Open-source license

The Open Source Definition, maintained by the Open Source Initiative (OSI), sets ten criteria that a license must meet to be considered open source. These include free redistribution, access to source code, permission to create derivative works, and no discrimination against fields of endeavor or persons or groups.¹ Licenses that meet these criteria are OSI-approved, and the OSI's certification is widely regarded as the benchmark for open-source status.

Open-source licenses differ from permissive licenses, which impose minimal obligations, and copyleft licenses, which require derivative works to be released under the same license. The choice of license affects how software can be adopted commercially and integrated into proprietary products.

The GNU General Public License (GPL) is the archetypal copyleft license, first released in 1989 by Richard Stallman. Its strong copyleft requires that any derivative work be distributed under the GPL, ensuring that modifications remain free. The GPL has been updated to version 3, which addresses issues like tivoization and patent retaliation.²

Permissive licenses, such as the MIT License and the Apache License 2.0, allow reuse with minimal conditions, typically only requiring preservation of copyright notices. The Apache License also includes an explicit patent grant, which protects users from patent litigation by contributors. The BSD licenses are another family of permissive licenses, with the 3-clause version prohibiting endorsement of derived products without permission.

Open-source licenses are copyright licenses, not contracts, which affects how they are enforced. In the United States, violations are typically addressed through copyright infringement claims, as seen in the landmark case Jacobsen v. Katzer (2008), which affirmed that license conditions are enforceable.³ In the European Union, the Court of Justice has ruled that open-source licenses can bind users even without explicit acceptance.

Compliance requires attention to attribution, copyleft obligations, and patent clauses. Companies often employ dedicated open-source compliance teams to manage license obligations, especially when combining code from multiple sources.

Beyond the mainstream licenses, there are niche variants and edge cases. The Creative Commons licenses are not recommended for software, but the CC0 public domain dedication is sometimes used. The Unlicense and the WTFPL are informal public-domain-like licenses, though they may be legally risky in some jurisdictions.

The Open Software License (OSL) and the European Union Public Licence (EUPL) are copyleft licenses with clauses tailored to specific legal systems. The EUPL is notable for being recognized by the European Commission and for its compatibility with the GPL. Another edge case is the Server Side Public License (SSPL), which is not OSI-approved but is used by MongoDB; it requires that software offering network services to third parties be released under the SSPL, a controversial provision.

Selecting a license depends on the goals of the project. For maximal adoption, permissive licenses like MIT or Apache are common. For ensuring that derivatives remain open, copyleft licenses like GPL or AGPL are chosen. The AGPL extends copyleft to network use, closing the 'SaaS loophole' that allows modified GPL code to be used as a service without disclosure.

Tools like choosealicense.com and the OSI's license list help developers compare options. Legal advice is recommended for complex scenarios, especially when contributing to existing projects or when patents are involved.

Copyleft

A licensing mechanism that requires derivative works to be distributed under the same license, ensuring that modifications remain free.

Permissive license

A license that imposes minimal restrictions on use, modification, and redistribution, often only requiring attribution.

Open Source Definition

A set of ten criteria that define open-source software, maintained by the Open Source Initiative.

Tivoization

The practice of using hardware restrictions to prevent users from running modified versions of software on their devices, addressed by GPLv3.

Patent grant

A clause in some licenses (e.g., Apache 2.0) that grants users a license to any patents held by contributors that are necessary to use the software.