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How Does HTTP Authentication Work

by Mozilla Contributors5mJune 15th, 2020

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This page shows an introduction to the HTTP framework for authentication and shows how to restrict access to your server using the HTTP "Basic" Scheme. 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: The server responds to a client with a 401 (Unauthorized) response status and provides information on how to authorize with a WWW-Authenticate response. A client that wants to authenticate itself with a server can then do so by including an Authorization request field.

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HTTP provides a general framework for access control and authentication. The most common HTTP authentication is based on the
"Basic" schema. This page shows an introduction to the HTTP framework
for authentication and shows how to restrict access to your server using the HTTP "Basic" schema.

The general HTTP authentication framework

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: The server responds to a client with a

(Unauthorized) response status and provides information on how to authorize with a

WWW-Authenticate

response header containing at least one challenge.

A client that wants to authenticate itself with a server can then do so by including an

Authorization

request header field with the credentials. Usually a client will present a password prompt to the user and will then issue the request including the correct

Authorization

header.

In the case of a "Basic" authentication like shown in the figure, the exchange must happen over an HTTPS (TLS) connection to be secure.

Proxy authentication

The same challenge and response mechanism can be used for proxy authentication. In this case, it is an intermediate proxy that requires 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

(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.

Access forbidden

If a (proxy) server receives valid credentials that are not adequate to gain access for a given resource, the server should respond with the

Forbidden

status code. Unlike

Unauthorized or

Proxy Authentication Required

, authentication is impossible for this user.

Authentication of cross-origin images

A potential security hole that has recently been fixed by browsers is 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.

Character encoding of HTTP authentication

Browsers use utf-8 encoding for usernames and passwords. Firefox used to use ISO-8859-1, but changed over to utf-8 for parity with other browsers, and to avoid potential problems as described in bug 1419658.

WWW-Authenticate and Proxy-Authenticate headers

The

WWW-Authenticate

and

Proxy-Authenticate

response headers define the authentication method that should be used to gain access to a resource. They need to 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.

Authorization and Proxy-Authorization headers

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>

Authentication schemes

The general HTTP authentication framework is used by several authentication schemes. Schemes can differ in security strength and in
their availability in client or server software.

The most common authentication scheme is the "Basic" authentication
scheme which is introduced in more details below. IANA maintains a list of authentication schemes, but there are other schemes offered by host services, such as Amazon AWS. Common authentication schemes include:

Basic (see RFC 7617, base64-encoded credentials. See below for more information.),
Bearer (see RFC 6750, bearer tokens to access OAuth 2.0-protected resources),
Digest (see RFC 7616, only md5 hashing is supported in Firefox, see bug 472823 for SHA encryption support),
HOBA (see RFC 7486, Section 3, HTTP Origin-Bound Authentication, digital-signature-based),
Mutual (see RFC 8120),
AWS4-HMAC-SHA256 (see AWS docs).

Basic authentication scheme

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

Security of basic authentication

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 in
conjunction with basic authentication. Without these additional security enhancements, basic authentication should not be used to protect sensitive or valuable information.

Restricting access with Apache and basic authentication

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 encrypted (md5 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/

Restricting access with nginx and basic authentication

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;
}

Access using credentials in the URL

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."