Allow Basic Authentication prompts from network proxies
This per-user policy controls whether Office applications accept sign-in prompts when a network proxy requests Basic Authentication.
since 0.9.8.11
Purpose
Show registry keys (1)
HKCU\software\policies\microsoft\office\16.0\common\identity\basicauthproxybehavior
This per-user policy controls whether Office applications accept sign-in prompts when a network proxy requests Basic Authentication.
When the policy is enabled, the proxy can trigger a prompt for a user name and password. This can allow users to open files stored on web servers when network access passes through a proxy that supports only Basic Authentication.
When the policy is disabled or not configured, Office suppresses these prompts. A file cannot be opened if reaching it requires Basic Authentication at the network proxy. Instead, Office displays a message indicating that the sign-in method is not allowed or might be insecure.
Notes
Security
Enabling this policy removes the default protection against Basic Authentication prompts from network proxies. Basic Authentication uses a user name and password for requests, does not support multifactor authentication, and provides less protection against credential capture and reuse than modern authentication methods.
The policy does not designate a proxy as trusted or restrict prompts to individually approved proxies. An unexpected or manipulated proxy response can therefore produce a convincing credential prompt. Credentials entered in response can be exposed if the proxy or the communication path is not trustworthy.
Possible side effects
When the policy is disabled or not configured, users might be unable to open files directly from web resources whose network path depends on a proxy using Basic Authentication. Resources that use other supported authentication methods are unaffected.
Notes
This setting applies only to Basic Authentication prompts originating from network proxies. Basic Authentication performed directly by a web server is governed by the separate policy that allows prompts for specified hosts.
Microsoft documents the associated blocking behavior for Access, Excel, OneNote, Outlook, PowerPoint, Project, Publisher, Visio, and Word on Windows. It does not apply to Outlook using Basic Authentication with on-premises Exchange Server or Exchange Online. Windows-hosted file shares using NTLM, SharePoint Online, OneDrive, and on-premises SharePoint Server configured for modern authentication are also outside its scope.
Microsoft documentation
Microsoft Learn: Basic authentication sign-in prompts are blocked by default in Microsoft 365 Apps