2032198 - Firefox is the only browser that includes "rv:" in its UA string, which causes it to be blocked by some sites (perhaps due to being misidentified as IE)

Status: NEW

(Core :: Networking: HTTP, defect, P3)

Description

[Daniel Holbert [:dholbert]]

In bug 2004679 comment 4, janbrasna observed that the rv: in Firefox's UA string seems to trigger a UA-sniffing-based-blockage in some ASP.NET backend (possibly due to being miscategorized as IE11, which also included rv:). Apparently no other modern browser has rv: in their UA string, and IE used to have it, so sites might take it as a sign of your browser being IE.

Perhaps we should remove/reformat that portion of our UA string, to take a step closer to other modern browsers' UA-string-formatting (and a step away from IE's formatting), to avoid inadvertently tripping over this sort of UA-string-based block?

(Note, I don't know offhand if bug 2004679 was a one-off at just a one-off issue affecting only that single site, vs. a sign of something broader where more sites might have the same logic via some shared tool/library. So we might not want to take any action here, unless/until we find evidence of this impacting other sites beyond the one in bug 2004679.)

[Note: I'm filing this in Networking:HTTP to match the other UA-string-tweak bugs that I found - e.g. bug 1861847. Feel free to reclassify if there's a better spot for this]

Comment 1

[Chris Peterson [:cpeterson]]

We had a similar issue (bug 1805967) where some websites blocked Firefox versions 110–119 because the websites' IE11 checks for rv:11 matched rv:110.0 in Firefox's UA string.

For reference, here is IE11's UA string:

Mozilla/5.0 (Windows NT 6.3; Trident/7.0; rv:11.0) like Gecko

See also "remove rv:" bug 588913, resolved WONTFIX 13 years ago.

If we knew of more than one website broken by rv:, I would suggest we try removing it. But with only one known broken website, I'm on the fence. I will ask some questions in the webcompat bug.

I know of one UA string parsing library (ua-parser-js) that get confused if Firefox's UA string doesn't include rv:. It still correctly identifies Firefox and its version, but doesn't recognize that it's a Gecko engine. We could upstream a fix to them.

https://uaparser.dev/
https://github.com/faisalman/ua-parser-js

Comment 2

[Randell Jesup [:jesup] (needinfo me)]

Chris - if this turns out to be worthwhile, please mark it for re-triage by removing the whiteboard tag and resetting priority/severity. Thanks!