AWS Lambda functions can provide temporary credentials for a user if the function is is assigned an execution role. These are passed via environment variables AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_SESSION_TOKEN.

The client authentication should check these environment variables if there is not an explicit user name and password provided before check the local instance metadata services.

References
https://docs.aws.amazon.com/lambda/latest/dg/configuration-envvars.html#configuration-envvars-runtime
https://boto3.amazonaws.com/v1/documentation/api/latest/guide/configuration.html#environment-variables