Managed Identity Authentication causing issue in Azure SQL Database stored procedure

Question

To disable local auth for Azure SQL, I made changes to my code to use system-assigned Managed Identity of the Azure app service for connection instead of the usual username/password in the connection string.

But now, one of the stored procedures is throwing an error

String or binary data would be truncated

I tried two ways to connect to SQL Server through the Azure app service:

1. Using the new SqlClient NuGet package: Microsoft.Data.SqlClient
2. Using older package (System.Data.SqlClient) and fetching an access token for authentication.

Initially, I thought the issue might be because of the newer NuGet package, but even after modifying the existing code to stick with System.Data.SqlClient and adding extra steps for the token, the issue persists.

Strangely, the problem seems to be tied to Managed Identity authentication. Current service is using connection string with username/password and System.Data.SqlClient nuget in Production environment and is running seamlessly.

I double-checked the stored procedures, but no luck finding issues with values going over limits into the SQL Server tables.

I am not understanding the connection between using ManagedIdentity and failure of stored procedure with a completely different error.

• Version of the solution: .NET 4.7.2
• Version of SQL Server database - v12.0.2000.8

Has anyone else ran into this? Any ideas on how to tackle it would be appreciated.

Answer 1

Problem: The issue corresponds directly to the error message, indicating that we were attempting to insert a string value exceeding the column's capacity. Specifically, we utilized the "SUSER_SNAME()" function to retrieve the currently logged-in user's username and attempted to store this value in a "CreatedBy" column, which is of type varchar. However, the "CreatedBy" column has different character limits across different tables.

Originally, when authentication was managed via username/password, the retrieved values, typically around 15 characters, were comfortably within the column's limits. The problem arose upon transitioning to ManagedIdentity, where the resulting value from "SUSER_SNAME()" took the form "AppId@TenantID", extending to approximately 73 characters, thereby exceeding the column's capacity.

Identifying the root cause of this issue was challenging and time-consuming, as the affected table was not directly referenced in the executed stored procedure. Instead, it was nested within multiple layers of triggers.

Resolution: We've increased the "CreatedBy" column size and standardized this limit across all tables.

Answer 2

string or binary data would be truncated

As per this

"string or binary data would be truncated" that means the field is NOT big enough to hold your data.

If the maximal length of the target column is shorter than the value you try to insert, for example if the column is only varchar(7), so the value 01-Jan-2024 exceeds that size then you may get the above error. You should check the columns and their datatypes.

• Make sure that the data you are trying to insert or update through the stored procedure does not exceed the defined length of the columns in the respective tables. Review the stored procedure parameters and the data being passed to them.

• Double-check the stored procedure code for any insert or update statements. Ensure that the length of the data being inserted or updated is compatible with the column definitions.

• Ensure that the Azure AD Managed Identity used by your app service has the necessary permissions to execute the stored procedure and perform the required operations on the tables.