Dual licensed (LGPL, commercial) library and contributions

Question

I have dual-licensed library, LGPL 3 and commercial license. Someone downloads the LGPL version, makes some improvements and sends back few patches. So far so good. But what now? It's legal to just merge those patches into code base and use them under commercial license?

I know some project (Qt, Canonical) have contributor agreements to handle exactly this cases. But what's the situation if this library doesn't have contributor agreements?

Thanks

Answer 1

IANAL, but as far as I understand you should ask the contributer to assign the copyrights of the changes to you, refusing to merge if they doesn't. Once the copyrights are you, you can distribute in whatever license you want.