Ubuntu Manpage: Net::LDAP::Security - Security issues with LDAP connections

Provided by: libnet-ldap-perl_0.6800+dfsg-1_all

NAME

       Net::LDAP::Security - Security issues with LDAP connections

SYNOPSIS

         none

DESCRIPTION

This document discusses various security issues relating to using LDAP and connecting to LDAP servers,
notably how to manage these potential vulnerabilities:

• do you know that you are connected to the right server

• can someone sniff your passwords/userids from the directory connection

• can someone sniff other confidential information from the directory connection

Net::LDAP provides ways to address these vulnerabilities: through the use of LDAPS, or LDAPv3 and TLS,
and/or the use of SASL. Each of these will be explained below.

How does an LDAP connection work
A normal LDAPv2 or LDAPv3 connection works by the client connecting directly to port 389 (by default),
and then issuing various LDAP requests like search, add, etc.

There is no way to guarantee that an LDAP client is connected to the right LDAP server. Hackers could
have poisoned your DNS, so 'ldap.example.com' could be made to point to 'ldap.hacker.com'. Or they could
have installed their own server on the correct machine.

It is in the nature of the LDAP protocol that all information goes between the client and the server in
'plain text'. This is a term used by cryptographers to describe unencrypted and recoverable data, so even
though LDAP can transfer binary values like JPEG photographs, audio clips and X.509 certificates,
everything is still considered 'plain text'.

If these vulnerabilities are an issue to, then you should consider the other possibilities described
below, namely LDAPS, LDAPv3 and TLS, and SASL.

How does an LDAPS connection work
LDAPS is an unofficial protocol. It is to LDAP what HTTPS is to HTTP, namely the exact same protocol (but
in this case LDAPv2 or LDAPv3) running over a secured SSL ("Secure Socket Layer") connection to port 636
(by default).

Not all servers will be configured to listen for LDAPS connections, but if they do, it will commonly be
on a different port from the normal plain text LDAP port.

Using LDAPS can potentially solve the vulnerabilities described above, but you should be aware that
simply "using" SSL is not a magic bullet that automatically makes your system "secure".

First of all, LDAPS can solve the problem of verifying that you are connected to the correct server. When
the client and server connect, they perform a special SSL 'handshake', part of which involves the server
and client exchanging cryptographic keys, which are described using X.509 certificates. If the client
wishes to confirm that it is connected to the correct server, all it needs to do is verify the server's
certificate which is sent in the handshake. This is done in two ways:

1. check that the certificate is signed (trusted) by someone that you trust, and that the certificate
hasn't been revoked. For instance, the server's certificate may have been signed by Verisign
(www.verisign.com), and you decide that you want to trust Verisign to sign legitimate certificates.

2. check that the least-significant cn RDN in the server's certificate's DN is the fully-qualified
hostname of the hostname that you connected to when creating the LDAPS object. For example if the
server is <cn=ldap.example.com,ou=My department,o=My company>, then the RDN to check is
cn=ldap.example.com.

You can do this by using the cafile and capath options when creating a Net::LDAPS object, and by setting
the verify option to 'require'.

To prevent hackers 'sniffing' passwords and other information on your connection, you also have to make
sure the encryption algorithm used by the SSL connection is good enough. This is also something that gets
decided by the SSL handshake - if the client and server cannot agree on an acceptable algorithm the
connection is not made.

Net::LDAPS will by default use all the algorithms built into your copy of OpenSSL, except for ones
considered to use "low" strength encryption, and those using export strength encryption. You can override
this when you create the Net::LDAPS object using the 'ciphers' option.

Once you've made the secure connection, you should also check that the encryption algorithm that is
actually being used is one that you find acceptable. Broken servers have been observed in the field which
'fail over' and give you an unencrypted connection, so you ought to check for that.

How does LDAP and TLS work
SSL is a good solution to many network security problems, but it is not a standard. The IETF corrected
some defects in the SSL mechanism and published a standard called RFC 2246 which describes TLS
("Transport Layer Security"), which is simply a cleaned up and standardized version of SSL.

You can only use TLS with an LDAPv3 server. That is because the standard (RFC 4511) for LDAP and TLS
requires that the normal LDAP connection (i.e., on port 389) can be switched on demand from plain text
into a TLS connection. The switching mechanism uses a special extended LDAP operation, and since these
are not legal in LDAPv2, you can only switch to TLS on an LDAPv3 connection.

So the way you use TLS with LDAPv3 is that you create your normal LDAPv3 connection using
"Net::LDAP::new()", and then you perform the switch using "Net::LDAP::start_tls()". The "start_tls()"
method takes pretty much the same arguments as "Net::LDAPS::new()", so check above for details.

How does SASL work
SASL is an authentication framework that can be used by a number of different Internet services,
including LDAPv3. Because it is only a framework, it doesn't provide any way to authenticate by itself;
to actually authenticate to a service you need to use a specific SASL mechanism. A number of mechanisms
are defined, such as CRAM-MD5.

The use of a mechanism like CRAM-MD5 provides a solution to the password sniffing vulnerability, because
these mechanisms typically do not require the user to send across a secret (e.g., a password) in the
clear across the network. Instead, authentication is carried out in a clever way which avoids this, and
so prevents passwords from being sniffed.

Net::LDAP supports SASL using the Authen::SASL class. Currently the only Authen::SASL subclasses (i.e.,
SASL mechanism) available are CRAM-MD5 and EXTERNAL.

Some SASL mechanisms provide a general solution to the sniffing of all data on the network vulnerability,
as they can negotiate confidential (i.e., encrypted) network connections. Note that this is over and
above any SSL or TLS encryption! Unfortunately, perl's Authen::SASL code cannot negotiate this.

ACKNOWLEDGEMENTS

       Jim Dutton <jimd@dutton3.it.siu.edu> provided lots of useful feedback on the early drafts.

AUTHOR

       Chris Ridd <chris.ridd@isode.com>

       Please report any bugs, or post any suggestions, to the perl-ldap mailing list <perl-ldap@perl.org>.

COPYRIGHT