Forcing Invoke-WebRequest to use TLS v1.2. See comments on line 42+.

post-to-slack.ps1

@@ -39,6 +39,15 @@ if($icon -ne "No icon given") {
 Write-Output "payload:"
 Write-Output "$payload"
 
+# 2020-05-23: As of 2020-05-04, Slack is blocking TLS connections using TLS v1.1 or lower.
+# This was causing Invoke-WebRequest to error out with a 404 / WebCmdletWebResponseException error.
+# This may only affect systems with certain version of .NET (which is what Invoke-WebRequest relies on under the hood), due to it affecting the system's default TLS version.
+# But the line below should force TLS v1.2 to be preferred.
+# A better solution would be to change the system default TLS version to something higher than v1.1.
+# https://api.slack.com/changelog/2019-07-deprecate-early-tls-versions
+# https://www.reddit.com/r/PowerShell/comments/7zr0yu/invokewebrequest_windows_powershell_51_is_scaring/
+[Net.ServicePointManager]::SecurityProtocol = "tls12, tls11, tls"
+
 # Send REST call
 # https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/invoke-webrequest?view=powershell-5.1
 #Invoke-WebRequest -Body (ConvertTo-Json -Compress -InputObject $payload) -UseBasicParsing -Method Post -Uri $webhook