Internet Engineering Task Force (IETF) J. Reschke
Request for Comments: 7617 greenbytes
Obsoletes: 2617 September 2015
Category: Standards Track
The 'Basic' HTTP Authentication Scheme
This document defines the "Basic" Hypertext Transfer Protocol (HTTP)
authentication scheme, which transmits credentials as user-id/
password pairs, encoded using Base64.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
This document defines the "Basic" Hypertext Transfer Protocol (HTTP)
authentication scheme, which transmits credentials as user-id/
password pairs, encoded using Base64 (HTTP authentication schemes are
defined in [RFC7235]).
This scheme is not considered to be a secure method of user
authentication unless used in conjunction with some external secure
system such as TLS (Transport Layer Security, [RFC5246]), as the
user-id and password are passed over the network as cleartext.
The "Basic" scheme previously was defined in Section 2 of [RFC2617].
This document updates the definition, and also addresses
internationalization issues by introducing the 'charset'
authentication parameter (Section 2.1).
Other documents updating RFC 2617 are "Hypertext Transfer Protocol
(HTTP/1.1): Authentication" ([RFC7235], defining the authentication
framework), "HTTP Digest Access Authentication" ([RFC7616], updating
the definition of the "Digest" authentication scheme), and "HTTP
Authentication-Info and Proxy-Authentication-Info Response Header
Fields" ([RFC7615]). Taken together, these four documents obsolete
1.1. Terminology and Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The terms "protection space" and "realm" are defined in Section 2.2
The terms "(character) repertoire" and "character encoding scheme"
are defined in Section 2 of [RFC6365].
2. The 'Basic' Authentication Scheme
The Basic authentication scheme is based on the model that the client
needs to authenticate itself with a user-id and a password for each
protection space ("realm"). The realm value is a free-form string
that can only be compared for equality with other realms on that
server. The server will service the request only if it can validate
the user-id and password for the protection space applying to the
The Basic authentication scheme utilizes the Authentication Framework
o The scheme name is "Basic".
o The authentication parameter 'realm' is REQUIRED ([RFC7235],
o The authentication parameter 'charset' is OPTIONAL (see
o No other authentication parameters are defined -- unknown
parameters MUST be ignored by recipients, and new parameters can
only be defined by revising this specification.
See also Section 4.1 of [RFC7235], which discusses the complexity of
parsing challenges properly.
Note that both scheme and parameter names are matched case-
For credentials, the "token68" syntax defined in Section 2.1 of
[RFC7235] is used. The value is computed based on user-id and
password as defined below.
Upon receipt of a request for a URI within the protection space that
lacks credentials, the server can reply with a challenge using the
401 (Unauthorized) status code ([RFC7235], Section 3.1) and the
WWW-Authenticate header field ([RFC7235], Section 4.1).
HTTP/1.1 401 Unauthorized
Date: Mon, 04 Feb 2014 16:50:53 GMT
WWW-Authenticate: Basic realm="WallyWorld"
where "WallyWorld" is the string assigned by the server to identify
the protection space.
A proxy can respond with a similar challenge using the 407 (Proxy
Authentication Required) status code ([RFC7235], Section 3.2) and the
Proxy-Authenticate header field ([RFC7235], Section 4.3).
To receive authorization, the client
1. obtains the user-id and password from the user,
2. constructs the user-pass by concatenating the user-id, a single
colon (":") character, and the password,
3. encodes the user-pass into an octet sequence (see below for a
discussion of character encoding schemes),
4. and obtains the basic-credentials by encoding this octet sequence
using Base64 ([RFC4648], Section 4) into a sequence of US-ASCII
The original definition of this authentication scheme failed to
specify the character encoding scheme used to convert the user-pass
into an octet sequence. In practice, most implementations chose
either a locale-specific encoding such as ISO-8859-1 ([ISO-8859-1]),
or UTF-8 ([RFC3629]). For backwards compatibility reasons, this
specification continues to leave the default encoding undefined, as
long as it is compatible with US-ASCII (mapping any US-ASCII
character to a single octet matching the US-ASCII character code).
The user-id and password MUST NOT contain any control characters (see
"CTL" in Appendix B.1 of [RFC5234]).
Furthermore, a user-id containing a colon character is invalid, as
the first colon in a user-pass string separates user-id and password
from one another; text after the first colon is part of the password.
User-ids containing colons cannot be encoded in user-pass strings.
Note that many user agents produce user-pass strings without checking
that user-ids supplied by users do not contain colons; recipients
will then treat part of the username input as part of the password.
If the user agent wishes to send the user-id "Aladdin" and password
"open sesame", it would use the following header field:
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
2.1. The 'charset' auth-param
In challenges, servers can use the 'charset' authentication parameter
to indicate the character encoding scheme they expect the user agent
to use when generating "user-pass" (a sequence of octets). This
information is purely advisory.
