An issue was discovered in IO FinNet tss-lib before 2.0.0. The parameter ssid for defining a session id is not used through the MPC implementation, which makes replaying and spoofing of messages easier. In particular, the Schnorr proof of knowledge implemented in sch.go does not utilize a session id, context, or random nonce in the generation of the challenge. This could allow a malicious user or an eavesdropper to replay a valid proof sent in the past.