Website: https://www.clamav.net/
Source Code: https://github.com/Cisco-Talos/clamav
"ClamAV® is an open-source antivirus engine for detecting trojans, viruses, malware & other malicious threats. It is the open-source standard for mail gateway-scanning software. It includes a multi-threaded scanner daemon, command-line utilities for on-demand file scanning and automatic signature updates. It supports multiple file formats and signature languages, as well as file and archive unpacking."