Exim 4.40 Specification chapter 34

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# 34. The plaintext authenticator

The plaintext authenticator can be configured to support the PLAIN and LOGIN authentication mechanisms, both of which transfer authentication data as plain (unencrypted) text (though base64 encoded). The use of plain text is a security risk. If you use one of these mechanisms without also making use of SMTP encryption (see chapter 37) you should not use the same passwords for SMTP connections as you do for login accounts.

## 34.1. Using plaintext in a server

When running as a server, plaintext performs the authentication test by expanding a string. It has the following options:

### server_prompts

Type: string, expanded
Default: unset

The contents of this option, after expansion, must be a colon-separated list of prompt strings. If expansion fails, a temporary authentication rejection is given.

### server_condition

Type: string, expanded
Default: unset

This option must be set in order to configure the driver as a server. Its use is described below.

The data sent by the client with the AUTH command, or in response to subsequent prompts, is base64 encoded, and so may contain any byte values when decoded. If any data is supplied with the command, it is treated as a list of strings, separated by NULs (binary zeros), which are placed in the expansion variables `$1`, `$2`, etc. If there are more strings in `server_prompts` than the number of strings supplied with the AUTH command, the remaining prompts are used to obtain more data. Each response from the client may be a list of NUL-separated strings.

Once a sufficient number of data strings have been received, `server_condition` is expanded. If the expansion is forced to fail, authentication fails. Any other expansion failure causes a temporary error code to be returned. If the result of a successful expansion is an empty string, “0”, “no”, or “false”, authentication fails. If the result of the expansion is “1”, “yes”, or “true”, authentication succeeds and the generic `server_set_id` option is expanded and saved in `$authenticated_id`. For any other result, a temporary error code is returned, with the expanded string as the error text.

Warning: If you use a lookup in the expansion to find the user's password, be sure to make the authentication fail if the user is unknown. There are good and bad examples at the end of the next section.

## 34.2. The PLAIN authentication mechanism

The PLAIN authentication mechanism (RFC 2595) specifies that three strings be sent as one item of data (that is, one combined string containing two NUL separators). The data is sent either as part of the AUTH command, or subsequently in response to an empty prompt from the server.

The second and third strings are a user name and a corresponding password. Using a single fixed user name and password as an example, this could be configured as follows:

      fixed_plain:
        driver = plaintext
        public_name = PLAIN
        server_prompts = :
        server_condition = \
          ${if and {{eq{$2}{username}}{eq{$3}{mysecret}}}{yes}{no}}
        server_set_id = $2

The `server_prompts` setting specifies a single, empty prompt (empty items at the end of a string list are ignored). If all the data comes as part of the AUTH command, as is commonly the case, the prompt is not used. This authenticator is advertised in the response to EHLO as

      250-AUTH PLAIN

and a client host can authenticate itself by sending the command

      AUTH PLAIN AHVzZXJuYW1lAG15c2VjcmV0

As this contains three strings (more than the number of prompts), no further data is required from the client. Alternatively, the client may just send

      AUTH PLAIN

to initiate authentication, in which case the server replies with an empty prompt. The client must respond with the combined data string.

The data string is base64 encoded, as required by the RFC. This example, when decoded, is `<NUL>username<NUL>mysecret`, where <NUL> represents a zero byte. This is split up into three strings, the first of which is empty. The `server_condition` option in the authenticator checks that the second two are `username` and `mysecret` respectively.

Having just one fixed user name and password, as in this example, is not very realistic, though for a small organization with only a handful of authenticating clients it could make sense.

