Open source software is software whose source code is publicly available for use, review, and modification. Anyone can inspect how it works, reuse parts of it, and contribute fixes or features, which is why OSS is widely used in applications, infrastructure tools, and security products.
In cyber security, OSS matters because it is part of the software supply chain. Defenders need to know which open source components are in use, who maintains them, and whether known vulnerabilities affect them. Attackers also target OSS ecosystems by exploiting unpatched dependencies, abusing compromised packages, or hiding malicious code in trusted projects. Security teams reduce risk with inventories such as SBOMs, trust assessment of projects, dependency monitoring, and fast patching. Used well, OSS improves transparency and defense; used carelessly, it can create exposure that spreads across many systems.