Access-Control-Expose-Headers HTTP Header: What You Need to Know
Access-Control-Expose-Headers is a response header in HTTP that allows servers to specify which response headers should be made available to scripts running in the browser. This header is particularly useful in the context of cross-origin requests, where a client application running on one domain requests resources from another domain. By default, only CORS-safelisted response headers are exposed to scripts running in the browser. However, if the server wants to allow clients to access other headers, it must list them using the Access-Control-Expose-Headers header.
The Access-Control-Expose-Headers header is closely related to the Access-Control-Allow-Headers header, which is used in response to a preflight request to indicate which HTTP headers can be used during the actual request. When a client sends a preflight request, it includes an Access-Control-Request-Headers header that lists the headers that it wants to include in the actual request. If the server allows these headers, it responds with the Access-Control-Allow-Headers header, which lists the headers that are allowed. If the server wants to expose any of these headers to scripts running in the browser, it must also include them in the Access-Control-Expose-Headers header.
What Is the Access-Control-Expose-Headers HTTP Header?
The Access-Control-Expose-Headers HTTP header is an optional response header that allows a server to indicate which headers should be exposed to the client in response to a cross-origin request. By default, only the CORS-safelisted response headers are exposed to scripts running in the browser.
When a client sends a cross-origin request, the server responds with a set of headers. However, the client can only access certain headers, known as the simple response headers, using the XMLHttpRequest 2 object’s getResponseHeader() method. The simple response headers include Cache-Control, Content-Language, and Content-Type.
If the server wants to expose additional headers to the client, it can include them in the Access-Control-Expose-Headers header. This header takes a comma-delimited list of header names that the server wants to expose.
It is essential to note that the Access-Control-Expose-Headers header does not grant access to the headers it exposes. It only allows the client to access headers that it would not have been able to access otherwise.
The Access-Control-Expose-Headers HTTP header, when configured with a literal header name, allows client-side web applications to access http authentication information and other response headers, which are typically composed of multiple headers separated by commas.
Access-Control-Expose-Headers Syntax and Example
The Access-Control-Expose-Headers header is a response header that is used to expose additional headers to a web application that are not included in the CORS-safelisted response headers. This header is used in conjunction with the Access-Control-Allow-Headers header, which specifies the headers that are allowed to be part of the request.
Syntax
The syntax for the Access-Control-Expose-Headers header is as follows:
Access-Control-Expose-Headers: header-name1, header-name2, ...
The header field consists of one or more header names separated by commas. These header names are the names of the headers that the server wants to expose to the client.
Example
Here is an example of how the Access-Control-Expose-Headers header can be used:
Access-Control-Allow-Origin: https://www.example.com
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: Authorization, Content-Type
Access-Control-Expose-Headers: X-Custom-Header
In this example, the server is allowing requests from https://www.example.com with the GET, POST, and OPTIONS methods. The server is also allowing the Authorization and Content-Type headers to be part of the request. Finally, the server is exposing the X-Custom-Header header to the client.
It is important to note that the headers that are exposed must also be allowed by the server in the Access-Control-Allow-Headers header. If a header is not allowed in the Access-Control-Allow-Headers header, it will not be exposed to the client, even if it is included in the Access-Control-Expose-Headers header.
Why Is the Access-Control-Expose-Headers HTTP Header Important
The Access-Control-Expose-Headers HTTP header is an essential component of Cross-Origin Resource Sharing (CORS). CORS is a security feature that allows web browsers to access resources on servers that are outside their domain. This feature is crucial because it enables web developers to create applications that can interact with resources from different servers without violating security protocols.
The Access-Control-Expose-Headers header is important because it allows servers to specify which response headers should be available to scripts running in the browser. This header is used in response to a cross-origin request, and it ensures that only the CORS-safelisted response headers are exposed by default. This header is essential because it provides an additional layer of security to the web application by preventing unauthorized access to sensitive data.
The Access-Control-Expose-Headers header is also important because it enables web developers to expose custom headers that are not part of the CORS-safelisted response headers. This feature is useful because it allows web developers to create custom headers that contain additional information that is relevant to their application. This information can be used to enhance the user experience or provide additional controls to the user.
The Access-Control-Expose-Headers header is compatible with most modern web browsers, but there are some compatibility issues with older browsers. Some older browsers may not support this header, which may cause issues with the application. However, this issue can be resolved by using alternative methods to expose headers.
How to Implement the Access-Control-Expose-Headers HTTP Header
Syntax
The syntax for the Access-Control-Expose-Headers HTTP header is as follows:
Access-Control-Expose-Headers: <header-name>, <header-name>, ...
Example
To expose the Authorization and Content-Type headers, the following syntax can be used:
Access-Control-Expose-Headers: Authorization, Content-Type
Browser Compatibility
The Access-Control-Expose-Headers header is supported by all major browsers, including Internet Explorer, Safari, Opera, and Firefox.
Fetch Standard
The Access-Control-Expose-Headers header is part of the Fetch standard and is used to control which headers are exposed to scripts running in the browser.
Header Coding
The Access-Control-Expose-Headers header should be included in the response headers of the server. It is important to note that the header must be included in the response headers of the server, even if the header is not actually exposed.
Cache Control
The Access-Control-Expose-Headers header is not cacheable and should not be included in the response headers of cached responses.
Content Negotiation
The Access-Control-Expose-Headers header does not affect content negotiation and should not be used to negotiate content.
Security Considerations
The Access-Control-Expose-Headers header should not be used to expose sensitive information, such as credentials or cookies. The header should only be used to expose headers that are necessary for the client to process the response.
XMLHttpRequest
The Access-Control-Expose-Headers header is supported by XMLHttpRequest and can be used in CORS requests.
Examples of Implementation
To implement the Access-Control-Expose-Headers header in a server response, the following code can be used in an Express.js application:
app.use(function(req, res, next) {
res.header("Access-Control-Expose-Headers", "Authorization, Content-Type");
next();
});
In this example, the Authorization and Content-Type headers are exposed to scripts running in the browser.
