Which method is NOT a common way to pass an API key to a server?

Passing an API key to a server in a secure and standard manner is crucial for protecting access and ensuring proper authentication. The most commonly accepted methods include sending the API key as a query string parameter, including it in a request header, or placing it in a cookie.

Using a query string parameter involves appending the API key to the URL being requested, making it visible in the URL. Including the key in a request header is the preferred method, as it keeps the key out of the URL and provides a cleaner and more secure way to transmit sensitive information. Cookies can also store API keys, especially in scenarios where session persistence is required.

In contrast, sending an API key as an email attachment is not a common or secure method to pass authentication credentials. Email attachments can be easily intercepted, compromised, or sent to unintended recipients, exposing the API key to significant security risks. Therefore, while the other methods align with standard, secure practices for API key transmission, using an email attachment does not fit into these common approaches.