• Package Name: @azure/identity
• Package Version: 1.3.0
• Operating system: Linux
• nodejs
  • version: 14
• browser
  • name/version:
• typescript
  • version: 4.4.0
• Is the bug related to documentation in
  • README.md
  • source code documentation
  • SDK API docs on https://docs.microsoft.com

Describe the bug
A clear and concise description of what the bug is.

When attempting to use the @azure/identity package in an Azure Function (Linux) with a VNet integration the GET request to the /msi/token endpoint fails with a 500 status code and the following error message:

More details:
unknown_error(status code 500).
More details:
An unknown error occurred and no additional details are available.
    at ManagedIdentityCredential.<anonymous> (/home/site/wwwroot/node_modules/@azure/identity/dist/index.js:1301:23)
    at Generator.throw (<anonymous>)
    at rejected (/home/site/wwwroot/node_modules/@azure/identity/node_modules/tslib/tslib.js:115:69)
    at process._tickCallback (internal/process/next_tick.js:68:7)
  statusCode: 500,
  errorResponse:
   { error: 'ManagedIdentityCredential authentication failed.',
     errorDescription:
      'unknown_error(status code 500).\nMore details:\nAn unknown error occurred and no additional details are available.',
     correlationId: undefined,
     errorCodes: undefined,
     timestamp: undefined,
     traceId: undefined },
  name: 'AuthenticationError' }

I can see that the request is made to the following endpoint: http://169.254.129.5:8081/msi/token?resource=https%3A%2F%2Fstorage.azure.com&api-version=2017-09-01

To Reproduce
Steps to reproduce the behavior:

1. Create a function app with a system-assigned managed identity
2. Integrate function app with VNet
3. Attempt to retrieve a token using the @azure/identity library (const defaultAzureCredential = new DefaultAzureCredential();)

Expected behavior
A clear and concise description of what you expected to happen.

The function app should successfully retrieve a token.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.