I believe [program] violates GPL, do I need to sue the author before using its source code?
<p>Let's say there's a program (&quot;ABC&quot;) which is licensed under <a href="https://en.wikipedia.org/wiki/GNU_General_Public_License" rel="nofollow noreferrer">GPL</a>. There's a similar program (&quot;CAD&quot;) on the market, which I believe is a modified ABC. It is therefore in violation of the GPL.</p>
<p>Am I allowed to take and use (or reverse engineer) CAD's source code without the author's consent, or do I need to sue CAD's author first for violating GPL?</p>
<p>Looks related: <a href="https://law.stackexchange.com/questions/25422/what-are-the-potential-legal-consequences-for-a-company-that-has-gpl-violations">What are the potential legal consequences for a company that has GPL violations?</a> The answer to that question makes me suspect the answer to this question is &quot;yes&quot;, but I'm not sure, since I can potentially be wrong about CAD.</p>

<p>It's possible that CAD has a separate licence from the authors of ABC that allows them to produce a closed source copy. If not, they have no right to distribute CAD. However two wrongs don't make a right, and so you don't get to violate the copyright of CAD.*</p>
<p>Unfortunately, unless you are one of the authors of ABC, you have no standing to sue the authors of CAD. You can only notify the authors of ABC and hope they do. If the authors of ABC don't have the resources to pursue the matter, you may be out of luck. That's one of the reasons the FSF gets copyright assignments for their projects.</p>
<p>* It turns out that this is a much more debateable issue than I first thought. Some courts have held that an unauthorized derivative work is not copyrightable.</p>

<p>Unless you are a copyright holder of ABC you have no rights to CAD whatsoever. You have no permission to copy their code, you have no standing to sue, and what you are planning can get you into deep legal trouble.</p>
<p>If you are a copyright holder of ABC you can sue them. You still have no rights to the portions of CAD that are not copies of your code.</p>
<p>That’s in case that CAD is indeed in violation of the GPL. But you cannot even be sure about that. The copyright holder of ABC could have sold them a proprietary license, which means you don’t even have an invalid excuse.</p>

copyright open-source-software gpl