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Generally, anyone who receives software under the GNU GPL may redistribute it however they please^*, and those recipients have the right to distribute it further, etc. This definitely allows recipients to post copies of the software to online servers to facilitate further distribution. It's not really correct to say this is done "without your permission" since you gave permission for your software to be redistributed in this way when you licensed it under the GPL.

The GPL FAQ further clarifies that the GPL does not even require a redistributor to notify you (nor does any FLOSS license require this).

In the case of GitHub in particular, there has been some speculation that their terms of service are technically incompatible with the GPL, but even the FSF itself has found the possibility very unlikely. (See Are the new GitHub Terms of service a kiss of death for open source projects?)

^* Redistribution is allowed as long as they meet the requirements to preserve the GPL notices/permissions downstream and that any binaries are accompanied by source; see MadHatter's answer as well.

APSillers' answer is excellent, but I'd add one thing: although by licensing under the GPL you gave permission to anyone to redistribute your code, you by the same mechanism required that such redistribution be under the relevant version of the GPL. If this github repository isn't clearly GPL'ed, then a copyright violation is occurring, and you as the rightsholder have every right to have this repository taken down.

Whether that's happening in this case we have no hope of saying without knowing the exact repo in question.

To address some of the questions in the follow-up edit:

For instance, what if Microsoft does something illegal

If someone is going to break the law and steal your code, then they are going to break the law and steal your code. It makes absolutely no difference whether the code is hosted on Github or any other site. Even if the code is only available on your personal website, somebody could download it from there and use it in violation of the open source license, for example by incorporating it into a closed-source project.

Licenses are not magic spells; they don't prevent somebody from using the code in an unlawful way. What they do is give you the ability to take that person to court if they violate your rights (assuming you find out about it).

like assign the copyright of my work to itself, or give itself rights that are not addressed by the GPL?

Microsoft cannot "assign" copyright of your work, nor can Microsoft "give itself rights" to your code, because it is not their copyright to assign in the first place. Only you as the copyright owner can legally assign your copyright to somebody else, and only you can choose to grant rights to other people through the choice of an open-source license.

If Microsoft were to claim rights to your code which you hadn't granted (either individually to Microsoft, or generally to everybody via the GPL) then they would be making a false claim and possibly breaking the law, at which point they could be sued or prosecuted depending on exactly what they did and which legal jurisdiction they did it in.

All Microsoft has to do is change their fine print for 1 day and they can claim that my code is theirs.

It doesn't work like that. Changing the terms on their website doesn't magically seize ownership of all code that is currently hosted on that site. If Microsoft did wish to change the terms of Github to require copyright assignment (which I'm 100% sure they will never do, because nobody would use it on these terms and the site would be dead in a heartbeat), the change would only apply to code that was uploaded after the new terms were put into effect, and would require explicit agreement by the copyright holder at the point where they uploaded the code. This would mean that Microsoft would probably need to delete all existing projects (including yours) that did not agree to copyright re-assignment.

Summary

Regardless of how you may personally feel about Microsoft, your concerns are unwarranted. Your code does not become vulnerable to some kind of "copyright seizure" simply by being on Github, because there is no such concept.

The only rights Microsoft have to your code is the rights you have chosen to grant via releasing the code as GPL. If you did not want them to have those rights, you shouldn't have released it as GPL in the first place.

Short answer: Probably not a violation.

Long answer (using the relevant part from GPL - since you did not specify the version):

You may copy and distribute verbatim copies of the Program's source
code as you receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice and
disclaimer of warranty; keep intact all the notices that refer to this
General Public License and to the absence of any warranty; and give any
other recipients of the Program a copy of this General Public License
along with the Program.

Did the uploader comply with the requirements above? Then it is not a copyright/license violation.

The ability to redistribute the source code code (to a site like GitHub) is a fundamental aspect of the GNU/GPL licence. If you wish to prevent someone from doing this, then perhaps releasing the project / code under a different licence would be worth while.

No, they can copy or fork your whole code into a new GPLed project, or copy or any parts of your code into other GPLed projects, and distribute it however they like.

I'm assuming that copyright notices remain intact on this copy of your project.

I think the only possible offense here is plagiarism, if they claim to have written it. That's a separate thing from copyright violation.

To simplify, let's consider another case: I publish a book of plays and poems. I put my name on it. But the works inside are word-for-word copies of Shakespeare, whose name doesn't appear anywhere. The work is in the public domain so there's no copyright claim to be made, but I've committed a new offense by claiming to have written them. (This is a kind of fraud against society, as well as or instead of against Shakespeare.)

This may not actually be a crime, I'm not sure. But if the project doesn't credit you, that's probably why you're feeling offended even though (from what you've said) there's no sign of a copyright violation.

See also https://en.wikipedia.org/wiki/Moral_rights for more about the moral rights of authors to get credit for their work, regardless of copyright law. Some countries even have legislation supporting that.

Possibly. The GPL requires distribution of build scripts and so forth. If source code only was uploaded and this was not sufficient to build the system then the licence conditions have not been met, so there would be no permission to distribute the source code.