A more sophisticated instance of this authenticator could use the user name in `$2` to look up a password in a file or database, and maybe do an encrypted comparison (see `crypteq` in chapter 11). Here is a example of this approach, where the passwords are looked up in a DBM file. Warning: This is an incorrect example:

      server_condition = \
        ${if eq{$3}{${lookup{$2}dbm{/etc/authpwd}}}{yes}{no}}

The expansion uses the user name (`$2`) as the key to look up a password, which it then compares to the supplied password (`$3`). Why is this example incorrect? It works fine for existing users, but consider what happens if a non-existent user name is given. The lookup fails, but as no success/failure strings are given for the lookup, it yields an empty string. Thus, to defeat the authentication, all a client has to do is to supply a non-existent user name and an empty password. The correct way of writing this test is:

      server_condition = ${lookup{$2}dbm{/etc/authpwd}\
        {${if eq{$value}{$3}{yes}{no}}}{no}}

In this case, if the lookup succeeds, the result is checked; if the lookup fails, authentication fails. If `crypteq` is being used instead of `eq`, the first example is in fact safe, because `crypteq` always fails if its second argument is empty. However, the second way of writing the test makes the logic clearer.

## 34.3. The LOGIN authentication mechanism

The LOGIN authentication mechanism is not documented in any RFC, but is in use in a number of programs. No data is sent with the AUTH command. Instead, a user name and password are supplied separately, in response to prompts. The plaintext authenticator can be configured to support this as in this example:

      fixed_login:
        driver = plaintext
        public_name = LOGIN
        server_prompts = User Name : Password
        server_condition = \
          ${if and {{eq{$1}{username}}{eq{$2}{mysecret}}}{yes}{no}}
        server_set_id = $1

Because of the way plaintext operates, this authenticator accepts data supplied with the AUTH command (in contravention of the specification of LOGIN), but if the client does not supply it (as is the case for LOGIN clients), the prompt strings are used to obtain two data items.

Some clients are very particular about the precise text of the prompts. For example, Outlook Express is reported to recognize only “Username:” and “Password:”. Here is an example of a LOGIN authenticator which uses those strings, and which uses the `ldapauth` expansion condition to check the user name and password by binding to an LDAP server:

      login:
        driver = plaintext
        public_name = LOGIN
        server_prompts = Username:: : Password::
        server_condition = ${if ldapauth \
          {user="cn=${quote_ldap_dn:$1},ou=people,o=example.org" \
          pass=${quote:$2} \
          ldap://ldap.example.org/}{yes}{no}}
        server_set_id = uid=$1,ou=people,o=example.org

Note the use of the `quote_ldap_dn` operator to correctly quote the DN for authentication. However, the basic `quote` operator, rather than any of the LDAP quoting operators, is the correct one to use for the password, because quoting is needed only to make the password conform to the Exim syntax. At the LDAP level, the password is an uninterpreted string.

## 34.4. Support for different kinds of authentication

A number of string expansion features are provided for the purpose of interfacing to different ways of user authentication. These include checking traditionally encrypted passwords from /etc/passwd (or equivalent), PAM, Radius, `ldapauth`, and pwcheck. For details see section 11.6.

## 34.5. Using plaintext in a client

The plaintext authenticator has just one client option:

### client_send

Type: string, expanded
Default: unset

The string is a colon-separated list of authentication data strings. Each string is independently expanded before being sent to the server. The first string is sent with the AUTH command; any more strings are sent in response to prompts from the server.

Note: you cannot use expansion to create multiple strings, because splitting takes priority and happens first.

Because the PLAIN authentication mechanism requires NUL (binary zero) bytes in the data, further processing is applied to each string before it is sent. If there are any single circumflex characters in the string, they are converted to NULs. Should an actual circumflex be required as data, it must be doubled in the string.

This is an example of a client configuration that implements the PLAIN authentication mechanism with a fixed user name and password:

      fixed_plain:
        driver = plaintext
        public_name = PLAIN
        client_send = ^username^mysecret

The lack of colons means that the entire text is sent with the AUTH command, with the circumflex characters converted to NULs. A similar example that uses the LOGIN mechanism is:

      fixed_login:
        driver = plaintext
        public_name = LOGIN
        client_send = : username : mysecret

The initial colon means that the first string is empty, so no data is sent with the AUTH command itself. The remaining strings are sent in response to prompts.

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