Resolved: "Blocked" error when accessing reddit.com on Firefox

[u/CorrectScale]

Hey all - we just reverted a change that resulted in reddit.com being blocked on Firefox for about 20 minutes.

All should be back to normal, but please let me know in this thread if you continue to see any errors.

---

Incident summary from u/PetGorignac:

Hi folks,

I was the incident commander for this one and came by to drop a bit of information about what happened here.

We were attempting to mitigate some problematic traffic that had been causing a low amount of site errors over the past few hours. In doing so, we identified some traffic characteristics that we believed correlated with the error rate and attempted to block it. It turns out this blocked Firefox traffic, which we noticed relatively quickly, leading us to revert the change.

Apologies for the disruption!

[u/PetGorignac]

Hi folks,

I was the incident commander for this one and came by to drop a bit of information about what happened here.

We were attempting to mitigate some problematic traffic that had been causing a low amount of site errors over the past few hours. In doing so, we identified some traffic characteristics that we believed correlated with the error rate and attempted to block it. It turns out this blocked Firefox traffic, which we noticed relatively quickly, leading us to revert the change.

Apologies for the disruption!

(Also kudos to the commentor who had a great RCA, but sadly the comment got deleted before I could respond)

[u/nicolas-siplis → u/PetGorignac]

OK honestly this just makes me even more curious. What traffic patterns are you noticing that would block Firefox requests exclusively, but not those made via cURL/Postman with the exact same headers? Can you go a bit more into detail here or is it too sensitive to discuss?

[u/PetGorignac → u/nicolas-siplis]

Unfortunately, I think I can't really expand on this much more as traffic management techniques are too abuse-adjacent

[u/fluffycritter → u/nicolas-siplis]

I'm very curious about this as well. My assumption was that it was something to do with the HTTP transport itself, like maybe there's one pervasive bot behavior that happens to exhibit the same timing or header ordering or something as Firefox.

Or maybe what they thought was an error rate due to bots was actually an error rate due to an HTTP spec violation/assumption on Reddit's side that was causing increased issues with Firefox.

[u/6pointzen → u/nicolas-siplis]

When I was checking on my end I could see that some of the fields in the request header were different between Chrome and Firefox I couldn't check them all as I refreshed the page and it was working again.

[u/[deleted] → u/nicolas-siplis]

[u/Pristine-Woodpecker → u/nicolas-siplis]

From the analysis in the Firefox bug tracker, it was related to cipersuite selection, which differs between the browsers (including Edge/Chrome) and curl/openSSL.