I've noticed that some FOSS licenses explicitly mention that they are irrevocable (as long as their conditions are met) and others are silent on this point. Does this matter? Are the licenses that don't mention it revocable at-will? Is the irrevocable term redundant? Or is it affected by other things?

Here's a few licenses that don't mention whether they are irrevocable:

Here are some that explicitly mention irrevocability:

  • GPL ("...and are irrevocable provided the stated conditions are met.")
  • Apache ("...a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to...")
  • CC-BY ("The licensor cannot revoke these freedoms as long as you follow the license terms.")
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    There's a similar question on Law.SE but the answer isn't great. – congusbongus Jun 14 '16 at 0:17
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    Great question! I think this would be most subject to the laws of the country of which the jurisdiction is. Also, that Law answer doesn't seem to go to far in-depth - it would be great to see some excellent answers here :) – Zizouz212 Jun 14 '16 at 1:16
  • This depends very much on the country and law system that you're interested in. The same license may be irrevocable in one country and revocable in another. It depends a lot on how "offer" and "acceptance" are handled. – Erwin Bolwidt Jun 14 '16 at 9:09
  • There was a great talk at FOSDEM 2018 about licences vs. contracts (bilateral and unilateral), and the implications for such issues as revocability, for three different jurisdictions (broadly, US, UK, and EU). LWN has an article on the talk (full disclosure: I wrote the article). – MadHatter Oct 26 '18 at 10:54

A non-exclusive copyright license (such as most FOSS licenses) can be revoked at any time only if there was no consideration involved. The United States Federal Circuit Court of Appeal took this on in Jacobsen v. Katzer in 2008 and ruled that there is consideration exchanged in the use of FOSS by a licensee. This indicates that an FOSS license that's silent on revocation is likely revocable only for violation of it's conditions.

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    This is a great answer; I learned a lot. My only recommendation is to include the relevant section about consideration directly in the answer here, which begins with: "Traditionally, copyright owners sold their copyrighted material in exchange for money. The lack of money changing hands in open source licensing should not be presumed to mean that there is no economic consideration, however..." (May I bribe you with a small bounty? :)) – apsillers Jul 18 '16 at 18:08
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    Wow! The judgment from the case is a pretty good read. Also, I edited your post to define what CAFC is - if the court has a different name, please edit it as such. One other thing, is that you could make this answer even more worthy of the bounty if you could quote various sections of that judgment and expand on the answer (hint hint) :) – Zizouz212 Jul 18 '16 at 23:02
  • Perhaps a stupid question: what does "consideration involved" mean? – GoodDeeds Oct 26 '18 at 9:49

This has been sitting idle for a while, but I came back upon it just now and happened to notice a similarity among the organizations listed in the Q. I would have just made it a comment, but it's too long. And having converted it to an answer it got even longer for completeness. Maybe it does help you understand what's being asked about, though.

Ignoring the WTFPL which doesn't talk about revocability because they are basically a wordy way of saying "This is Public Domain".

The two organizations that don't say anything about revocability have large staffs of lawyers, who write all their legal stuff; usually for B2B licenses. The others have 0 or 1 lawyer and are mostly concerned with end-user (i.e. consumer) licenses. I understand why this distinction between them results in the different language.

That understanding comes from having worked as support staff at MIT and having talked to one of the MIT licensing lawyers about some code I wrote to do my job and my desire to distribute it with a GPL license. At one stage the MIT lawyer was all set to allow it but with one final question, he had no idea what the phrase "TO THE EXTENT PERMITTED BY APPLICABLE LAW" (which at the time only occurred once and in all caps, now there are several variants in several places) was there for. To his mind having it there meant that the reverse might be true without it. But you obviously can't grant something in the license that is not "permitted by applicable law". So what was it saying? Well, the answer (from FSF's lawyer) was that this was standard required boilerplate in licenses to consumers who are not expected to realize this exemption and have lawyers to research it for them, whereas the MIT lawyer and all the company lawyers he deals with are such lawyers. There was case law (in the consumer realm) that a license without that disclaimer can be dismissed wholesale as an illegal license. Or more accurately chopped into little bits and discarded that way. And, since I was putting it up for FTP (it was pre-web) publicly on the net meant that the consumer oriented text was the right thing.

I expect there is a quite similar story behind the correlation between the irrevocable terminology and the organizations that included it. It may well be one of those case law in the consumer realm but never in B2B licenses differences. In fact, there may be a strong correlation in licenses that have one phrase also having the other.

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