I'd like to write a computer program that runs on a server.

This program leverages the popular Express.js library which is released under the Creative Commons Attribution-Sharealike 3.0 USA license.

Usage is subject to the following term:

If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.

Does this mean that any project or program using Express.js must also be licensed as CC BY-SA 3.0?

What are the implications of closed-source, commercial works that use Express.js? Are they legal?

  • Express is under the MIT license, which is very permissive. Only the website/documentation is under a CC license.
    – amon
    Nov 2, 2020 at 11:32

1 Answer 1


First of all, if the library is in fact under the MIT license (as a comment says, I haven't checked) then the only requirement is to include the proper copyright and permission statements with the software. There is no requirement to release associated software under the same license.

But assuming that this, or some similar library, is in fact released under CC-BY-SA 3.0, the question arises as to whether the developer's software "remix[es], transform[s], or build[s] upon the material" that is under the CC license. Only if it does so must it be released under the same license (the "share-alike" part of the license).

If the package includes a modified or altered version of the library, then it is clearly a derivative work, and must be made available under the same license. However, if it merely makes calls to the library, which could be installed separately, or indeed might already be on the machine where it is to be used, that would not be a derivative work, and would not require the closed-source software to be relicensed. If the supplier of the closed source software also includes a copy of the library, that library must be distributed under CC-BY-3.0, and the documentation must make that clear, but other separate software need not be.

I have been part of commercial development projects where proprietary code made calls to open-source libraries, and we had to get a sign-off from a company lawyer that this was OK and would not place the commercial product under a free license, and this was the essence of the legal opinion we got.

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