Directory servers are often used in multi-tier applications to store user profiles, preferences, or other information useful to the application. Oftentimes the web application includes an administrative console to assist in the management of that data; allowing operations such as user creation or password reset. Multi-tier environments pose a challenge, however, as it is difficult to determine the identity of the user that actually performed the operation as opposed to the user that simply showed up in the log file(s).
Consider the relationship between the user logging into the web application and the interaction between the web application and a directory server such as OpenDJ.
There are two general approaches that many web applications follow when performing actions against the directory server; I will refer to these as Application Access and User Access. In both scenarios, the user must first log in to the web application. Their credentials may be validated directly against the directory server (using local authentication) or they may be accessing the web application using single sign-on. In either pattern, the user must first prove his identity to the web application before they are allowed to perform administrative tasks. The differences become apparent post authentication and can be found in the manner in which the web application integrates with the directory server to perform subsequent administrative tasks.
Note: The following assumes that you are already familiar with OpenDJ access control. If this is not the case, then it is highly advisable that you review the following: OpenDJ Access Control Explained.
In the case of the Application Access approach, all operations against the directory server are performed as an application owner account configured in the directory server. This account typically has a superset of privileges required by all Web Application administrators in order to perform the tasks required of those users. In this scenario, the Web Application binds to the directory server using its Web Application service account and performs the operation. A quick look in the directory server log files demonstrates that all operations coming from the Web Application are performed by the service account and not the user who logged in to the Web Application.
[27/Mar/2015:16:37:40 +0000] BIND REQ conn=2053 op=0 msgID=1 version=3 type=SIMPLE dn=”uid=WebApp1,ou=AppAccounts,dc=example,dc=com”
[27/Mar/2015:16:37:40 +0000] BIND RES conn=2053 op=0 msgID=1 result=0 authDN=”uid=WebApp1,ou=AppAccounts,dc=example,dc=com” etime=1
[27/Mar/2015:16:37:40 +0000] SEARCH REQ conn=2053 op=1 msgID=2 base=”ou=People,dc=example,dc=com” scope=wholeSubtree filter=”(l=Tampa)” attrs=”ALL”
[27/Mar/2015:16:37:40 +0000] SEARCH RES conn=2053 op=1 msgID=2 result=0 nentries=69 etime=2
While easiest to configure, one drawback to this approach is that you need to reconcile the directory server log files with the Web Application log files in order to determine the identity of the user performing the action. This makes debugging more difficult. Not all administrators have the same access rights; so another problem with this approach is that entitlements must be maintained and/or recognized in the Web Application and associated with Web Application users. This increases complexity in the Web Application as those relationships must be maintained in yet another database. Finally, some security officers may find this approach to be insecure as the entry appearing in the log files is not indicative of the user performing the actual operation.
The User Access approach is an alternative where the Web Application impersonates the user when performing operations. Instead of the Web Application binding with a general service account, it takes the credentials provided by the user, crafts a user-specific distinguished name, and then binds to the directory server with those credentials. This approach allows you to manage access control in the directory server and the logs reflect the identity of the user that performed the operation.
[27/Mar/2015:17:01:01 +0000] BIND REQ conn=2059 op=0 msgID=1 version=3 type=SIMPLE dn=”uid=bnelson,ou=Administators,dc=example,dc=com”
[27/Mar/2015:17:01:01 +0000] BIND RES conn=2059 op=0 msgID=1 result=0 authDN=” uid=bnelson,ou=Administators,dc=example,dc=com ” etime=1
[27/Mar/2015:17:40:40 +0000] SEARCH REQ conn=2059 op=1 msgID=2 base=”ou=People,dc=example,dc=com” scope=wholeSubtree filter=”(l=Tampa)” attrs=”ALL”
[27/Mar/2015:17:40:40 +0000] SEARCH RES conn=2059 op=1 msgID=2 result=0 nentries=69 etime=2
A benefit to this approach is that entitlements can be maintained in the directory server, itself. This reduces the complexity of the application but requires that you configure appropriate access controls for each user. This can easily be performed at the group level, however, and even dynamically configured based on user attributes. A drawback to this approach is that the Web Application is acting as if they are the user – which they are not.
The Browser is essentially the user and the Browser is not connecting directly to the directory server. So while the log files may reflect the user, they are somewhat misleading as the connection will always be from the Web Application. The other problem with this approach is the user’s credentials must be cached within the Web Application in order to perform subsequent operations against the directory server.
