disable Global Discovery (if possible): For manually configured setups, disable global discovery to reduce unsolicited network exposure.

Attack Surface: Unsecured Web UI Exposure

  • Description: Syncthing includes a Web UI for management. If exposed to untrusted networks without proper security measures, it becomes a highly vulnerable entry point for web-based attacks.
  • Syncthing Contribution: Syncthing provides a built-in Web UI, enabled by default, accessible on port 8384 (default). This UI manages critical Syncthing functions.
  • Example: A Syncthing Web UI is exposed to the internet without HTTPS or strong authentication. An attacker exploits a Cross-Site Scripting (XSS) vulnerability in the Web UI to take over an administrator's session and gain full control of the Syncthing instance.
  • Impact: Full control over Syncthing configuration, data manipulation, potential data breach, denial of service.
  • Risk Severity: Critical (if exposed to the internet or highly untrusted networks without HTTPS and strong authentication).
  • Mitigation Strategies:
    • Bind Web UI to Loopback: Configure Syncthing to bind the Web UI only to 127.0.0.1. Access it securely via SSH tunneling or a VPN for remote management.
    • Enforce HTTPS: Always access the Web UI over HTTPS. Configure Syncthing to use HTTPS or use a reverse proxy for HTTPS termination.
    • Strong Authentication: Set a strong, unique password for Web UI access.
    • Restrict Access by IP (if possible): Limit Web UI access to specific trusted IP addresses or networks in Syncthing's configuration.

Attack Surface: Unsecured API Exposure

  • Description: Syncthing's REST API allows programmatic control. Exposing this API without robust authentication and authorization mechanisms creates a significant risk of unauthorized access and manipulation.
  • Syncthing Contribution: Syncthing offers a REST API for automation and integration, enabled by default and accessible on the same port as the Web UI. This API provides powerful control over Syncthing.
  • Example: A Syncthing API is exposed without proper API key protection. An attacker gains access to a weak or leaked API key and uses it to programmatically download synchronized files or disrupt synchronization processes.
  • Impact: Unauthorized data access, data manipulation, denial of service, potential system compromise depending on API capabilities and vulnerabilities.
  • Risk Severity: High (if API is exposed without strong API key protection and authorization).
  • Mitigation Strategies:
    • Strong API Key Generation and Management: Generate strong, unique API keys. Store and manage API keys securely, avoiding insecure transmission or storage.
    • API Access Control and Authorization: Implement proper authorization checks on API endpoints to ensure API keys are only granted the necessary permissions.
    • Restrict API Access by IP (if possible): Limit API access to specific trusted IP addresses or networks in Syncthing's configuration.

Attack Surface: Disabled Encryption for Synchronized Folders

  • Description: Syncthing offers the option to disable encryption for synchronized folders. Disabling encryption, especially over untrusted networks, exposes data in transit to eavesdropping and interception.
  • Syncthing Contribution: Syncthing allows users to disable encryption per folder for performance reasons. This is a configuration choice within Syncthing itself.
  • Example: A user disables encryption for a synchronized folder containing sensitive data. An attacker intercepts network traffic between Syncthing devices and captures the plaintext data being transmitted.
  • Impact: Data breach, compromise of confidentiality.
  • Risk Severity: High (if encryption is disabled for sensitive data synchronized over untrusted networks).
  • Mitigation Strategies:
    • Always Enable Encryption: Unless there is a very specific and well-justified reason, always enable encryption for synchronized folders, especially when dealing with sensitive data or untrusted networks.
    • Regular Configuration Review: Periodically review Syncthing folder configurations to ensure encryption is enabled where required and that no folders are inadvertently configured without encryption.