Let's say there's a program ("ABC") which is licensed under GPL. There's a similar program ("CAD") on the market, which I believe is a modified ABC. It is therefore in violation of the GPL.

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?

Looks related: What are the potential legal consequences for a company that has GPL violations? The answer to that question makes me suspect the answer to this question is "yes", but I'm not sure, since I can potentially be wrong about CAD.

  • In addition to the possibility of a separate license from ABC authors to CAD, do you have access to the source of CAD so you can reliably judge the similarity of the code? Copyright does not protect the ideas of how to approach or solve certain problems, it protects only the implementation. The authors of CAD can legally write their own implementation to even provide a similar user interface (unless the UI is protected by a patent). Commented Jul 3, 2020 at 19:48
  • 1
    @cbeleitesunhappywithSX yes, I have access to the source code.
    – Allure
    Commented Jul 4, 2020 at 1:37

2 Answers 2


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.*

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.

* 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.

  • "Two wrongs do not make a right" is the start of circular logic here. It starts by assuming the second action is wrong, which you then use to show the second action is wrong. You first need to show that the distributors of CAD have a copyright that can be violated. Secondly, you'd need to show that the GPL permissions granted by ABC are insufficient to copy CAD.
    – MSalters
    Commented Jul 3, 2020 at 12:32
  • @MSalters The author of CAD almost certainly has copyright on it, unless they are the US federal government or the changes were so small as to be insignficant. Copyright pretty much applies to all works of authorship.
    – D M
    Commented Jul 3, 2020 at 12:42
  • @MSalters Thanks. I thought it was obvious that the authors of CAD held the copyright to their derivative work (if not a useful one without ABC's permisison). It seems it's not that clear.
    – richardb
    Commented Jul 3, 2020 at 12:51
  • The bar for copyright is indeed low, and indeed the general rule would be to assume a copyright exists unless there are specific reasons to suspect otherwise. However, the existence of ABC gives a ground to challenge CAD's copyright claim, and at that point the burden of proof is on CAD. They might prove it, which is why I pointed out that there is also a second proof required for CAD to win. As the answer points out, the OP can't sue CAD, but that does mean any lawsuit between OP and CAD would have to be initiated by CAD and therefore CAD has the burden of proof.
    – MSalters
    Commented Jul 3, 2020 at 12:56
  • If the Software Freedom Conservancy wins their lawsuit against Vizio, then third parties will have a right to sue if they receive GPL code without source.
    – Someone
    Commented Dec 6, 2022 at 6:21

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.

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.

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.

  • There is a "clean hands" doctrine which gives OP an equitable defense, though, in the case where CAD does not have a proprietary license. Then CAD's copyright claim would be based on a prior copyright violation, which means CAD does not come to court with clean hands. It's even the worst possible case of dirty hands, since it's founded in the very same law. But yes, the OP cannot simply assume CAD is acting in bad faith, and there's no way to discover that before being sued.
    – MSalters
    Commented Jul 3, 2020 at 13:08
  • You forget ABCs copyright claim :-) And CAD does indeed have the copyright for everything CAD wrote, and GPL doesn’t allow you to take for example GPL licensed code on my hard drive and copy it - only I have the license to give you a copy.
    – gnasher729
    Commented Jul 4, 2020 at 16:40

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