One could argue that you could simply keep the connection between the Web Application and the directory server open, and that is certainly an option, but you would need to keep it open for the user’s entire session to prevent them from having to re-authenticate. This could lead to performance problems if you have extended session durations, a large number of administrative users, or a number of concurrent sessions by each administrative user.
There are both benefits and drawbacks to each of the previously mentioned approaches, but I would like to offer up an alternative proxy-based approach that is essentially a hybrid between the two. RFC 4370 defines a proxied authorization control (2.16.840.1.113730.3.4.18) that allows a client (i.e. the Web Application) to request the directory server (i.e. OpenDJ) to perform an operation not based on the access control granted to the client, but based on another identity (i.e. the person logging in to the Web Application).
The proxied authorization control requires a client to bind to the directory server as themselves, but it allows them to impersonate another entry for a specific operation. This control can be used in situations where the application is trusted, but they need to perform operations on behalf of different users. The fact that the client is binding to the directory server eliminates the need to cache the user’s credentials (or re-authenticate for each operation).
The fact that access is being determined based on that of the impersonated user means that you can centralize entitlements in the directory server and grant access based on security groups. This is essentially the best of both worlds and keeps a smile on the face of your security officer (as if that were possible).
So how do you configure proxy authorization? I am glad you asked.
Before configuring proxied access, let’s return to the example of performing a search based on Application Access. The following is an example of a command line search that can be used to retrieve information from an OpenDJ server. The search operation uses the bindDN and password of the WebApp1 service account.
./ldapsearch -h localhost -D “uid=WebApp1,ou=AppAccounts,dc=example,dc=com ” -w password -b “ou=People,dc=example,dc=com” “l=Tampa”
The response to this search would include all entries that matched the filter (l=Tampa) beneath the container (ou=People). My directory server has been configured with 69 entries that match this search and as such, the OpenDJ access log would contain the following entries:
[27/Mar/2015:16:37:40 +0000] SEARCH REQ conn=2053 op=1 msgID=2 base=”ou=People,dc=example,dc=com” scope=wholeSubtree filter=”(l=Tampa)” attrs=”ALL”
[27/Mar/2015:16:37:40 +0000] SEARCH RES conn=2053 op=1 msgID=2 result=0 nentries=69 etime=2
As previously mentioned, these are the results you would expect to see if the search was performed as the WebApp1 user. So how can you perform a search impersonating another user? The answer lies in the parameters used in the search operation. The LDAP API supports a proxied search, you just need to determine how to access this functionality in your own LDAP client.
Note: I am using ldapsearch as the LDAP client for demonstration purposes. This is a command line tool that is included with the OpenDJ distribution. If you are developing a web application to act as the LDAP client, then you would need to determine how to access this functionality within your own development framework.
The OpenDJ search command includes a parameter that allows you to use the proxy authorization control. Type ./ldapsearch –help to see the options for the ldapsearch command and look for the -Y or –proxyAs parameter as follows.
Now perform the search again, but this time include the proxy control (without making any changes to the OpenDJ server). You will be binding as the WebApp1 account, but using the -Y option to instruct OpenDJ to evaluate ACIs based on the following user: uid=bnelson,ou=People,dc=example,dc=com.
./ldapsearch -h localhost -D “uid=WebApp1,ou=AppAccounts,dc=example,dc=com” -w password –Y “uid=bnelson,ou=People,dc=example,dc=com” -b “ou=People,dc=example,dc=com” “l=Tampa”
You should see the following response:
SEARCH operation failed
Result Code: 123 (Authorization Denied)
Additional Information: You do not have sufficient privileges to use the proxied authorization control The request control with Object Identifier (OID) “2.16.840.1.113730.3.4.18” cannot be used due to insufficient access rights
The corresponding entries in OpenDJ’s access log would be as follows:
[27/Mar/2015:10:47:18 +0000] SEARCH REQ conn=787094 op=1 msgID=2 base=”ou=People,dc=example,dc=com” scope=wholeSubtree filter=”(l=Tampa)” attrs=”ALL”
[27/Mar/2015:10:47:18 +0000] SEARCH RES conn=787094 op=1 msgID=2 result=123 message=”You do not have sufficient privileges to use the proxied authorization control You do not have sufficient privileges to use the proxied authorization control” nentries=0 etime=1
The key phrase in these messages is the following:
You do not have sufficient privileges to use the proxied authorization control
The key word in that phrase is “privileges” as highlighted above; the WebApp1 service account does not have the appropriate privileges to perform a proxied search and as such, the search operation is rejected. The first step in configuring proxied access control is to grant proxy privileges to the Application Account.
