How to return a 401 Unauthorized response from AWS Custom Lambda Authorizer

If you are using the async handler then, to return a 401 you have to:

• Set Lambda Environment Variable UNWRAP_AGGREGATE_EXCEPTIONS to 1 (Only if you are using the async handler)
• throw exceptions with a message starting with “Unauthorized”

So for the following async Handler:

public async Task<APIGatewayCustomAuthorizerResponse> Handler(
    APIGatewayCustomAuthorizerRequest request,
    ILambdaContext context)
    {
        throw new Exception("Unauthorized");
    }

The environment variable is set as follows:

Why?

Seems as though Amazon’s Api gateway uses a regex expression to look for the word “Unauthorized”^[1]. However, when exceptions are thrown by AWS Lambda, then they are wrapped in an AggregateException^{[2] and needs to be unwrapped.}

References:

[1] https://github.com/aws/aws-lambda-dotnet/issues/293

[2] https://hackernoon.com/how-to-unwrap-an-aggregateexception-thrown-by-aws-lambda-6652fd085952

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