SYSTEM INITIATED:
Bridge Protocol Online.
The Trezor Bridge software operates as a shielded daemon, eliminating the vulnerability gap between your secure hardware and the untrusted host operating system. It’s the mandatory, cryptographic gatekeeper for all Trezor interactions. **Security is a service, not a feature.**
Analyze Threat Model V3.1THREAT MODEL: Hostile Local Environment
We operate under the assumption that the host machine—your PC, laptop, or server—is inherently compromised. Malware, keyloggers, remote access tools (RATs), and browser exploits represent **Layer 7 and Layer 4** attacks that aim to intercept or manipulate data before it reaches the hardware. The Bridge is designed to neutralize this entire threat vector, acting as an impenetrable membrane around the USB connection. By enforcing a **Strict Local Loopback Policy**, the Bridge ensures that commands can only originate from the authenticated Trezor Suite process, running on the same machine, and never from remote IP addresses or unverified local ports.
The primary threats mitigated include **USB Eavesdropping (Sniffing)**, where malicious drivers attempt to read raw USB packets, and **Process Impersonation**, where unauthorized software attempts to masquerade as the legitimate Trezor client to send fraudulent transaction requests. The Bridge's lightweight binary code minimizes its own attack surface, meaning less code for an attacker to analyze and exploit compared to a complex browser API implementation. This dedicated security microservice is the key to maintaining the integrity of the data handshake process.
**Threat Vector Closure:** Browser/OS kernel direct communication eliminated. Interface restricted to authenticated WSS/JSON-RPC relay.
01 // Protocol Decoupling (Isolation)
The Bridge physically and logically decouples the volatile browser stack from the reliable USB stack. The browser only interacts with a high-level WebSocket API. The critical **Trezor Wire Protocol (TWP)** segmentation, packetization, and USB handling logic is confined solely within the hardened Bridge process, isolated from the browser's memory space and inherent vulnerabilities. This separation is paramount in preventing cross-process contamination.
02 // Input Command Sanitization
Every JSON-RPC command received by the Bridge is subjected to an exhaustive validation schema check. This protects against **Fuzzing and Malformed Data Injection**. If a payload contains unexpected data types, out-of-spec length fields, or attempts to execute unauthorized commands (e.g., direct firmware manipulation), the Bridge drops the packet immediately and logs a security violation. This ensures the hardware device only receives clean, validated requests.
03 // Persistent Session Integrity Check
Throughout a signing session (which may take minutes for multisig or complex DeFi transactions), the Bridge continuously verifies the integrity of the connection. It detects unexpected termination signals or unauthorized connection attempts on the same port, prioritizing the established session and immediately terminating any concurrent, potentially hostile connections to maintain the single source of truth for the transaction flow.
CRYPTOGRAPHIC VALIDATION: Signature Enforcement Protocol
Binary Code Signing Verification
The very first layer of trust is the binary itself. Trezor Bridge binaries for Windows and macOS are authenticated using high-assurance, extended validation (**EV**) code signing certificates. The operating system's kernel performs a cryptographic check during installation and launch to confirm that the executable file's hash matches the digital signature provided by SatoshiLabs. Any tampering with the compiled code, even a single bit flip, invalidates the signature, and the OS will refuse to run the service. This defends against sophisticated **supply chain attacks** where malware attempts to replace the official executable with a backdoored version, a crucial defense often overlooked by less security-focused applications. The user should always verify the publisher name during installation.
Local WSS (TLS) Encryption Layer
While the connection is local (`127.0.0.1`), the Bridge mandatorily utilizes a **WebSocket Secure (WSS)** connection, meaning all traffic is encrypted via TLS 1.2 or higher. The Bridge generates a unique, self-signed SSL certificate upon first run. The Trezor Suite client is configured to accept connections only using this established protocol. This prevents local eavesdropping by other processes on the host's operating system which might try to sniff TCP/IP packets on the loopback interface. Furthermore, the use of TLS ensures forward secrecy and prevents replay attacks. The encryption protects data integrity and confidentiality during the transit of the unsigned transaction data and the vital signed output. This cryptographic membrane is the core defense against user-space malware.
Device & Host Identity Validation
Upon connection, the Bridge interrogates the connected hardware device to retrieve its unique identifier (UID) and firmware signature. This data is relayed to the Trezor Suite. Simultaneously, the Trezor Suite client authenticates its own connection using a unique **Ephemeral Session Key**. This two-way validation confirms three critical things: **(1)** The hardware is a genuine Trezor device, **(2)** the Bridge is communicating with the authorized Trezor Suite process, and **(3)** the entire channel is mutually authenticated before any sensitive payload transfer begins. If the device firmware is unverified or the Suite process signature is invalid, the Bridge refuses to open the USB communication tunnel, signaling a severe security anomaly to the user.
DIAGNOSTICS & FORENSICS: Bridge Log Analysis
Understanding Bridge Log Output
The Bridge maintains a detailed, time-stamped log of all its activities, communication attempts, and security events. This log file (usually found in the user's application data directory, e.g., `%APPDATA%/Trezor Bridge/log.txt`) is the essential tool for advanced troubleshooting and forensic analysis. When submitting a support ticket, including these logs allows our security engineers to precisely pinpoint the point of failure: whether it was a driver conflict, a failed cryptographic handshake, or an unauthorized connection attempt. The logging level is configurable, but the default setting is designed to capture all critical security state changes without logging sensitive payload data. **No private keys or transaction specifics are ever written to the log file.**
A healthy log shows consistent "Connection Established," "WSS Handshake Success," and "TWP Packet Acknowledged" entries. Anomalies like repeated "Client Authentication Failed" or "USB Access Denied" point directly to configuration or malware interference. The Bridge's logging mechanism is built with security in mind: it uses append-only writing and implements time-based rotation to ensure logs are never overwritten in a way that could conceal a sustained attack. Analyzing the log for unexpected outbound connections is also key, as the Bridge should **never** attempt to contact external network addresses, only the local loopback interface. This logging discipline adds a verifiable, auditable trail to every interaction between your software environment and your secure hardware, thereby completing the final component of the zero-trust architecture.
Example Log Console Snippet
DEPLOYMENT: Installation Integrity Check
The installation process is as critical as the software itself. We ensure full deployment integrity through digitally signed installers and verified checksums, completing the trust chain from our servers to your operating system.
DOWNLOAD_EXECUTABLE_V2.4.1File integrity hash: SHA256:32F85C...A34B7D (Verify upon download)
Installer uses EV Code Signing. Requires Admin rights for initial driver installation only. Runs as low-privilege service after reboot.
Fully Apple Notarized. Runs as a launch agent. Verify the application bundle integrity post-install using the macOS security settings.
Requires correct UDEV rule implementation for USB access rights. Packages are GPG signed for repository security validation.