It has come to light that two GTA 5 mods come bundled with malware.
GTA5-Mods, who verified the malware, explained it this way:
The Angry Planes mod was downloaded and run by thousands of people on GTA5-Mods.com and various other community mod hosting sites and forums. It was even featured in Kotaku and PC Gamer!. The mod did exactly what it advertised, however it also spawned a thread that installed malware on your machine behind the scenes. The threat was not picked up by running the .asi file through VirusTotal, which made it difficult to detect before running. This was an extremely sneaky attack, something I can’t say I’ve seen in 12 years of GTA modding.
If you used these mods, GTA5-Mods recommends you delete theme and look for a file called Fade.exe.