There is a another way of controlling this using back-end procedure but it is not recommended because this feature is already in-built within Apex.

Setup
APEX: 3.2.1
Web Server: Apache (OHS)
Database:

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Prod
PL/SQL Release 10.2.0.3.0 - Production
CORE 10.2.0.3.0 Production
TNS for 32-bit Windows: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production

First I fired up the web server:

C:oraclehttpopmnbin>opmnctl start
opmnctl: opmn started

C:oraclehttpopmnbin>opmnctl startproc process-type=HTTP_Server
opmnctl: starting opmn managed processes...

Opened up APEX, and created a new application. For authentication schemes I chose "No Authentication."

After I had created the application, I went into Shared Components –> Authentication Schemes –> Create

Select the default and click Next

step 1

Select "Show Login Page and Use LDAP Directory Credentials" and click Next

step 2

I’ve already done this so I’m selecting my current Login page, 11, click Next

step 3

Enter your LDAP Host and your DN:

step 4

Your DN String should look something like this (from article above):

cn=%LDAP_USER%,l=amer,dc=oracle,dc=com

Make sure you use the %LDAP_USER% after the cn= portion of the string.

Name it ldap_test, click Create Scheme:

step 5

You will then be redirected back to the list of Authentication Schemes, ldap_test should now be current

Fini!

To test it just run your application and login using your LDAP (AD) credentials

login

Success!

success!!

 

Thanks to http://www.oraclenerd.com/2010/03/apex-ldap-authentication.html