The only allowed value is "UTF-8"; it is to be matched case-
insensitively (see [RFC2978], Section 2.3). It indicates that the
server expects character data to be converted to Unicode
Normalization Form C ("NFC"; see Section 3 of [RFC5198]) and to be
encoded into octets using the UTF-8 character encoding scheme
For the user-id, recipients MUST support all characters defined in
the "UsernameCasePreserved" profile defined in Section 3.3 of
[RFC7613], with the exception of the colon (":") character.
For the password, recipients MUST support all characters defined in
the "OpaqueString" profile defined in Section 4.2 of [RFC7613].
Other values are reserved for future use.
Note: The 'charset' is only defined on challenges, as Basic
authentication uses a single token for credentials ('token68'
syntax); thus, the credentials syntax isn't extensible.
Note: The name 'charset' has been chosen for consistency with
Section 2.1.1 of [RFC2831]. A better name would have been
'accept-charset', as it is not about the message it appears in,
but the server's expectation.
In the example below, the server prompts for authentication in the
"foo" realm, using Basic authentication, with a preference for the
UTF-8 character encoding scheme:
WWW-Authenticate: Basic realm="foo", charset="UTF-8"
Note that the parameter value can be either a token or a quoted
string; in this case, the server chose to use the quoted-string
The user's name is "test", and the password is the string "123"
followed by the Unicode character U+00A3 (POUND SIGN). Using the
character encoding scheme UTF-8, the user-pass becomes:
't' 'e' 's' 't' ':' '1' '2' '3' pound
74 65 73 74 3A 31 32 33 C2 A3
Encoding this octet sequence in Base64 ([RFC4648], Section 4) yields:
Thus, the Authorization header field would be:
Authorization: Basic dGVzdDoxMjPCow==
Or, for proxy authentication:
Proxy-Authorization: Basic dGVzdDoxMjPCow==
2.2. Reusing Credentials
Given the absolute URI ([RFC3986], Section 4.3) of an authenticated
request, the authentication scope of that request is obtained by
removing all characters after the last slash ("/") character of the
path component ("hier_part"; see [RFC3986], Section 3). A client
SHOULD assume that resources identified by URIs with a prefix-match
of the authentication scope are also within the protection space
specified by the realm value of that authenticated request.
A client MAY preemptively send the corresponding Authorization header
field with requests for resources in that space without receipt of
another challenge from the server. Similarly, when a client sends a
request to a proxy, it MAY reuse a user-id and password in the Proxy-
Authorization header field without receiving another challenge from
the proxy server.
For example, given an authenticated request to:
requests to the URIs below could use the known credentials:
while the URIs
would be considered to be outside the authentication scope.
Note that a URI can be part of multiple authentication scopes (such
as "http://example.com/" and "http://example.com/docs/"). This
specification does not define which of these should be treated with
3. Internationalization Considerations
User-ids or passwords containing characters outside the US-ASCII
character repertoire will cause interoperability issues, unless both
communication partners agree on what character encoding scheme is to
be used. Servers can use the new 'charset' parameter (Section 2.1)
to indicate a preference of "UTF-8", increasing the probability that
clients will switch to that encoding.
The 'realm' parameter carries data that can be considered textual;
however, [RFC7235] does not define a way to reliably transport non-
US-ASCII characters. This is a known issue that would need to be
addressed in a revision to that specification.
4. Security Considerations
The Basic authentication scheme is not a secure method of user
authentication, nor does it in any way protect the entity, which is
transmitted in cleartext across the physical network used as the
carrier. HTTP does not prevent the addition of enhancements (such as
schemes to use one-time passwords) to Basic authentication.
The most serious flaw of Basic authentication is that it results in
the cleartext transmission of the user's password over the physical
network. Many other authentication schemes address this problem.
Because Basic authentication involves the cleartext transmission of
passwords, it SHOULD NOT be used (without enhancements such as HTTPS
[RFC2818]) to protect sensitive or valuable information.
A common use of Basic authentication is for identification purposes
-- requiring the user to provide a user-id and password as a means of
identification, for example, for purposes of gathering accurate usage
statistics on a server. When used in this way it is tempting to
think that there is no danger in its use if illicit access to the
protected documents is not a major concern. This is only correct if
the server issues both user-id and password to the users and, in
particular, does not allow the user to choose his or her own
password. The danger arises because naive users frequently reuse a
single password to avoid the task of maintaining multiple passwords.
If a server permits users to select their own passwords, then the
threat is not only unauthorized access to documents on the server but
also unauthorized access to any other resources on other systems that
the user protects with the same password. Furthermore, in the
server's password database, many of the passwords may also be users'
passwords for other sites. The owner or administrator of such a
system could therefore expose all users of the system to the risk of
unauthorized access to all those other sites if this information is
not maintained in a secure fashion. This raises both security and
privacy concerns ([RFC6973]). If the same user-id and password
combination is in use to access other accounts, such as an email or
health portal account, personal information could be exposed.
