The GNU Affero General Public License (AGPL), version 3, released November 19, 2007, closes the saas-loophole — the gap in gpl-v2 and gpl-v3 that allows companies to run modified GPL software as a network service without releasing the modifications to users.
The loophole arises from the GPL's trigger: copyleft obligations attach when software is "distributed." Running software on a server and letting users interact with it over a network is not "distribution" in the GPL's legal sense — no copy of the software is transferred to the user. A company can therefore take GPL-licensed server software, make extensive proprietary improvements, and run it as a service indefinitely without ever releasing those improvements. This became an increasingly significant issue as web services supplanted desktop software during the gplv3-and-later-career-2006-present era.
The AGPL addresses this with section 13: if modified AGPL software is used to provide a network service, users interacting with the service must be provided a way to obtain the source code of the modified version they are interacting with. The trigger shifts from "distribution" to "use to provide a service to remote users."
The Affero General Public License existed before stallman and eben-moglen formalized it into the GNU license family. The Affero organization had released version 1 in 2002 under a similar provision as an amendment to GPL v2. The FSF's AGPL v3 of 2007 replaced this with a fully integrated license compatible with gpl-v3.
The AGPL has been adopted by a notable range of database, infrastructure, and application projects that want to ensure service providers cannot "embrace and extend" their code without contributing back: MongoDB (before its controversial SSPL shift), Nextcloud, Mastodon, and others. Its practical impact is debated: many large cloud providers simply avoid AGPL software entirely rather than comply, which is itself a form of the license working as designed — it makes free-riding by cloud providers more costly. stallman endorses the AGPL as the appropriate license whenever the GPL's freedoms need to extend to users of network services, not just recipients of distributed software. The essay who-does-that-server-really-serve expresses the philosophical case for this extension.