Projects/KerberosDelegationACL
Background
Customers would like greater access controls for constrained delegation. The current implementation provides ACLs for S4U2Proxy requests to restrict which target services the KDC can issue for proxy principals, but does not have provisions for which user principals that the proxy principal can request service tickets on behalf of.
Design
The proposed solution is implemented in the LDAP back-end only. The proposed attributes and classes are:
attributeTypes: ( 2.16.840.1.113730.3.8.11.20
NAME 'memberPrincipal'
DESC 'Principal names member of a groupOfPrincipals group'
EQUALITY caseIgnoreMatch
SUBSTR caseIgnoreSubstringsMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.15)
objectClasses: ( 2.16.840.1.113730.3.8.12.6
NAME 'groupOfPrincipals'
SUP top AUXILIARY
MUST ( cn )
MAY ( memberPrincipal ))
attributeTypes: ( 2.16.840.1.113730.3.8.11.21
NAME 'AllowToImpersonate'
DESC 'Principals that can be impersonated'
SUP distinguishedName)
attributeTypes: ( 2.16.840.1.113730.3.8.11.22
NAME 'AllowedTarget'
DESC 'Target principals alowed to get a ticket for'
SUP distinguishedName)
objectClasses: ( 2.16.840.1.113730.3.8.12.7
NAME 'Krb5DelegationACL'
SUP groupOfPrincipals STRUCTURAL
MAY ( AllowToImpersonate $ AllowedTarget ))
Groupings are proposed given that one instance could be used across multiple entries instead of having to define these multiple times for each entry.
groupOfPrincipals is first created (which may contain a single principal) for clients and targets, and then joining all together into an 'ACL object'.
Note that in the schema, lack of AllowToImpersonate means ALL clients can be impersonated. This may not be adhere to conservative security practices, but is in line with the general behavior for constrained delegation access, has better usability, and performance. We haven't implemented it yet, but also you could have regexes in memberPrincipal for additional flexibility.
This schema allows us to resolve the ACL using a single LDAP query (if your LDAP server supports the dereference control). The implementation could try the dereference control query first and then fall-back to a join control query after the first query fails.
We find this schema was the optimal compromise between the flexibility we needed and the complexity we wanted to allow, and simpler ones would prevent us from doing what's needed in a useful manner.
The only drawback is that you cannot fit this into kadmin as it is, because it requires to be able to represent grouping mechanism and ACL objects, it would be nice if kadmin could be extended so that it is flexible enough to allow this kind of representation.
Use Cases
The main use case is to restrict which user principals are allowed to be impersonated by the proxy principal for the target service principal. Administrators that want to limit access to certain users for target services can use the ACL design without needing to update target service applications/systems to use interpret authorization data. This assumes a non-cross realm environment.
This use case can be expanded to allow for a set of multiple users to be impersonated by a set of proxy principals for a set of target principals. This is accomplished with out having to maintain separate ACL lists for various target services by using groupings similar to network groups.
Example
We want to allow our HTTP server cluster to impersonate 2 users (shawn and simo) against the LDAP server cluster:
First we create a group of users:
dn: cn=allowed-users,SUFFIX objectclass: groupOfPrincipals cn: cn=allowed-users memberPrincipal: shawn@KERBEROS.ORG memberPrincipal: simo@KERBEROS.ORG
Then a group of target servers (we assume there may be multiple copies all equally accessible):
dn: cn=LDAP-Servers,SUFFIX objectclass: groupOfPrincipals cn: LDAP-Servers memberPrincipal: ldap/ldap-server-1.kerberos.org@KERBEROS.ORG memberPrincipal: ldap/ldap-server-2.kerberos.org@KERBEROS.ORG
Finally we create the allow rule which binds clients, targets, and the proxy service:
dn: cn=http-ldap-delegation,SUFFIX objectclass: groupOfPrincipals objectclass: Krb5DelegationACL cn: http-ldap-delegation AllowToImpersonate: cn=allowed-users,SUFFIX AllowedTarget: cn=LDAP-Servers,SUFFIX memberPrincipal: HTTP/http-proxy-1.kerberos.org@KERBEROS.ORG memberPrincipal: HTTP/http-proxy-2.kerberos.org@KERBEROS.ORG memberPrincipal: HTTP/http-proxy-3.kerberos.org@KERBEROS.ORG
Potential Conflicts
After investigating w2k12, there could be a way to deploy something with similar functionality.
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In regards to Windows Server 2012: The "Delegation" tab is still present through the AD Users and Computers interface, but has the traditional behavior of the front-end selecting which resource services it can delegate to, instead of selecting which front-end services can delegate to the resource service. The security tab is not present in the AD Users and Computers interface, but rather through the AD Administrative Center. So to emulate the functionality of the Delegation ACL (using the same actors as described in the KerberosDelegationACL project page), in the AD environment:
1. Create a group "allowed-users" with "shawn@KERBEROS.ORG" and "simo@KERBEROS.ORG" as members. 2. Select "ldap-server-1"s and "ldap-server-2"s security tab and add "allowed-users". 3. Create another group "http-ldap-delegation" with "HTTP/http-proxy-1.kerberos.org@KERBEROS.ORG", "HTTP/http-proxy-2.kerberos.org@KERBEROS.ORG", and "HTTP/http-proxy-3.kerberos.org@KERBEROS.ORG" as members. 4. Then restrict the delegation group to the resource with the following invocation: set-adcomputer ldap-server-1 -PrincipalsAllowedToDelegateToAccount http-ldap-delegation set-adcomputer ldap-server-2 -PrincipalsAllowedToDelegateToAccount http-ldap-delegation
So above, the KDC will restrict the resource ticket issuance for S4U2Proxy requests to http-ldap-delegation. The user access to the resource will be controlled by the allowed-users group, which differs from the Delegation ACL project by the fact that the authorization is performed by the application rather than enforced by the KDC. Note that the above should also be supported through x-realms.
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The benefits of the proposed project justifies having differences between the vendor implementations:
1. Policy decisions can be made by the KDC w/o having to rely on updating application servers to perform the policy enforcement against the user. 2. Bindings are/can be user-to-service controlled providing finer granular authorization to said services.
