In the footer of Stack Exchange sites, it is stated that user contributions are licensed under CC-BY-SA 3.0 with a contradictory and incompatible attribution requirement addendum.
The “attribution required” text links to a blog post dated 25-Jun-2009 by Jeff Atwood, which appears to be more of a personal rant than a serious license specification.
The post quotes part of an outdated human-readable summary of CC-BY-SA 3.0 that was revised on 26-Nov-2013 to better reflect the actual license wording. The post then sets out attribution requirements that are incompatible with, specifically voided by, or otherwise nulled by the Creative Commons license.
Section 3 (Subscriber Content) of the Network Terms of Service reiterates the attribution required blog post requirements as a list of 4 points:
a. You will ensure that any such use of Subscriber Content visually displays or otherwise indicates the source of the Subscriber Content as coming from the Stack Exchange Network. This requirement is satisfied with a discreet text blurb, or some other unobtrusive but clear visual indication.
b. You will ensure that any such Internet use of Subscriber Content includes a hyperlink directly to the original question on the source site on the Network (e.g., https://stackoverflow.com/questions/12345)
c. You will ensure that any such use of Subscriber Content visually display or otherwise clearly indicate the author names for every question and answer so used.
This specification of linking mechanics is extraordinary and beyond the scope of the license attribution requirements.
License modifications are excluded by the CC license legal code (the license wording), and Creative Commons trademarks and logos may not be used in conjunction with any derivative or adapted license.
Jeff clearly misunderstood the license and what the former human-readable summary meant by “attribute the work in the manner specified by the author”, which is (and always has been) expressly prescribed and limited in the license terms.
Creative Commons licenses are designed to unify and simplify licensing so that people know exactly how they can use content. Inserting arbitrary additional conditions defeats the object and spirit of Creative Commons.
Such a prominent case of distorting Creative Commons licensing, by specifying exact linking mechanics, erodes the purpose of Creative Commons, and encourages others to attempt to impose their own idea of “fair” linking with animated GIF images required.
Furthermore, there are distinct classes of copyright works involved and a lack of clarity in licensing.
Individual Post - each individual question, answer or comment, licensed by the contributing users (original author and any editing users). Copyright owned by contributor(s) and clearly licensed under CC-BY-SA 3.0.
Question Thread - each single question thread, made up of a question and one or more answers and/or comments. Copyright owned by all contributors who shape the thread and possibly partly owned by Stack Exchange (to the extent that the platform facilitates the structured creation of the thread as a Collection). This is a grey area. Is a Question Thread a collective/derivative work - an evolving document, with each post/contribution deriving context from the thread - or merely a collection of separate works? If it is considered a collection, the collection itself in this case would still be subject to the CC license, as it is within the scope of “user contributions” (plural) and users definitely build the collection document by their contributions.
Collection of Questions - substantial set made up of multiple questions or question threads and associated metadata - as a whole (entire database) or subsets organized by metadata (e.g. date, tag, status). Copyright in the collection is surely owned by Stack Exchange. How is this licensed? A collection of CC licensed works is not itself subject to CC licensing. Does Stack Exchange expressly license its rights in such collections, which is outside the scope of “user contributions”? Where is this license grant?
If Stack Exchange wants to impose its own attribution requirements or use restrictions, it should do so under a separate license agreement for its intellectual property rights in Collections not covered by CC licensing.