Leaked giveaway - how to track down the traitor ?

The NSA could do the trick.

Well, Mr. Sarif, why don't you ask your head of security?

Sounds like an alt

I don't know how you can track down the traitor. But at least report the mystery guest who joined for the points you mentioned.
Regifting, tricking account value and multiple accounts are all not allowed. (Everything will be confirmed by support of course. Especially since you cannot confirm multiple accounts yourself)

I suggest to hire one of my dwarf warriors mystery solvers. They will find and punish traitor in no time!

Maybe support can help you see what IPs accessed the giveaway, and where from.

One could just brute force guess giveaway links. Unlikely, but possible.

There is no way to track who gave it away without having a keylogger on the traitor's PC at the time of the leak.

I suggest that the next time you create a giveaway that is meant only for certain known people is that you make a steam group, add them to the group, then create a group giveaway.

Contact FBI. They know all, see all. (Being paranoid doesn't mean they aren't out to get you) :P

its probably one of the people you shared with using multiple accounts to get more entires. so they didnt leak they just cheated

Don't know how many people you shared the giveaway with, but maybe check who of them has most of the games, the invalid guest has won. Chances are he used the won games on his main account.

check and see if he's friends with anyone that entered

Just reroll if its him

Close the giveaway, make a private group - ask everyone who you shared the giveaway to enter the group.
You wouldn't find out who did that, but you would be protected from this in the future .
As for finding out - as mentioned before, ask the support .

thanks for your advice everyone, closing now.