You have probably noticed that all Macs equipped with a webcam have a small green LED that lights up whenever the camera is active / in use. This visual indicator, after all, is a very simple and effective way of preventing malware access the camera without the user's authorization, since the LED, as it is physically connected to the webcam, always lights up whenever it is on, calling the user's attention to the suspicious behavior.
However, if the Mac's camera is already active when the user is using FaceTime or Skype, for example, it is difficult to detect if there is a malicious source capturing this content secretly. That's what the former NSA employee discovered Patrick Wardle, which created a malware as a proof of concept to demonstrate that it is possible for a hacker to have access to the images and sounds captured by the Mac every time the camera / microphone is activated.
To alert users and fight invasions, Warden is making available a small Mac application called OverSight. What the utility does is to monitor the activity of the webcam and microphone, notifying the user whenever a new process accesses these components and giving the option to allow or block this access.
In other words, if a malicious program wants to record the contents of your webcam or microphone while you are using another application that activates these components, OverSight reports the malware and you can block it right away. It seems to be a slightly more elegant solution than Mark Zuckerberg's, for example and you can download it for free from the Warden website.
(tip from Vitor Coutinho)