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Currently, all user submitted content is licensed under "cc by-sa 3.0 with attribution required". This shows up at the bottom of each page:

user contributions licensed under cc by-sa 3.0 with attribution required

Is material that is submitted to Stack Exchange under this license compatible with the GPL?

The reason that I ask is to clear up confusion about attribution and what seems to be something akin to the BSD-3 clause license (the "obnoxious BSD advertising clause") that isn't GPL compatible.

I am wondering if the requirement to add and maintain an attribution within the source in a way that is specified by the licensor falls into the same classification as the BSD 3-clause license's endorsement. From "Attribution required" Stack Overflow blog post:

Attribution — You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work).

...

So let me clarify what we mean by attribution. If you republish this content, we require that you:

  1. Visually indicate that the content is from Stack Overflow or the Stack Exchange network in some way. It doesn't have to be obnoxious; a discreet text blurb is fine.
  2. Hyperlink directly to the original question on the source site (e.g., http://stackoverflow.com/questions/12345)
  3. Show the author names for every question and answer
  4. Hyperlink each author name directly back to their user profile page on the source site (e.g., http://stackoverflow.com/users/12345/username)
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Warning: It seems that this isn’t possible, because the provision (that only "the last license applied" has to be followed) was introduced with CC BY-SA 4.0, so CC BY-SA 3.0 doesn’t allow it. Please see Trevor’s answer.


If you adapt¹ content licensed under CC BY-SA 3.0, you can (additionally) license your contribution under GPLv3 (but not the other way around).

See creativecommons.org’s Compatible Licenses:

  • CC BY-SA 3.0 is compatible with CC BY-SA 4.0.

  • CC BY-SA 4.0 is compatible with GPLv3.

(¹ See my question What counts as adaptation for using code licensed under CC BY-SA 4.0 in software licensed under GPLv3?)

  • 2
    Are you sure that is a valid path? While additional contributions in CC BY-SA 3.0 may be licensed as part of a CC BY-SA 4.0 work. The wording at creativecommons.org/compatiblelicenses# is "Your contributions to adaptations of BY-SA 3.0 materials may only be licensed under BY-SA 3.0, or a later version of the BY-SA license" - but this doesn't mean that the original material is now CC BY-SA 4.0. CC BY-SA 3.0 doesn't allow relicensing of the material. My reading of this is that the original material is still CC BY-SA 3.0 and no non-CC license are compatible with it. – user756 Dec 18 '15 at 0:58
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    @MichaelT: I’m not sure I understand … do you wonder if it’s allowed to take two steps at once, or do you question if even the first step is allowed? -- As far as I understand it: if I adapt a work licensed under CC BY-SA 3.0, I can share my adaptation of this work under CC BY-SA 4.0. And if I adapt a work licensed under CC BY-SA 4.0, I can share my adaptation of this work under GPLv3. And taking both steps at once should be possible, too (you could think of publishing an intermediate step with the first half of your adaptation). All of this doesn’t change the license of the original work. – unor Dec 18 '15 at 18:25
  • It is my understanding from wiki.creativecommons.org/wiki/4.0/Treatment_of_adaptations that from "The creator of Adapted Material may choose the license that applies to his or her contributions. However, both the CC license on the original work and the license that the adapter uses for the new contribution apply to the use of the Adapted Material (in addition to whatever third-party rights may apply)." the original CC BY-SA 3.0 license requirements apply to the original work. – user756 Dec 20 '15 at 1:44
  • ... This is again stated in "License obligations of downstream users" that "As explained above, when someone creates an adaptation of a CC-licensed work and licenses it, the license on the adaptation only covers the adapter’s contributions and does not extend to the original content. ... In other words, the person using the adapted work is the licensee under two separate licenses -- one from the adapter with respect to the new elements (i.e. the adaptation), and one from the original creator with respect to the original." – user756 Dec 20 '15 at 1:44
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    @MichaelT: In ShareAlike compatibility there is the FAQ How does ShareAlike compatibility work?, which says: "[…] However, downstream users of the adaptation may choose to look ‘only at the compatible license (that is, the last license applied) to determine what their attribution and ShareAlike obligations are under both licenses when they reuse the work." -- So if I understand it correctly, while both licenses apply, others (who want to share/adapt) only have to follow the rules from the compatible license. – unor Dec 20 '15 at 6:11
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No, none of the CC-BY-SA v3.0 licenses (Unported and Ported alike) are compatible with any of the GPL licenses (unless you have a fair use exception such as a very short code snippet).