Basic authentication is also vulnerable to spoofing by counterfeit
servers. If a user can be led to believe that she is connecting to a
host containing information protected by Basic authentication when,
in fact, she is connecting to a hostile server or gateway, then the
attacker can request a password, store it for later use, and feign an
error. Server implementers ought to guard against this sort of
counterfeiting; in particular, software components that can take over
control over the message framing on an existing connection need to be
used carefully or not at all (for instance: NPH ("Non-Parsed Header")
scripts as described in Section 5 of [RFC3875]).
Servers and proxies implementing Basic authentication need to store
user passwords in some form in order to authenticate a request.
These passwords ought to be stored in such a way that a leak of the
password data doesn't make them trivially recoverable. This is
especially important when users are allowed to set their own
passwords, since users are known to choose weak passwords and to
reuse them across authentication realms. While a full discussion of
good password hashing techniques is beyond the scope of this
document, server operators ought to make an effort to minimize risks
to their users in the event of a password data leak. For example,
servers ought to avoid storing user passwords in plaintext or as
unsalted digests. For more discussion about modern password hashing
techniques, see the "Password Hashing Competition"
The use of the UTF-8 character encoding scheme and of normalization
introduces additional security considerations; see Section 10 of
[RFC3629] and Section 6 of [RFC5198] for more information.
5. IANA Considerations
IANA maintains the "Hypertext Transfer Protocol (HTTP) Authentication
Scheme Registry" ([RFC7235]) at <http://www.iana.org/assignments/
The entry for the "Basic" authentication scheme has been updated to
reference this specification.
Appendix A. Changes from RFC 2617
The scheme definition has been rewritten to be consistent with newer
specifications such as [RFC7235].
The new authentication parameter 'charset' has been added. It is
purely advisory, so existing implementations do not need to change,
unless they want to take advantage of the additional information that
previously wasn't available.
Appendix B. Deployment Considerations for the 'charset' Parameter
B.1. User Agents
User agents not implementing 'charset' will continue to work as
before, ignoring the new parameter.
User agents that already default to the UTF-8 encoding implement
'charset' by definition.
Other user agents can keep their default behavior and switch to UTF-8
when seeing the new parameter.
Servers that do not support non-US-ASCII characters in credentials do
not require any changes to support 'charset'.
Servers that need to support non-US-ASCII characters, but cannot use
the UTF-8 character encoding scheme will not be affected; they will
continue to function as well or as badly as before.
Finally, servers that need to support non-US-ASCII characters and can
use the UTF-8 character encoding scheme can opt in by specifying the
'charset' parameter in the authentication challenge. Clients that do
understand the 'charset' parameter will then start to use UTF-8,
while other clients will continue to send credentials in their
default encoding, broken credentials, or no credentials at all.
Until all clients are upgraded to support UTF-8, servers are likely
to see both UTF-8 and "legacy" encodings in requests. When
processing as UTF-8 fails (due to a failure to decode as UTF-8 or a
mismatch of user-id/password), a server might try a fallback to the
previously supported legacy encoding in order to accommodate these
legacy clients. Note that implicit retries need to be done
carefully; for instance, some subsystems might detect repeated login
failures and treat them as a potential credentials-guessing attack.
B.3. Why not simply switch the default encoding to UTF-8?
There are sites in use today that default to a local character
encoding scheme, such as ISO-8859-1 ([ISO-8859-1]), and expect user
agents to use that encoding. Authentication on these sites will stop
working if the user agent switches to a different encoding, such as
Note that sites might even inspect the User-Agent header field
([RFC7231], Section 5.5.3) to decide which character encoding scheme
to expect from the client. Therefore, they might support UTF-8 for
some user agents, but default to something else for others. User
agents in the latter group will have to continue to do what they do
today until the majority of these servers have been upgraded to
always use UTF-8.
This specification takes over the definition of the "Basic" HTTP
Authentication Scheme, previously defined in RFC 2617. We thank John
Franks, Phillip M. Hallam-Baker, Jeffery L. Hostetler, Scott
D. Lawrence, Paul J. Leach, Ari Luotonen, and Lawrence C. Stewart for
their work on that specification, from which significant amounts of
text were borrowed. See Section 6 of [RFC2617] for further
The internationalization problem with respect to the character
encoding scheme used for user-pass was reported as a Mozilla bug back
in the year 2000 (see
<https://bugzilla.mozilla.org/show_bug.cgi?id=41489> and also the
more recent <https://bugzilla.mozilla.org/show_bug.cgi?id=656213>).
It was Andrew Clover's idea to address it using a new auth-param.
We also thank the members of the HTTPAUTH Working Group and other
reviewers, namely, Stephen Farrell, Roy Fielding, Daniel Kahn
Gillmor, Tony Hansen, Bjoern Hoehrmann, Kari Hurtta, Amos Jeffries,
Benjamin Kaduk, Michael Koeller, Eric Lawrence, Barry Leiba, James
Manger, Alexey Melnikov, Kathleen Moriarty, Juergen Schoenwaelder,
Yaron Sheffer, Meral Shirazipour, Michael Sweet, and Martin Thomson
for feedback on this revision.