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SharePoint suddenly unable to login - starts multiple credential prompts - able to login on premise however.

Suddenly we are unable to login to SharePoint externally to our network.
Internally it works fine.
What could this be?
We had a similar issue at the same time with another server, our ADFS server.
It did the same but wouldn't allow login either on or off network.
I set ADFS to forms-based authentication and it now works but I don't have a root cause.
These must be related to SharePoint, correct??

Thank you for your help in advance!
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Most Valuable Expert 2015
Distinguished Expert 2018

FBA needs to be enabled for Extranet, if that was disabled for whatever reason, it explains the issue.

Is this SharePoint Online or on-prem? What version of AD FS are you on and what are you using for reverse proxy? Does the issue only affect SP, or other workloads (i.e. Outlook, Lync, etc)? Any changes on the AD FS server, claims rules, WIA accepted strings, anything else?


Vasi, it's an old on-prem SharePoint.  l thank you so much for the reply but I just figured it out.
I had two servers with repeated credential prompts (three prompts and then 401.1).
1.  ADFS
2. SharePoint

Turns out patch Tuesday hit Sunday night for this customer.
KB 3002657 :: So beware of issues with ADFS & SharePoint etc.  
I thought SPNs were the issue but it was the patch KB3002657. Removed and rebooted. Done.
Most Valuable Expert 2015
Distinguished Expert 2018
Yeah, KB3002657 broke more than just SP. Time to move away from those 2003 DCs :)