Prompted by this Meta Stack Exchange discussion

Stack Exchange uses the Creative Commons Attribution-ShareAlike 3.0 Unported license for user contributions. But in the footer of every page is a link to a blog post specifying how attribution should be given. Some of the restrictions specified are:

  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., https://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., https://stackoverflow.com/users/12345/username)

By “directly”, I mean each hyperlink must point directly to our domain in standard HTML visible even with JavaScript disabled, and not use a tinyurl or any other form of obfuscation or redirection. Furthermore, the links must not be nofollowed.

The relevant section (4c) of the licence itself says:

If You Distribute, or Publicly Perform the Work or any Adaptations or Collections, You must, unless a request has been made pursuant to Section 4(a), keep intact all copyright notices for the Work and provide, reasonable to the medium or means You are utilizing: (i) the name of the Original Author (or pseudonym, if applicable) if supplied, and/or if the Original Author and/or Licensor designate another party or parties (e.g., a sponsor institute, publishing entity, journal) for attribution ("Attribution Parties") in Licensor's copyright notice, terms of service or by other reasonable means, the name of such party or parties; (ii) the title of the Work if supplied; (iii) to the extent reasonably practicable, the URI, if any, that Licensor specifies to be associated with the Work, unless such URI does not refer to the copyright notice or licensing information for the Work; and (iv) , consistent with Ssection 3(b), in the case of an Adaptation, a credit identifying the use of the Work in the Adaptation (e.g., "French translation of the Work by Original Author," or "Screenplay based on original Work by Original Author"). The credit required by this Section 4(c) may be implemented in any reasonable manner; provided, however, that in the case of a Adaptation or Collection, at a minimum such credit will appear, if a credit for all contributing authors of the Adaptation or Collection appears, then as part of these credits and in a manner at least as prominent as the credits for the other contributing authors. For the avoidance of doubt, You may only use the credit required by this Section for the purpose of attribution in the manner set out above and, by exercising Your rights under this License, You may not implicitly or explicitly assert or imply any connection with, sponsorship or endorsement by the Original Author, Licensor and/or Attribution Parties, as appropriate, of You or Your use of the Work, without the separate, express prior written permission of the Original Author, Licensor and/or Attribution Parties.

In the Meta Stack Exchange discussion it is argued that Stack Exchange are trying to assert restrictions which are not part of the CC BY-SA licence:

  1. To hyperlink each author, in addition to the page itself
  2. To use a direct link rather than a shortener
  3. To not use nofollow

Does the CC BY-SA licence allow primary distributors to specify restrictions like these, or can secondary distributors ignore them and still be compliant with the licence?

  • Does the license allow it? Sure. Is the requirement in line with the purpose of the license? That's an opinion and in my opinion no. – jqning Jun 7 '15 at 13:33

The Stack Exchange "requirement" of a particular type of attribution is unlikely to be enforceable, given the terms of the CC BY-SA license.

The actual Creative Commons license, the part that governs the relationship between the parties, is the "legal code" license agreement. The CC BY-SA agreement contains two clauses relevant to the form of attribution.

One, as the question points out, is paragraph 4(c), which states that "credit may be implemented in any reasonable manner." This language is much less restrictive than the StackExchange blog post.

The other relevant clause is the last clause of the agreement, paragraph 8(e), which is a standard integration clause:

This License constitutes the entire agreement between the parties with respect to the Work licensed here. There are no understandings, agreements or representations with respect to the Work not specified here. Licensor shall not be bound by any additional provisions that may appear in any communication from You. This License may not be modified without the mutual written agreement of the Licensor and You.

In other words, any changes to the license, which would include any additional restrictions on the nature of the attribution that may be given (besides the "reasonable manner" specified in the license) are effective only if both parties agree to do so in writing.

The specific law applied from jurisdiction to jurisdiction is going to vary, but in general, in the United States, when you have a fully integrated agreement such as this license, a court will not look at anything outside the four corners of the agreement to determine the parties' rights and duties.

If a court finds this to be a fully integrated agreement, and refuses to consider "parol evidence," meaning statements outside of the contract, then StackExchange can post a page asking people to attribute content in a particular way, but no, they cannot require it.

  • 8(e) is very clear, isn't it! – curiousdannii Jun 7 '15 at 23:16
  • 1
    That said, Stackoverflow might be seen as offering an opinion that adding "nofollow" to the links would be "unreasonable" and hence would not satisfy the requirements of the license. Even if they're completely wrong, it's still their opinion what is a reasonable manner to give credit. "All" they need to do to enforce it is persuade the court that nofollow links are not "the URI specified by the licensor", or something of the sort. I can't really tell from the blog post whether their legal position is "nofollow links are unreasonable", or that they believe they can add new conditions. – Steve Jessop Sep 1 '15 at 7:59
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    @SteveJessop, with regard to user contributions, StackExchange is not the licensor. StackExchange is merely a licensee. – sampablokuper Dec 25 '15 at 3:50

You mention specifically the following stipulations as being perhaps beyond the scope of CC-BY-SA 3.0:

  • To hyperlink each author, in addition to the page itself
  • To use a direct link rather than a shortener
  • To not use nofollow

Let's go through them in turn, in relation to 4(c).

To hyperlink each author, in addition to the page itself

According to 4(c)(i), the name or pseudonym of the Original Author and/or of any Licensor designated parties must be supplied, but there is no requirement in CC-BY-SA 3.0 to hyperlink each author. I do not know if it could plausibly be argued that the URL for the relevant SO/SE user page for the author is a valid pseudonym for that author. It seems to me that you can satisfy 4(c)(i) by giving the author's SO/SE username, with no need to additionally supply the user's profile page URL.

To use a direct link rather than a shortener

According to 4(c)(iii), you must provide "the URI, if any, that Licensor specifies to be associated with the Work, unless such URI does not refer to the copyright notice or licensing information for the Work".

In the case of SO/SE, it might reasonably be argued that by submitting a contribution, the licensor is specifying a URI to be associated with the work, because every contribution to SO/SE is provided with its own unique URI. (For example, your question has the unique URI http://law.stackexchange.com/q/419 , which is manifestly not the same thing as e.g. http://v.gd/kuUea2 even though the latter points to the former.)

So, this stipulation seems to represent a plausible interpretation of the license.

To not use nofollow

The only ground on which this is a vaguely plausible interpretation of the license is if one considers the use of nofollow to be a violation of 4(c)(iii). I think a court would be unlikely to be convinced that it is a violation, but courts are surprising and unpredictable creatures. A cautious and reasonable person might well think it prudent to avoid the use of nofollow here.


Creative Commons Corp Human Readable Layer defines attribute (attribution) under SE's license type as follows:

Under the following terms:

Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.

ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original. No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.


You do not have to comply with the license for elements of the material in the public domain or (if) where your use (it) is permitted by an applicable exception or limitation.

No warranties are given. The license may not give you all of the permissions necessary for your intended use. For example, other rights such as publicity, privacy, or moral rights may limit how you use the material.

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