Below is a long explanation on why one cannot "license wash" CC-BY-SA v3.0 code to the GPL v3.0 via adaption of a CC-BY-SA v3.0 project by a new CC-BY-SA v4.0 contribution and then further adaption of that adaption by a GPL v3.0 contribution and then downstream users choosing to comply with the terms of the CC-BY-SA v3.0, CC-BY-SA v4.0, and GPL v3.0 license stack by only complying with the terms of the GPL v3.0 license since GPL v3.0 contributions are simply not allowed to occur in the first place by the terms of the CC-BY-SA v3.0 license. Note that although the CC-BY-SA v4.0 has a mechanism to allow adaptions to effectively be re-licensed under compatible licenses including one-way adaptions to the GPL v3.0 and CC-BY-SA v3.0 has a mechanism to allow contributions to an adapted work be licensed under later versions of the CC-BY-SA licenses such as the CC-BY-SA v4.0 license if you do adapt a CC-BY-SA v3.0 in such a manner the licenses stack (i.e. the original code in the adaption are licensed under the CC-BY-SA v3.0 and the new contributed code are licensed under the CC-BY-SA v4.0 license) and there is no provision in the CC-BY-SA v3.0 where downstream users can elect to comply with it by only complying with just the terms of the CC-BY-SA v4.0 (this kind of provision was new with v4.0). Hence if someone adapts CC-BY-SA v3.0 code by making a CC-BY-SA v4.0 contribution another person cannot further adapt the adaption by then making a contribution under the GPL v3.0 since the GPL v3.0 has not been deemed by Creative Commons to be compatible with the CC-BY-SA v3.0 and hence such a contribution is simply not allowed under the entire license stack of the adaption. Relevant quotes from Creative Commons:

In version 4.0, CC added a provision in the ShareAlike licenses that enables downstream licensees to refer only to the adapter’s license when using adapted material that contains the copyrightable contributions of multiple authors. This feature is designed to minimize complexity for reusers where they are using a later version of the ShareAlike license or a compatible license as their adapter's license. In 4.0, users need only refer to a single set of conditions contained in the last license applied to reuse adapted material, rather than parsing the conditions of the original and other adapter's licenses (to the extent the licenses differ).

In all cases, the licenses stack (the later license does not supplant all previously-applied licenses) when adapted material is created. In particular, the license originally applied to the material being remixed continues to apply once remixed, however permission is given in 4.0 for licensees to meet the conditions of the 4.0 license with reference to those in the adapter's license.

Prior to the 4.0 versioning process, CC had not always been clear that the ShareAlike licenses stacked just as they stack for the BY and BY-NC licenses, and reasonable minds do differ on this point. CC believes, however, that this is the best reading of its all of its licenses that permit adaptations prior to 4.0 and, now, has made that explicit in version 4.0.

https://wiki.creativecommons.org/wiki/License_Versions#Compatibility_mechanism_in_BY-SA_licenses

  • Interesting, glad you answered :) I guess I don’t fully get it yet, I hope you don’t mind my questions. -- If I understand the FAQ How does ShareAlike compatibility work? correctly: if someone made BY-SA 4.0 contributions to a BY-SA 3.0 work, both licenses apply. But people that get this adaptation may decide to follow only "the last license applied", so they may attribute it only according to BY-SA 4.0. Now, if someone gets this work, he never learns that it’s also licensed under BY-SA 3.0. -- Would you agree? – unor Apr 26 '18 at 22:17
  • The "last license applied" provision was only added in CC BY-SA 4.0 so that if someone adapts a CC BY-SA 4.0 work under a compatible license a downstream user may select that compatible license. That provision is not present in CC BY-SA 3.0 and downstream users may not choose to only comply with the "last license applied" but must comply with both CC BY-SA 3.0 as well as the "last license applied". Furthermore, the user should know that it is also licensed under CC BY-SA 3.0 since proper attribution is a requirement of the license and license details should be part of an attribution. – Trevor Apr 26 '18 at 22:22
  • Technically a downstream user could have to comply with a whole stack of licenses like CC BY-SA 2.0 plus CC BY-SA 2.5 plus CC BY-SA 3.0 plus the "last CC BY-SA 4.0 compatible license applied" which is why the provision got added to CC BY-SA 4.0 in order to simplify matters for downstream users but does not retroactively apply to earlier CC BY-SA licenses. – Trevor Apr 26 '18 at 22:24
  • One could alternatively think in terms of compatible "license stacks". CC BY-SA 4.0 + GPLv3 is a compatible stack as is CC BY-SA 3.0 + CC BY-SA 4.0 as is CC BY-SA 2.0 + CC BY-SA 2.5 + CC BY-SA 3.0 + CC BY-SA 4.0. However CC BY-SA 3.0 + GPLv3 is an incompatible stack as is CC BY-SA 3.0 + CC BY-SA 4.0 + GPLv3 as is CC BY-SA 1.0 + CC BY-SA 4.0. – Trevor Apr 26 '18 at 22:30
  • I see (I thought the "With version 4.0" in the linked FAQ applied only to the sentence it appears in, but reading it now again, I guess it applies to everything that follows). -- About the draft where they added this provision in 4.0, CC commented: "This change, which we believe comports with common practice under 3.0, […]". Do you have an idea what they meant with this? Did they assume that it would be allowed for 3.0, too, even if not stated? – unor Apr 26 '18 at 22:38

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