Location: Vienna, Austria
Date: July 18th
Daytime: around 5pm
Begaviours: none, unfortunately. Just walking
I take these to be six legs and some kind of appendices. But I am not entirely sure.
This is a true bug larvae - thus, an insect - you correctly counted six legs :) Characteristics of true bug larvae are the combination of the rostrum, the wing pockets and the dorsal scent glands (although the latter both are hard to see on the photos).
I suspect the specimen to be a member of the family Coreidae as it exhibits typical (larval) characteristics of this family: a rhomb shaped body and long legs and antenna. Also spines are a common feature of this family.
Although a bit hard to tell from the photos, I find it to be most similar to Coreus marginatus, a common species in Central Europe. Comparing two of the specimen's photos with those showing C. marginatus from a similar perspective, shows that - besides the general habitus and coloration - the number and position of the dorsal spines (2 x 2 central and 2 x 3 lateral) match well. Also, the antenna are relatively large in comparison to the body (compare also this photo).
In comparison, a Leptoglossus occidentalis larvae (as this species was suggested in another answer) would have a generally more elongated habitus with thinner antennae. And, most strikingly, L. occidentalis larvae have a more-than-body-long rostrum.
For comparison, here are more photos of...
I think it could be this animal. It is the nymph of Leptoglossus occidentalis, the Western Conifer Seed Bug.
It fits in terms of size and the greenish-brown colour of the abdomen. Also the position of the spikes and the long antennae are a match, eventhough the antenna look a bit more flat and broad in your picture and less hairy.
This invasive species is originally from the US. It was first recorded in Vienna in 2006.