Security Principles

Security is one of the primary project goals. This means that it should not be possible for an attacker to join a cluster uninvited, and it should not be possible to extract private information from intercepted traffic. Currently this is implemented as follows.

All device to device traffic is protected by TLS. To prevent uninvited devices from joining a cluster, the certificate fingerprint of each device is compared to a preset list of acceptable devices at connection establishment. The fingerprint is computed as the SHA-256 hash of the certificate and displayed in a human-friendly encoding, called Device ID.

Incoming requests for file data are verified to the extent that the requested file name must exist in the local index and the global model.

For information about ensuring you are running the code you think you are and for reporting security vulnerabilities, please see the official security page.

Information Leakage

Global Discovery

When global discovery is enabled, Syncthing sends an announcement every 30 minutes to the global discovery servers so that they can keep a mapping between your device ID and external IP. The announcement contain the device ID and listening port(s). Also, when connecting to other devices that have not been seen on the local network, a query is sent to the global discovery servers containing the device ID of the requested device. The connection to the discovery server is encrypted using TLS and the discovery server certificate is verified, so the contents of the query should be considered private between the device and the discovery server. The discovery servers are currently hosted by @calmh. Global discovery defaults to on.

When turned off, devices with dynamic addresses not on the local network cannot be found and connected to.

An eavesdropper on the Internet can deduce which machines are running Syncthing with global discovery enabled, and what their device IDs are.

The operator of the discovery server can map arbitrary device addresses to IP addresses, and deduce which devices are connected to each other.

If a different global discovery server is configured, no data is sent to the default global discovery servers.

Local Discovery

When local discovery is enabled, Syncthing sends broadcast (IPv4) and multicast (IPv6) packets to the local network every 30 seconds. The packets contain the device ID and listening port. Local discovery defaults to on.

An eavesdropper on the local network can deduce which machines are running Syncthing with local discovery enabled, and what their device IDs are.

When turned off, devices with dynamic addresses on the local network cannot be found and connected to.

Upgrade Checks

When automatic upgrades are enabled, Syncthing checks for a new version at startup and then once every twelve hours. This is by an HTTPS request to the download site for releases, currently hosted by @calmh. Automatic upgrades default to on (unless Syncthing was compiled with upgrades disabled).

Even when automatic upgrades are disabled in the configuration, an upgrade check as above is done when the GUI is loaded, in order to show the “Upgrade to …” button when necessary. This can be disabled only by compiling Syncthing with upgrades disabled.

The actual download, should an upgrade be available, is done from GitHub, thus exposing the user to them.

The upgrade check (or download) requests do not contain any identifiable information about the user or device.

Usage Reporting

When usage reporting is enabled, Syncthing reports usage data at startup and then every 24 hours. The report is sent as an HTTPS POST to the usage reporting server, currently hosted by @calmh. The contents of the usage report can be seen behind the “Preview” link in settings. Usage reporting defaults to off but the GUI will ask once about enabling it, shortly after the first install.

The reported data is protected from eavesdroppers, but the connection to the usage reporting server itself may expose the client as running Syncthing.

Sync Connections (BEP)

Sync connections are attempted to all configured devices, when the address is possible to resolve. The sync connection is based on TLS 1.2 or TLS 1.3. The TLS certificates can be obtained by an eavesdropper, although it is more difficult to do so in TLS 1.3. This means that the contents of the certificate are visible, which includes certificate Common Name (by default syncthing).

An eavesdropper can deduce that this is a Syncthing connection and under certain circumstances calculate the device IDs involved based on the hashes of the sent certificates.

Likewise, if the sync port (default 22000) is accessible from the internet, a port scanner may discover it, attempt a TLS negotiation and thus obtain the device certificate. This provides the same information as in the eavesdropper case.

Relay Connections

When relaying is enabled, Syncthing will look up the pool of public relays and establish a connection to one of them (the best, based on an internal heuristic). The selected relay server will learn the connecting device’s device ID. Relay servers can be run by anyone in the general public. Relaying defaults to on. Syncthing can be configured to disable relaying, or only use specific relays.

If a relay connections is required between two devices, the relay will learn the other device’s device ID as well.

Any data exchanged between the two devices is encrypted as usual and not subject to inspection by the relay.

Web GUI

If the web GUI is accessible, it exposes the device as running Syncthing. The web GUI defaults to being reachable from the local host only.

In Short

Parties doing surveillance on your network (whether that be corporate IT, the NSA or someone else) will be able to see that you use Syncthing, and your device IDs are OK to share anyway, but the actual transmitted data is protected as well as we can. Knowing your device ID can expose your IP address, using global discovery.

Protecting your Syncthing keys and identity

Anyone who can access the Syncthing TLS keys and config file on your device can impersonate your device, connect to your peers, and then have access to your synced files. Here are some general principles to protect your files:

1. If a device of yours is lost, make sure to revoke its access from your other devices.

2. If you’re syncing confidential data on an encrypted disk to guard against device theft, put the Syncthing config folder on the same encrypted disk to avoid leaking keys and metadata. Or, use whole disk encryption.