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Posted 18 Nov 2004

Authorize and authenticate users with AD

, 18 Nov 2004
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How much time do you spend to ensure user permissions? Ease the job and let Windows and Active Directory do it for you.


I always had interest in security issues, specially about application use security. I believe user authentication and authorization is one of the main thoughts in application development (even if not the first to be coded).

Despite all my interest, just recently, I got the time to study the resources and advantages .NET offers to this matter. And there are many things I found.

Active Directory and LDAP

Since MS launched Windows 2000 family, there is the Active Directory (AD). Whoever has studied it is aware this is based on a less used Internet protocol called LDAP. Its job is, basically, manage users, groups and other security stuff on a domain in a simple way. The greater advantage is interoperability, since one can replace AD for another LDAP server given some work.

For developers, .NET comes with a full namespace to ease working with both AD and LDAP, System.DirectoryServices, which includes LDAP v3. On the following samples, I will use a fake domain called AD1.

private static string domain = "AD1"; 

public static bool LogonValid(string userName, string password) {
  DirectoryEntry de = new DirectoryEntry(null, domain +
    "\\" + userName, password);
  try {
    object o = de.NativeObject;
    DirectorySearcher ds = new DirectorySearcher(de);
    ds.Filter = "samaccountname=" + userName;
    SearchResult sr = ds.FindOne();
    if(sr == null) throw new Exception();
    return true;
  } catch {
    return false;

public static bool IsInRole(string userName, string role) {
  try {
    role = role.ToLowerInvariant(); 
    DirectorySearcher ds = new DirectorySearcher(new DirectoryEntry(null));
    ds.Filter = "samaccountname=" + userName;
    SearchResult sr = ds.FindOne();
    DirectoryEntry de = sr.GetDirectoryEntry();
    PropertyValueCollection dir = de.Properties["memberOf"];
    for(int i = 0; i < dir.Count; ++i) {
      string s = dir[i].ToString().Substring(3);
      s = s.Substring(0, s.IndexOf(',')).ToLowerInvariant();
      if(s == role) return true;
    throw new Exception();
  } catch {
    return false;

These methods are implemented to work with a single application. The sources provided with this article are full implementations of IIdentity and IPrincipal interfaces. They are more suitable for developing ASP.NET Forms Authentication based on AD.

Let it to Windows

Applications can benefit more efficiently and easily from Active Directory by using another set of classes, also avoiding to create their own logon process. Developers should keep in mind this is a Windows-dependent solution.

But how is that possible? This time, we'll use Windows logon itself to authenticate the user, using only two classes in the System.Security.Principals namespace. Yet, this can be done in two ways:

IIdentity wi = WindowsIdentity.GetCurrent();
IPrincipal wp = new WindowsPrincipal((WindowsIdentity)wi); 
// ...or... 
IPrincipal wp = Thread.CurrentPrincipal;
IIdentity wi = wp.Identity;

From now on, we can check if a user belongs to a user group by simply calling the method IsInRole defined by the IPrincipal interface. It's important to remember that domain groups must specify the domain (e.g. "AD1\Administrators"), or the group evaluated will belong to the machine running the code.

A call to IsInRole is an alternate when the group name is known only at runtime. Once it is known during design time, methods and even full classes can be blocked using PrincipalPermissionAttribute. This can allow access to specific groups (roles), users, or simply user is authenticated (remember, in Windows 9x/ME, the user can cancel logon).

[PrincipalPermission(SecurityAction.Demand, Role="AD1\\Administrators")]

[PrincipalPermission(SecurityAction.Demand, User="AD1\\harkos")]

[PrincipalPermission(SecurityAction.Demand, Authenticated=true)]

The Windows identity can also be used to authenticate users on intranet sites. This configuration requires no code at all but adjusting the web.config file to the following lines:

<authentication mode="Windows"/> 
   <allow roles="AD1\Administrators"/> 

<identity impersonate="true"/>

The last line is not mandatory, but it makes the ASP.NET process to impersonate the user accessing the site, thus making the site more secure and allowing the use of PrincipalPermissionAttributes through your ASP.NET code.


