What is the dirty cow exploit?
The Dirty Cow exploit takes advantage of a recently discovered vulnerability in the Linux kernel that has existed since the inception of the operating system. COW stands for copy-on-write and it is a mechanism in the Linux kernal that manages the way modifiable resources are shared among multiple callers. The exploit allows a user to perform privilege escalation and gain root access to the system.
This issue also affects Raspbian (and thus most Raspberry Pi users), but luckily it's easy to Fix the Dirty Cow Exploit on the Raspberry Pi.