RFC 7235 defines the HTTP authentication framework, which can be used by a server to challenge a client request, and by a client to provide authentication information.

The challenge and response flow works like this:

1. The server responds to a client with a 401 (Unauthorized) response status and provides information on how to authorize with a WWW-Authenticate response header containing at least one challenge.
2. A client that wants to authenticate itself with the server can then do so by including an Authorization request header with the credentials.
3. Usually a client will present a password prompt to the user and will then issue the request including the correct Authorization header.

The general message flow above is the same for most (if not all) authentication schemes. The actual information in the headers and the way it is encoded does change!

The same challenge and response mechanism can be used for proxy authentication. As both resource authentication and proxy authentication can coexist, a different set of headers and status codes is needed. In the case of proxies, the challenging status code is 407 (Proxy Authentication Required), the Proxy-Authenticate response header contains at least one challenge applicable to the proxy, and the Proxy-Authorization request header is used for providing the credentials to the proxy server.

If a (proxy) server receives invalid credentials, it should respond with a 401 Unauthorized or with a 407 Proxy Authentication Required, and the user may send a new request or replace the Authorization header field.

If a (proxy) server receives valid credentials that are inadequate to access a given resource, the server should respond with the 403 Forbidden status code. Unlike 401 Unauthorized or 407 Proxy Authentication Required, authentication is impossible for this user and browsers will not propose a new attempt.

In all cases, the server may prefer returning a 404 Not Found status code, to hide the existence of the page to a user without adequate privileges or not correctly authenticated.

A potential security hole (that has since been fixed in browsers) was authentication of cross-site images. From Firefox 59 onwards, image resources loaded from different origins to the current document are no longer able to trigger HTTP authentication dialogs (bug 1423146), preventing user credentials being stolen if attackers were able to embed an arbitrary image into a third-party page.

Browsers use utf-8 encoding for usernames and passwords.

Firefox once used ISO-8859-1, but changed to utf-8 for parity with other browsers and to avoid potential problems as described in bug 1419658.

The WWW-Authenticate and Proxy-Authenticate response headers define the authentication method that should be used to gain access to a resource. They must specify which authentication scheme is used, so that the client that wishes to authorize knows how to provide the credentials.

The syntax for these headers is the following:

WWW-Authenticate: <type> realm=<realm>
Proxy-Authenticate: <type> realm=<realm>

Here, <type> is the authentication scheme ("Basic" is the most common scheme and introduced below). The realm is used to describe the protected area or to indicate the scope of protection. This could be a message like "Access to the staging site" or similar, so that the user knows to which space they are trying to get access to.

The Authorization and Proxy-Authorization request headers contain the credentials to authenticate a user agent with a (proxy) server. Here, the <type> is needed again followed by the credentials, which can be encoded or encrypted depending on which authentication scheme is used.

Authorization: <type> <credentials>
Proxy-Authorization: <type> <credentials>

The general HTTP authentication framework is the base for a number of authentication schemes.

IANA maintains a list of authentication schemes, but there are other schemes offered by host services, such as Amazon AWS.

Some common authentication schemes include:

Basic

See RFC 7617, base64-encoded credentials. More information below.

Bearer

See RFC 6750, bearer tokens to access OAuth 2.0-protected resources

Digest

See RFC 7616. Firefox 93 and later support the SHA-256 algorithm. Previous versions only support MD5 hashing (not recommended).

HOBA

See RFC 7486, Section 3, HTTP Origin-Bound Authentication, digital-signature-based

Mutual

See RFC 8120

Negotiate / NTLM

See RFC4599

VAPID

See RFC 8292

SCRAM

See RFC 7804

AWS4-HMAC-SHA256

See AWS docs. This scheme is used for AWS3 server authentication.

Schemes can differ in security strength and in their availability in client or server software.

The "Basic" authentication scheme offers very poor security, but is widely supported and easy to set up. It is introduced in more detail below.

The "Basic" HTTP authentication scheme is defined in RFC 7617, which transmits credentials as user ID/password pairs, encoded using base64.

As the user ID and password are passed over the network as clear text (it is base64 encoded, but base64 is a reversible encoding), the basic authentication scheme is not secure. HTTPS/TLS should be used with basic authentication. Without these additional security enhancements, basic authentication should not be used to protect sensitive or valuable information.

To password-protect a directory on an Apache server, you will need a .htaccess and a .htpasswd file.

The .htaccess file typically looks like this:

AuthType Basic
AuthName "Access to the staging site"
AuthUserFile /path/to/.htpasswd
Require valid-user

The .htaccess file references a .htpasswd file in which each line consists of a username and a password separated by a colon (:). You cannot see the actual passwords as they are hashed (using MD5-based hashing, in this case). Note that you can name your .htpasswd file differently if you like, but keep in mind this file shouldn't be accessible to anyone. (Apache is usually configured to prevent access to .ht* files).

aladdin:$apr1$ZjTqBB3f$IF9gdYAGlMrs2fuINjHsz.
user2:$apr1$O04r.y2H$/vEkesPhVInBByJUkXitA/

For Nginx, you will need to specify a location that you are going to protect and the auth_basic directive that provides the name to the password-protected area. The auth_basic_user_file directive then points to a .htpasswd file containing the encrypted user credentials, just like in the Apache example above.

location /status {
    auth_basic           "Access to the staging site";
    auth_basic_user_file /etc/apache2/.htpasswd;
}

Many clients also let you avoid the login prompt by using an encoded URL containing the username and the password like this:

https://username:[email protected]/

The use of these URLs is deprecated. In Chrome, the username:password@ part in URLs is even stripped out for security reasons. In Firefox, it is checked if the site actually requires authentication and if not, Firefox will warn the user with a prompt "You are about to log in to the site "www.example.com" with the username "username", but the website does not require authentication. This may be an attempt to trick you."

• WWW-Authenticate
• Authorization
• Proxy-Authorization
• Proxy-Authenticate
• 401, 403, 407