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• Description of the vulnerability
• Steps to reproduce
• Potential impact
• Suggested fix (if any)

xfr is a network bandwidth testing tool. When running the server:

• The server accepts connections from any client by default
• Use --psk for mutual pre-shared key authentication and protected control messages
• Use --allow / --deny for IP-based access control lists
• Use --rate-limit to prevent abuse from individual IPs
• Consider firewall rules to restrict access
• Use --one-off mode for single-use testing

With PSK enabled, peers authenticate each other using HMAC-SHA256 proofs and post-auth control messages are protected with ChaCha20-Poly1305. TCP and UDP bulk payloads remain plaintext.

QUIC encrypts each TLS connection, but xfr uses a self-signed server certificate without PKI verification. Without a PSK, server identity is not verified and an active relay can terminate and forward the connection. With a PSK, xfr mixes the RFC 9266 TLS exporter into the authentication proof and protected-control keys, binding mutual authentication to the exact QUIC connection that carries the bulk streams. A relay cannot splice authentication across two QUIC connections without the PSK.

This binding is mandatory for QUIC + PSK: client and server refuse the session unless both advertise quic_channel_binding_v1, so upgrade both peers together. The self-signed certificate is still not a public PKI identity; use an authenticated VPN when you need centrally managed endpoint identities or isolation between different holders of a shared PSK.

To combine QUIC transport encryption with PSK-protected control:

xfr serve --psk "secretkey"
xfr <host> -Q --psk "secretkey"

Use a strong, high-entropy PSK (16+ random characters) to prevent offline brute-force attacks.

When running the client:

• Only connect to trusted servers
• Use --psk when connecting to authenticated servers
• Use -Q (QUIC) for encrypted transport
• Be aware that bandwidth tests can saturate network links