A bit of digging seems to show that this is related to the autofill functionality:
Perhaps its prepopulating your contacts to autofill?
I havent seen it yet.
Not sure. Can you point me to the procedure for getting a dump of the unencrypted https traffic? I guess that'd tell me what these clients are sending.
However, I just tested with a coworker who was hitting the proxy HARD to that site about a dozen per second and had them turn off autocomplete. Nothing chnaged until he restarted the browser then things were clean. Had he reenable autocomplete and restart the browser with the same tab set and ... presto, the 403's were back in force. So, your theory appears to be dead on.
To replicate it make an upload media types rule that denies application/x-gzip as a MIME ensured type on requests and embedded objects. (mine's in a rule called global media type filtering that's old enough I have no idea if it's a library rule or not). And login to gmail from Chrome on Windows. In my case in sparse throwaway gmail account I only saw a few such 403 blocks. Other users are just killin it with lots of requests per second. I wonder if that implies more google contacts? *shrug* I coudln't isolate the requests to any given site making them happen.
We've encountered this issue as well. Two PCs were crushing our two 5500 MWG servers. Disabling 'autofill' has mitigated the issue...for now.
I say "mitigated" because the issue isn't resolved. I'm concerned that all of the sudden we'll have 1,000 PCs doing this. Does anyone know if this is related to a specific version of Chrome? Is there any other information on the cause? Is there a permanent fix?
I'm the original poster and Jon was quite correct as turning off autofill functionality in the affected version of Chrome did alleviate the pain (but we didn't do it company wide).
Our problem went away with an ensuing Chrome update. Are you still seeing the issue in the latest Chrome or is it some slightly back level Chrome that's plaguing you?
We're using version 49.0.2623.75 m. Was your issue resolved in a previous release? Have re-enabled autofill?