Bug: AWS::Serverless::Api automatically sets "arn:aws:iam::*:user/*" as integration execution role #8276

Description:

When creating an AWS::Serverless::Api with the following SAM template, the API Gateway integration request execution role is automatically set to arn:aws:iam::*:user/*.

This causes API Gateway → Lambda invocations to fail with a 500 error until the execution role is manually cleared in the console.

RestApi:
  Type: AWS::Serverless::Api
  Properties:
    Name: !Sub "${AWS::StackName}"
    Description: Application API
    StageName: !Ref Env
    Auth:
      DefaultAuthorizer: AWS_IAM
      AddDefaultAuthorizerToCorsPreflight: false

RestApiFunction:
  Type: AWS::Serverless::Function
  Properties:
    Runtime: nodejs22.x
    CodeUri: functions/rest-api
    Handler: src/index.handler
    Events:
      ApiGatewayProxy:
        Type: Api
        Properties:
          RestApiId: !Ref RestApi
          Path: /{proxy+}
          Method: ANY

Steps to reproduce:

1. Deploy the above template with sam deploy.
2. Inspect the API Gateway method integration in the AWS console.
3. Observe that the integration request execution role is set to "arn:aws:iam:::user/".
4. Call the endpoint → results in HTTP 500.
5. Manually clear the execution role in the console → the API works as expected.

Observed result:

• integration request contains "arn:aws:iam:::user/" as the execution role.
• API Gateway fails to invoke the Lambda function, returning 500 errors.

Expected result:

• For Lambda proxy integration, the execution role should remain empty.
• Setting DefaultAuthorizer: AWS_IAM should not populate any execution role on the integration.

Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS:MacOS
2. SAM CLI, version 1.132.0
3. AWS region: ap-northeast-1

{
  "version": "1.132.0",
  "system": {
    "python": "3.11.10",
    "os": "macOS-26.0-arm64-arm-64bit"
  },
  "additional_dependencies": {
    "docker_engine": "Not available",
    "aws_cdk": "Not available",
    "terraform": "Not available"
  },
  "available_beta_feature_env_vars": [
    "SAM_CLI_BETA_FEATURES",
    "SAM_CLI_BETA_BUILD_PERFORMANCE",
    "SAM_CLI_BETA_TERRAFORM_SUPPORT",
    "SAM_CLI_BETA_RUST_CARGO_LAMBDA"
  ]
}