Fingerprint openx.net what is that?

[Black Tiger]

I'm just looking at my site and my little shield (next to the padlock) is purple just like it's here on this forum.

When having a look at it, is says fingerprints are present, which is: rtb.openx.net

but I've got no clue what that is. I don't have odd things on my forums, only Google Adsense.

I did use Viglink in the past but disabled that again. Also cleared cache, but no difference.

Anybody a clue on where that rtb.openx.net fingerprint is coming from or when it's not from Adsense, how to remove that from my forums?