Your understanding is sort of mistaken. Mosh does an initial handshake over ssh, but after that it’s a custom UDP protocol. There have been and could be more security bugs in that UDP protocol.
The only mosh CVE [1] was in the terminal emulator (a DoS that could only be triggered by a local user), not in the protocol. There have been no vulnerabilities in mosh's UDP protocol.
[1] https://nvd.nist.gov/vuln/detail/CVE-2012-2385