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We want to develop an Outlook-Addin with a connection to a On-Premise SharePoint. The SharePoint uses NTLM as standard Authentication-Method. The question is, is it possible to use NTLM in the Outlook-Addin as Authentication-Method for the SharePoint from the client side (JavaScript)?

I found a project called ntlm.js on GitHub but it didn't worked for me, because I get no challenge back in the WWW-authenticate header field.

I'm pretty clueless. Is it possible in anyway to perform an Authentication via NTLM with JavaScript from an Outlook Addin?

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Any Ajax/HTTP JavaScript request you'll made from the browser/OL add-in will automatically support NTLM authentication (with a behavior -SSO or credentials-prompt) depending on the client.

However, if your JavaScript code is loaded from say domain A, unrelated to your on-prem SharePoint farm, calls to domain B (your on-prem SP farm) will fail because of the "cross-domain call problem" aka "same origin policy" that forbids such scenarios. But this has nothing to do with NTLM/SharePoint/add-ins: it's a matter of browser security.

See https://dev.office.com/docs/add-ins/develop/addressing-same-origin-policy-limitations for possible workarounds.

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