Years ago somebody created a bunch of really awesome and popular scripts. But they were not updated for a long, now they no-longer work (target platform were updated, and some changes are needed).

He hasn't released it under any license.  I want to fix the bug (currently, many of the target users can't use it), and post it on GitHub, preferably under a public-domain style OSS license. I wonder what the legal ramifications might be? 

I have sent an email to the author, but (let's say) he didn't reply to my email.

What we should do in the following this into 2 cases:


If the script is posted on a private website (without any source control).
If the script is posted on GitHub (without any licensing hints).


However, one can clearly see, that it seems open source - intended to be used/modified/whatever.  

Short answer: absolutely not.

Everything a person writes, whether it is software or text, is automatically under copyright.  The default state of any text is that it is completely owned by the author and no one has rights to do anything with it without express permission of the author.  A few decades ago, an author used to have to assert copyright in order to retain it, but this is no longer the case.

You can even see on sites like this legal text down there that states that I agree that this post I am typing is available under a certain license.  If that wasn't there, I'd retain all rights under the law.

Thus, if you cannot find any license information, then you cannot copy or modify it for any reason other than personal use.

Making something "open source" is a deliberate act and for you to treat it as such, you have to have found a license that tells you explicitly what your rights to the software are.  This is even true of "public domain" software.  That is, something is only "public domain" if it has either expired copyright (which mostly means it was written decades ago) or if the author has explicitly placed it in the public domain in writing.

In the case you describe, your only recourse is to contact the author and request that he allow you to do what you ask.  To do otherwise is flatly illegal and in theory could lead to damages.  (In practice, of course, you'd have to get caught.)

Edit: IANAL.  Talk to one if you intend to do this.
