The HTTP fallback for SIP connection in Lync must be disabled.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-223345	O365-LY-000002	SV-223345r1043178_rule		Medium

Description

Prevents from HTTP being used for SIP connection in case TLS or TCP fail.

STIG	Date
Microsoft Office 365 ProPlus Security Technical Implementation Guide	2025-03-05

Details

Check Text (C-25018r442254_chk)

Verify the policy value for Computer Configuration >> Administrative Templates >> Skype for Business 2016 >> Microsoft Lync Feature Policies "Disable HTTP fallback for SIP connection" is set to "Enabled".

Use the Windows Registry Editor to navigate to the following key:

HKLM\Software\Policies\Microsoft\office\16.0\lync

If the value disablehttpconnect is REG_DWORD = 1, this is not a finding.

Fix Text (F-25006r442255_fix)

Set the policy value for Computer Configuration >> Administrative Templates >> Skype for Business 2016 >> Microsoft Lync Feature Policies "Disable HTTP fallback for SIP connection" to "Enabled".