I think you might be misunderstanding me.

According to the CVE Numbering Athorities, there can be vulnerabilities that result in service being denied, and they refer to them as a denial-of-service vulnerability. For example, there can be a bug in a program that causes it to crash if you perform a certain set of steps/actions, thus resulting in the service being denied. Whereas traditionally, a DoS/DDoS attack is simply flooding a target with more bandwidth than they have available downstream bandwidth. Sending massive amounts of data to overwhelm a service is not the same thing as finding a unique set of actions to cause the program to crash.

So in theory, yes, a memory leak could amount to and result in a security vulnerability, like if the memory leak is reproducible and so severe it causes a service to crash.