User authentication and authorization using Windows/Active Directory is the best way to protect applications running inside a corporation, like a webmail or ERP application, easing management and task delegation and avoiding multiple passwords. Of course, nothing here applies if users should or must not be associated with domain user accounts, like a blog or an event registration. In these cases, a larger implementation with or without databases is more suitable.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Leonardo Pessoa
Systems Engineer
Brazil Brazil
I may seem nice but I rewrite other people's code from scratch.

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Comments and Discussions

Questionregarding authentication tools Pin
kailasapu murali4-Jun-13 20:22
memberkailasapu murali4-Jun-13 20:22 
QuestionHow to get Windows Identity Pin
Member 72740730-Nov-09 13:07
memberMember 72740730-Nov-09 13:07 
AnswerRe: How to get Windows Identity Pin
Harkos1-Dec-09 1:08
memberHarkos1-Dec-09 1:08 
GeneralRe: How to get Windows Identity Pin
Member 7274071-Dec-09 5:02
memberMember 7274071-Dec-09 5:02 
Generalldap java Pin
Naciye2-Jul-08 5:15
memberNaciye2-Jul-08 5:15 
AnswerRe: ldap java Pin
Harkos2-Jul-08 11:13
memberHarkos2-Jul-08 11:13 
GeneralRe: ldap java Pin
Naciye2-Jul-08 22:13
memberNaciye2-Jul-08 22:13 
QuestionHow to use the source code? Pin
Jason Law27-Nov-07 20:09
memberJason Law27-Nov-07 20:09 
Sorry for such silly question but I really have no idea how to use the source code attached in my windows application.
Can you please give me an example?
Thanks in advance.

Jason Law

QuestionFailing to authenticate Pin
Bogo Mip22-Feb-07 16:02
memberBogo Mip22-Feb-07 16:02 
AnswerRe: Failing to authenticate Pin
Harkos23-Feb-07 0:56
memberHarkos23-Feb-07 0:56 
GeneralExtranet Pin
dave at b22-Nov-06 7:34
memberdave at b22-Nov-06 7:34 
GeneralRe: Extranet Pin
dave at b22-Nov-06 7:35
memberdave at b22-Nov-06 7:35 
AnswerRe: Extranet Pin
Harkos23-Nov-06 1:16
memberHarkos23-Nov-06 1:16 
GeneralMethod LogonValid Pin
stancrm3-Nov-06 4:34
memberstancrm3-Nov-06 4:34 
GeneralRe: Method LogonValid Pin
Harkos3-Nov-06 11:47
memberHarkos3-Nov-06 11:47 
Generalnewbie in ldap Pin
LouPadrino25-Jul-06 0:33
memberLouPadrino25-Jul-06 0:33 
AnswerRe: newbie in ldap [modified] Pin
Harkos25-Jul-06 3:47
memberHarkos25-Jul-06 3:47 
GeneralRe: newbie in ldap Pin
LouPadrino25-Jul-06 4:24
memberLouPadrino25-Jul-06 4:24 
GeneralObject O Pin
James Curran13-Jun-06 13:51
memberJames Curran13-Jun-06 13:51 
AnswerRe: Object O Pin
Harkos14-Jun-06 2:21
memberHarkos14-Jun-06 2:21 
QuestionHow to get results from command "net session" using C#? Pin
Pi po10-Apr-06 19:15
memberPi po10-Apr-06 19:15 
AnswerRe: How to get results from command "net session" using C#? Pin
Harkos20-Apr-06 3:40
memberHarkos20-Apr-06 3:40 
GeneralSecurity accessing AD/LDAP Pin
Harkos15-Sep-05 3:38
memberHarkos15-Sep-05 3:38 
GeneralCreate user on remote IIS server via LDAP Pin
dragomir14-Sep-05 9:02
memberdragomir14-Sep-05 9:02 
GeneralRe: Create user on remote IIS server via LDAP Pin
Harkos15-Sep-05 3:19
memberHarkos15-Sep-05 3:19 

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