https://mosh.org/#faq:~:text=Q%3A%20What%20is%20Mosh%27s%20s...
The cryptography is standard AES-128 in OCB3 mode. It's been around long enough, and has had enough security scrutiny to at least discover a few minor DoS vulnerabilities, that it isn't entirely unreviewed.
For the cipher itself, see https://en.wikipedia.org/wiki/OCB_mode#Attacks