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Currently there's a discussion on Meta over whether or not StackExchange was allowed to retrospectively change their licensing policy from CC SA 3.0 to CC SA 4.0. Presuming that it is indeed an open question, how can people sharing content from StackExchange sites properly attribute back to the original author? The official CC wiki recommends the following format:

"10th Birthday Celebration San Francisco" by tvol is licensed under CC BY 2.0

But if I'm not sure if 3.0 or 4.0 applies to SE content, how do I write a proper attribution? Some possibilities include:

  1. is licensed under CC SA 3.0/4.0

  2. is licensed under CC SA 3.0 (possibly 4.0)

  3. is licensed under CC SA // ignore version altogether

  4. is licensed under CC SA (exact version unknown)

References to official statements by Creative Commons or to relevant copyright lawsuits are appreciated, if any are available on this subject.

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    Normally, the licence(s) that apply to material are available when you get them. May I ask how one could be sure material was under CC-BY-SA but not sure which version? – MadHatter supports Monica Nov 8 at 8:22
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    @MadHatter Well, just as in the motivating SE example: (1) the copyright holder, A, releases the content under CC-BY-SA 3.0. (2) B redistributes the content, but slaps a CC-BY-SA 4.0 licence notice on it. (3) You, C, get the content from B. – Emil Jeřábek supports Monica Nov 8 at 13:54
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The question seems slightly disingenuous to me. It purports to be about uncertainty in the licence version that applies, but in a specific case: SE's forced relicensing of all SE sites' content. The ambiguity here is not the version which is declared at distribution time, since that is clearly indicated. Instead, it seems to me that the heart of the question is

a) whether content contributed under CC BY-SA 3.0, with the contributor futher granting certain unlimited rights to SE "in order to provide the Services", can be relicensed by SE for general distribution under CC BY-SA 4.0, and

b) if it cannot, which licence should people reusing content from SE assume applies, and label their reused content with.

While there is uncertainty as to the answer to part (a), it is not an uncertain point. Either the terms on which contributions were made permit the relicensing - in which case it's now under CC BY-SA 4.0 - or SE has no permission, express or implied, to do it - in which case all contributions made prior to the change are under CC BY-SA 3.0. So I don't like any of the attribution options listed above.

Unfortunately, I'm no lawyer, and it would seem to me to have got way past the point where they should have been involved. I agree with the point made in the leading answer to the linked meta.SE post that, had SE any formal justification for this, they might reasonably be expected by now to have presented it, and that as the ones trying to initiate the change the onus is on them to do so.

So proceeding on the assumption that the relicensing was ineffective, I turn my attention to question (b). There is further ambiguity about when the licence change for newly-contributed content was effective: this post says that the ToS change to CC BY-SA 4.0 was made 2019-04-30, but those ToS as I read them don't specify a CC BY-SA version number at all. Moreover, the linked post noted that the individual site footers - which is all many contributors see - kept displaying "CC BY-SA 3.0" until 2019-09-05 by "oversight". That further blurs the status of contributions made between 2019-04-30 and 2019-09-05, though I lean towards concluding that contributions on individual sites were made under CC BY-SA 3.0 until 2019-09-05.

CC BY-SA 3.0 provides in s4b(ii) that derivative works can be relicenced under later versions of CC BY-SA, so posts originally predating the relicensing that have received substantive contributions after 2019-09-05 (and thus under CC BY-SA 4.0) can reasonably be taken to have been so relicensed.

It seems to me that any honest agent reusing content entirely predating 2019-09-05 should label it, on reuse, as subject to CC BY-SA 3.0. Content entirely postdating 2019-09-05 should be labelled CC BY-SA 4.0. Content predating 2019-09-05 but with substantive contributions after that date can reasonably be labelled CC BY-SA 4.0.

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You absolutely need to be sure which exact license applies before you use some content.

If you are not sure which license applies it means you don't have a license to use the content at all and would break the law by doing so!

The copyright law almost everywhere says that you are not allowed to copy/use/... anything someone else created. If you want to use the intellectual property of someone else, you need the permission of the creator/owner of the intellectual property. The burden of proof that you actually have this permission (a license) lies with you.

If you copy my intellectual property from this site and I sue you for copyright violation, you must be able to prove in a court of law that you have my permission to use my intellectual property.

"It's some version of CC SA. Maybe 3.0, maybe 4.0, I don't know, who cares anyway? I just copied it from this website where I knew they changed the license although it wasn't allowed by the former license and I don't care much." might not do to convince the judge that you actually have my permission.

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    But I am sure it's either 3.0 or 4.0. It cannot be anything else. What then? – JonathanReez Supports Monica Nov 8 at 15:08
  • @JonathanReezSupportsMonica then you need to find out the exact terms under which you are allowed to use the content and be able to prove that you actually got a valid license. – Josef Nov 8 at 19:47

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