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That XKCD is pretty darn relevant as always.
Your mention of window size/resolution etc reminded me of LibreWolf, a Firefox fork dedicated to countering fingerprinting methods like that and for general privacy. I can log in to Discover through that browser (no errors yet, but I haven't attempted to repeatedly test it for consistency ie logging in and out repeatedly), which brings me to the standard text message 2FA code. However, after *verifying with the text 2FA code* I was then presented with a requirement to verify with a government issued ID using some awful third party service and a phone's camera/app. I'm assuming this is *yet another* "security" addition, one which I won't be consenting to.
After waiting a few weeks since discovering that LibreWolf "works" (relevant to the issue at hand), it seems like it'll let me back in with LibreWolf now without this third party verification service. So, it seems like it's absolutely something on their end fingerprinting Firefox (and sometimes multiple Chromium browsers) for one reason or another. It might not be worth trying to chase that rabbit down any further at this point, especially since Discover could change what they're looking at for fingerprinting at any point. Still fails as of present in Chromium based browsers.
It's kind of ironic that LibreWolf *fixed* the issue here, as usually these counter-fingerprinting methods cause breakage instead. I wonder if it's just a simple screen resolution "issue" as you've stated? That's one of the only things that would be different from this computer to the others I've tested on, and LibreWolf does mask/spoof it. It also *might* offer some explanation to the inconsistent nature of this issue, perhaps I was running the Chromium and FlatPak Firefox browsers on different monitors under different sizes without realizing it? I did test to see if the monitor made a difference at points, but perhaps I didn't restart the browser between tests or fully reset the configuration between tests, or something. Not sure if that's exclusively the issue or not. Never would've expected that to be a reason for a bank to block my access though - it's so asinine that you couldn't make it up as a joke. What bot is going to use a less standard and more unique screen resolution? I wonder if Wayland vs X11 would have any impact on the resolution which Firefox would claim in multi-monitor setups? I would assume any resolution information that a server could see is purely from the browser window making the connection to the server, but perhaps X11 could claim a wider resolution due to the offsets of multiple monitors, or "adding" the resolutions together? I wouldn't know, but would assume that Firefox would only care about the active window of the tab and that X11 vs Wayland *should* be irrelevant.
I wonder if there's a way to try to isolate fingerprinting related issues to try to narrow in on the actual issues they're looking for. Then again, doing that might actually be bot-like actions, and could cause more problems than it's worth (but they're already seeing my normal logins as a bot login, so it's probably not going to cause any additional issues which aren't already present). Thank you all again tremendously for the help!
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For what its worth, I have the exact same issue: I can't use firefox nor chromium to log in to bank.discover.com. I tried new profiles with both browsers including without any extensions. I keep getting "We're sorry. Your request cannot be completed at this time. We sincerely apologize for this inconvenience and are working to resolve this issue soon.". Needless to say, I'm seeing that message for several months now.
Firefox on the pixel 4a with lineage OS works every time. Not great for CSV downloads but at least I can move my money.
I had previously spent time with discover's support without any (real) luck: They stopped helping once it worked on the phone.
Then it just occurred to me to try "Web" (I think it used to be called epiphany at some point) and that works every time.
Let me know if you want me to try some combination or such.
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