First DoS Flaw?

The CTSS project used a modified 7094 equipped with extra interrupts such as a timer interrupt. The instruction set included XEC, or ‘execute’ which addressed a command to be executed after which the instruction after the XEC would be obeyed. The instruction “XEC *” would execute itself and would never finish even for a pending interrupt. A user mode program issuing such a command would hang the system. This was perhaps the first Denial of Service flaw. What had changed to make this a flaw was the new notion that a system should remain in service despite obeying malicious programs. CTSS survived this wart fairly well.

Here is an oblique reference to the problem.