Sysax Multi Server versions prior to 5.55 contains a stack-based buffer overflow in its SSH service. When a remote attacker supplies an overly long username during authentication, the server copies the input to a fixed-size stack buffer without proper bounds checking. This allows remote code execution under the context of the service.
A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).