4. Security Considerations

Now that your machines (and possibly other services) are authenticating against your LDAP server, this server needs to be protected at least as well as /etc/master.passwd would be on a regular server, and possibly even more so since a broken or cracked LDAP server would break every client service.

Remember, this section is not exhaustive. You should continually review your configuration and procedures for improvements.

4.1. Setting Attributes Read-only

Several attributes in LDAP should be read-only. If left writable by the user, for example, a user could change his uidNumber attribute to 0 and get root access!

To begin with, the userPassword attribute should not be world-readable. By default, anyone who can connect to the LDAP server can read this attribute. To disable this, put the following in slapd.conf:

Example 8. Hide Passwords
access to dn.subtree="ou=people,dc=example,dc=org"
  attrs=userPassword
  by self write
  by anonymous auth
  by * none

access to *
  by self write
  by * read

This will disallow reading of the userPassword attribute, while still allowing users to change their own passwords.

Additionally, you'll want to keep users from changing some of their own attributes. By default, users can change any attribute (except for those which the LDAP schemas themselves deny changes), such as uidNumber. To close this hole, modify the above to

Example 9. Read-only Attributes
access to dn.subtree="ou=people,dc=example,dc=org"
  attrs=userPassword
  by self write
  by anonymous auth
  by * none

access to attrs=homeDirectory,uidNumber,gidNumber
  by * read

access to *
  by self write
  by * read

This will stop users from being able to masquerade as other users.

4.2. root Account Definition

Often the root or manager account for the LDAP service will be defined in the configuration file. OpenLDAP supports this, for example, and it works, but it can lead to trouble if slapd.conf is compromised. It may be better to use this only to bootstrap yourself into LDAP, and then define a root account there.

Even better is to define accounts that have limited permissions, and omit a root account entirely. For example, users that can add or remove user accounts are added to one group, but they cannot themselves change the membership of this group. Such a security policy would help mitigate the effects of a leaked password.

4.2.1. Creating a Management Group

Say you want your IT department to be able to change home directories for users, but you do not want all of them to be able to add or remove users. The way to do this is to add a group for these admins:

Example 10. Creating a Management Group
dn: cn=homemanagement,dc=example,dc=org
objectClass: top
objectClass: posixGroup
cn: homemanagement
gidNumber: 121 # required for posixGroup
memberUid: uid=tuser,ou=people,dc=example,dc=org
memberUid: uid=user2,ou=people,dc=example,dc=org

And then change the permissions attributes in slapd.conf:

Example 11. ACLs for a Home Directory Management Group
access to dn.subtree="ou=people,dc=example,dc=org"
  attr=homeDirectory
  by dn="cn=homemanagement,dc=example,dc=org"
  dnattr=memberUid write

Now tuser and user2 can change other users' home directories.

In this example we have given a subset of administrative power to certain users without giving them power in other domains. The idea is that soon no single user account has the power of a root account, but every power root had is had by at least one user. The root account then becomes unnecessary and can be removed.

4.3. Password Storage

By default OpenLDAP will store the value of the userPassword attribute as it stores any other data: in the clear. Most of the time it is base 64 encoded, which provides enough protection to keep an honest administrator from knowing your password, but little else.

It is a good idea, then, to store passwords in a more secure format, such as SSHA (salted SHA). This is done by whatever program you use to change users' passwords.

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