The first step in allowing the WebApp1 service account to perform a proxied search is to give that account the proxied-auth privilege. You can use the ldapmodify utility to perform this action as follows:
./ldapmodify -D “cn=Directory Manager” -w password
dn: uid=WebApp1,ou=AppAccounts,dc=example,dc=com
changetype: modify
add: ds-privilege-name
ds-privilege-name: proxied-auth
Processing MODIFY request for uid=WebApp1,ou=AppAccounts,dc=example,dc=com
MODIFY operation successful for DN uid=WebApp1,ou=AppAccounts,dc=example,dc=com
Now repeat the proxied search operation.
./ldapsearch -h localhost -D “uid=WebApp1,ou=AppAccounts,dc=example,dc=com” -w password –Y “uid=bnelson,ou=People,dc=example,dc=com” -b “ou=People,dc=example,dc=com” “l=Tampa”
Once again your search will fail, but this time it is for a different reason.
SEARCH operation failed
Result Code: 12 (Unavailable Critical Extension)
Additional Information: The request control with Object Identifier (OID) “2.16.840.1.113730.3.4.18” cannot be used due to insufficient access rights
The corresponding entries in OpenDJ’s access log would be as follows:
[27/Mar/2015:11:39:17 +0000] SEARCH REQ conn=770 op=1 msgID=2 base=” ou=People,dc=example,dc=com ” scope=wholeSubtree filter=”(l=Tampa)” attrs=”ALL”
[27/Mar/2015:11:39:17 +0000] SEARCH RES conn=770 op=1 msgID=2 result=12 message=”” nentries=0 authzDN=”uid=bnelson,ou=People,dc=example,dc=com” etime=3
As discussed in OpenDJ Access Control Explained, authorization to perform certain actions may consist of a combination of privileges and ACIs. You have granted the proxied-auth privilege to the WebApp1 service account, but it still needs an ACI to allow it to perform proxy-based operations. For the purposes of this demonstration, we will use the following ACI to grant this permission.
(targetattr=”*”) (version 3.0; acl “Allow Proxy Authorization to Web App 1 Service Account”; allow (proxy) userdn=”ldap:///uid=WebApp1,ou=AppAccounts,dc=example,dc=com”;)
This ACI will be placed at the root suffix for ease of use, but you should consider limiting the scope of the ACI by placing it at the appropriate branch in your directory tree (and limiting the targetattr values).
Once again, you can use the ldapmodify utility to update OpenDJ with this new ACI.
./ldapmodify -D “cn=Directory Manager” -w password
dn: dc=example,dc=com
changetype: modify
add: aci
aci: (targetattr=”*”) (version 3.0; acl “Allow Proxy Authorization to Web App 1 Service Account”; allow (proxy) userdn=”ldap:///uid=WebApp1,ou=AppAccounts,dc=example,dc=com”;)
Processing MODIFY request for dc=example,dc=com
MODIFY operation successful for DN dc=example,dc=com
Now repeat the proxied search a final time.
./ldapsearch -h localhost -D “uid=WebApp1,ou=AppAccounts,dc=example,dc=com” -w password –Y “uid=bnelson,ou=People,dc=example,dc=com” -b “ou=People,dc=example,dc=com” “l=Tampa”
This time you should see the results of the search performed correctly. But how do you know that this was a proxied search and not simply one performed by the WebApp1 as before? The clue is once again in the OpenDJ access log file. Looking in this file, you will see the following entries:
[27/Mar/2015:11:40:23 +0000] SEARCH REQ conn=797 op=1 msgID=2 base=”ou=People,dc=example,dc=com” scope=wholeSubtree filter=”(l=Tampa)” attrs=”ALL”
[27/Mar/2015:11:40:23 +0000] SEARCH RES conn=797 op=1 msgID=2 result=12 message=”” nentries=69 authzDN=”uid=bnelson,ou=people,dc=example,dc=com” etime=1
The authzDN value contains the DN of the entry used for authorization purposes. This is a clear indicator that access control was based on the uid=bnelson entry and not uid=WebApp1.
Still not convinced? You can verify this by removing the rights for the uid=bnelson entry and running your search again. Add the following ACI to the top of your tree.
(targetattr=”*”)(version 3.0;acl ” Deny Access to BNELSON”; deny (all)(userdn = “ldap:///uid=bnelson,out=people,dc=example,dc=com”);)
Now run the search again. This time, you will not see any errors, but you will also not see any entries returned. While you are binding as the WebApp1 service account, for all intents and purposes, you are impersonating the uid=bnelson user when determining access rights.
The following steps should be performed when configuring OpenDJ for proxied access control.
Create the Application Account in OpenDJ (i.e. WebApp1)
