The GNU General Public License (GPL) is a copyleft software license. It allows code to be used, modified, and redistributed, but any derivative work must keep the same license terms and provide corresponding source code. That makes the GPL different from permissive licenses, which impose fewer redistribution rules.
In cyber security, GPL matters because licensing affects software supply chains, code reuse, and how trusted components are shipped inside products. Security teams need to know whether a library or tool can be safely embedded in a proprietary system, audited, or replaced. Attackers can also exploit confusion around license obligations when organizations mix third-party code without tracking provenance. Defenders use GPL compliance checks, dependency inventories, and build reviews to keep the trusted core predictable. In operating systems, removing GPL-licensed code from a base system can simplify governance and clarify what belongs in the most trusted layer.