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TL;DR: Is it legal to use a modified version of the product licensed under LGPL 2.1, for commercial purposes and without the permission of the author (because the authors cannot be reached), and following the conditions as stated in the license.

Some context: I am a software engineer, not a lawyer, need help on understanding license LGPL 2.1.

  1. The project ZedGraph is licensed under LGPL 2.1, I made modifications to the software to fix a bug (meaning corrected a software mistake).
  2. Since I suspect the project is basically abandoned by the authors, I don't want to bother with the process of contacting the authors, waiting for the author to review my changes, integrate the changes, and finally publish a release (is it even necessary?)
  3. Thus, I want to update the current version of ZedGraph used by the company software, to the modified version that I made, without going through the process above.

Is this legal under LGPL 2.1? More specifically, is it legal to modify the product, without permission from the author, and use it for commercial purposes, given that we meet the conditions in the license?

According to the license, point 2 states:

"You may modify your copy or copies of the Library or any portion of it... provided that you also meet all of these conditions...", here "You" refers to the licensee (that's us?). I assume given that we follow strictly to the conditions, we may modify it, but the license never states if the modified version can be used for commercial purposes, less the author's permission.

Then on point 8:

"You may not copy, modify, sublicense, link with, or distribute the Library except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, link with, or distribute the Library is void, and will automatically terminate your rights under this License...", which I think just means if we follow the conditions in (2) for modifying the product, we are good to use it regardless if in a commercial product?

The real confusion for me is that the license never states whether using it in a commercial product is even allowed. I don't think the lack of clarification automatically means it is legal that modified versions of the product can be used for commercial purposes without the permission of the author, unless I am misunderstanding.

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Yes, you may do this. I would summarize your obligations as

  1. Ensure recipients of your software are given or offered the modified library code
  2. Ensure that recipients can rebuild your software using a further modified version of the library. This generally means offering the at least the compiled, linkable object code of your own application code alongside the library.

the license never states if the modified version can be used for commercial purposes

The (L)GPL is generally agnostic to commercial use. You must follow requirements regarding license/copyright notices and permitting specific downstream use, but whether distribution is done for pay or free of charge does not matter.

I assume"commercial use" here means redistribution for sale. If you never distribute software (e.g., your "commercial use" is running a private instance of some software that never leaves your workplace) then you have no obligations whatsoever.

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    One tidbit to consider is the name of the library. If you fork a project without consent you might consider to use a different name for the library while making very clear in the docs where this is forked from. See e.g. PIL vs pillow – planetmaker Mar 17 at 6:08

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