Testing for LDAP Injection (WSTG-INPV-06)

ID
WSTG-INPV-06

Summary

The Lightweight Directory Access Protocol (LDAP) is used to store information about users, hosts, and many other objects. LDAP injection is a server-side attack, which could allow sensitive information about users and hosts represented in an LDAP structure to be disclosed, modified, or inserted. This is done by manipulating input parameters afterwards passed to internal search, add, and modify functions.

A web application could use LDAP in order to let users authenticate or search other users' information inside a corporate structure. The goal of LDAP injection attacks is to inject LDAP search filters metacharacters in a query which will be executed by the application.

Rfc2254 defines a grammar on how to build a search filter on LDAPv3 and extends Rfc1960 (LDAPv2).

An LDAP search filter is constructed in Polish notation, also known as Polish notation prefix notation.

This means that a pseudo code condition on a search filter like this:

find("cn=John & userPassword=mypass")

will be represented as:

find("(&(cn=John)(userPassword=mypass))")

Boolean conditions and group aggregations on an LDAP search filter could be applied by using the following metacharacters:

Metachar	Meaning
&	Boolean AND
|	Boolean OR
!	Boolean NOT
=	Equals
~=	Approx
>=	Greater than
<=	Less than
*	Any character
()	Grouping parenthesis

More complete examples on how to build a search filter can be found in the related RFC.

A successful exploitation of an LDAP injection vulnerability could allow the tester to:

• Access unauthorized content

• Evade application restrictions

• Gather unauthorized information

• Add or modify Objects inside LDAP tree structure

Test Objectives

• Identify LDAP injection points.

• Assess the severity of the injection.

How to Test

Example 1: Search Filters

Let's suppose we have a web application using a search filter like the following one:

searchfilter="(cn="+user+")"

which is instantiated by an HTTP request like this:

http://www.example.com/ldapsearch?user=John

If the value John is replaced with a *, by sending the request:

http://www.example.com/ldapsearch?user=*

the filter will look like:

searchfilter="(cn=*)"

which matches every object with a 'cn' attribute equals to anything.

If the application is vulnerable to LDAP injection, it will display some or all of the user's attributes, depending on the application's execution flow and the permissions of the LDAP connected user.

A tester could use a trial-and-error approach, by inserting in the parameter (, |, &, * and the other characters, in order to check the application for errors.

Example 2: Login

If a web application uses LDAP to check user credentials during the login process and it is vulnerable to LDAP injection, it is possible to bypass the authentication check by injecting an always true LDAP query (in a similar way to SQL and XPATH injection ).

Let's suppose a web application uses a filter to match LDAP user/password pair.

searchlogin= "(&(uid="+user+")(userPassword={MD5}"+base64(pack("H*",md5(pass)))+"))";

By using the following values:

user=*)(uid=*))(|(uid=*
pass=password

the search filter will results in:

searchlogin="(&(uid=*)(uid=*))(|(uid=*)(userPassword={MD5}X03MO1qnZdYdgyfeuILPmQ==))";

which is correct and always true. This way, the tester will gain logged-in status as the first user in LDAP tree.

Tools

• Softerra LDAP Browser

References

• LDAP Injection Prevention Cheat Sheet

Whitepapers

• Sacha Faust: LDAP Injection: Are Your Applications Vulnerable?

• IBM paper: Understanding LDAP

• RFC 1960: A String Representation of LDAP Search Filters

• LDAP injection