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# Get token in system.security.securestring format and decode it
$securestring = (Get-AzAccessToken).token
$bstr = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($secureString)
$plaintext = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($bstr)

$token = $plaintext.Split(".")[1].Replace('-', '+').Replace('_', '/')

while ($token.Length % 4) {$token += "="}
# Decode the token, and match the ObjectIds returned by Get-AzRoleAssignment with oid section of JWT. You can also manually decode the JWT and pass oid to Get-AzRoleAssignment -ObjectId $oid
Get-AzRoleAssignment | where ObjectId -EQ ([System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String($token)) | ConvertFrom-Json).oid