🍪 Cookies and CORS — When Are Cookies Actually Sent?

In the previous article , we briefly discussed the relationship between Cookies and CORS . In this article, we'll take a closer look at how browsers decide whether a Cookie should be included in a Cross-Origin request. One of the most common misconceptions is that once CORS is configured correctly, Cookies are automatically sent with every request. In reality, that's not how browsers work. 📌 Default Browser Behavior When a Cross-Origin request is made using fetch() or XMLHttpRequest , browsers do not send Cookies, Authorization headers, or other credentials by default. For example: fetch ( " https://api.example.com/profile " ) Even if the user is already logged into api.example.com , the browser will not include any Cookies with this request. This default behavior helps prevent authentication data from being unintentionally leaked across different Origins. 📌 How Can We Send Cookies? If you want the browser to include Cookies in a Cross-Origin request, you must explicitly use the credentials option. For example: fetch ( " https://api.example.com/profile " , { credentials : " include " }) Using credentials: "include" does not guarantee that Cookies will be sent. Instead, it tells the browser: "If there are any Cookies that are eligible to be sent with this request, include them." 📌 What Makes a Cookie Eligible? Even with credentials: "include" , the browser still evaluates the Cookie before sending it. Some of the most important checks include: Domain Path SameSite For example: If the Cookie's Domain doesn't match the request destination, it won't be sent. If the request path doesn't satisfy the Cookie's Path attribute, it won't be sent. If the Cookie's SameSite policy blocks Cross-Site requests, it won't be sent. In other words, credentials is only the first requirement , not the final decision. 📌 Server Configuration Matters Too If your application expects JavaScript to access the response while using Cookies, the server must also be configured correctly. For exampl