I will focus on Wikipedia in this answer to illustrate the argument made by Creative Commons and the Wikimedia Foundation that attribution-by-link-only may indeed be sufficient in some situations – of course, this is not legal advice by any means.
As user6726 already wrote (and contrary to the answer by Jack Aidley) "Wikipedia" is not the licensor. The Wikimedia Foundation's Terms of Use stated in their earliest version from 2009 (i.e., when content on Wikipedias started to be dual-licensed under GFDL and CC BY-SA 3.0):
Therefore, for any text you hold the copyright to, by submitting it, you agree to license it under the Creative Commons Attribution-ShareAlike 3.0 Unported License. For compatibility reasons, you are also required to license it under the GNU Free Documentation License (unversioned, with no invariant sections, front-cover texts, or back-cover texts). Re-users can choose the license(s) they wish to comply with.
For re-users pursuant to CC BY-SA 3.0, the individual authors of an article are the (only) licensors, see section 4(a) of the license: "You may not sublicense the Work."
Regarding attribution, section 4(c) of the license says the following (edited for readability, emphasis added):
c. If You Distribute [...] the Work [...], You must [...] 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 [...];
- (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 Section 3(b), in the case of an Adaptation, a credit identifying the use of the Work in the Adaptation [...].
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. [...]
In addition, the Wikimedia Foundation's Terms of Use suggest three means of attribution probably deemed "reasonable" (edited for readability, emphasis added):
As an author, you agree to be attributed in any of the following fashions:
- a) through a hyperlink (where possible) or URL to the article or articles you contributed to,
- b) through a hyperlink (where possible) or URL to an alternative, stable online copy which is freely accessible, which conforms with the license, and which provides credit to the authors in a manner equivalent to the credit given on this website, or
- c) through a list of all authors. (Any list of authors may be filtered to exclude very small or irrelevant contributions.)
In a blog post from March 9, 2009, Mike Linksvayer from Creative Commons writes that the attribution requirement by link has been introduced in version 2.0 of the licenses with wikis in mind. Erik Möller, deputy director of the Wikimedia Foundation in 2008–2015, explains in an email on March 20, 2009 how the Terms of Use are compliant with the license: Attribution-by-link-only works if authors do "not supply an author name for the purposes of attribution" (slightly longer explanation in a follow-up email).
Of course, if CC-licensed content is imported from third parties into Wikipedias, these parties have not agreed to the Wikimedia Foundation's Terms of Use. The English Wikipedia, for example, advises to use special templates to properly attribute imported works in the article. The German Wikipedia suggests to put such information in the edit summary (which the English Wikipedia also suggests for copying within Wikipedia).
Since another update to the Terms of Use on June 7, 2023, edited articles are licensed under CC BY-SA 4.0. This license even provides an example of a reasonable attribution in section 3(a)(2) (emphasis added):
You may satisfy the conditions in Section 3(a)(1) in any reasonable manner based on the medium, means, and context in which You Share the Licensed Material. For example, it may be reasonable to satisfy the conditions by providing a URI or hyperlink to a resource that includes the required information.
(In contrast, the OSMF Licence Working Group still decided to ask for explicit permission to attribute incoming data via their website which are licensed under CC BY 4.0.
See also: What does "in any reasonable manner" of the Creative Commons Attribution clause mean?)
The license "update" from 3.0 to 4.0 was possible because section 4(b) of CC BY-SA 3.0 provides as follows:
You may Distribute or Publicly Perform an Adaptation only under: [...] a later version of this License with the same License Elements as this License [...].
Note, however, that section 8(b) also says:
Each time You Distribute or Publicly Perform an Adaptation, Licensor offers to the recipient a license to the original Work on the same terms and conditions as the license granted to You under this License.
Therefore, re-users of such edited articles still receive the grant for the original material under CC BY-SA 3.0. It is sufficient, however, to only indicate CC BY-SA 4.0, per section 2(a)(5)(B) of that 4.0 license:
Every recipient of Adapted Material from You automatically receives an offer from the Licensor to exercise the Licensed Rights in the Adapted Material under the conditions of the Adapter’s License You apply.
Note that the text excerpt you quote...
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).
...is from an old version of the license summary, which is not a substitute for the license CC BY-SA 3.0. On November 26, 2013 the summary has been updated by Creative Commons to better reflect the actual license:
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.
