Open-source software is software whose source code is publicly available for anyone to inspect, modify, and redistribute under a permissive license. In cybersecurity, that transparency matters because defenders can review how the code handles encryption, authentication, logging, and network connections, and independent researchers can spot flaws that a closed system might hide.
Open-source is not the same as automatically secure. Public code can still contain bugs, weak defaults, or unsafe dependencies, and attackers may also study the same code to look for exploitable mistakes. The security benefit comes from reviewability: teams can audit the code, verify claims, and detect backdoors or hidden telemetry. In practice, open-source tools are common in VPNs, firewalls, and forensic utilities, where trust and technical scrutiny are part of the security model.