Kerberos Unconstrained Delegation

This lab explores a security impact of unrestricted kerberos delegation enabled on a domain computer.
Overview
Unrestricted kerberos delegation is a privilege that can be assigned to a domain computer or a user;
Usually, this privilege is given to computers (in this lab, it is assigned to a computer IIS01) running services like IIS, MSSQL, etc.;
Those services usually require access to some back-end database (or some other server), so it can read/modify the database on the authenticated user's behalf;
When a user authenticates to a computer that has unresitricted kerberos delegation privilege turned on, authenticated user's TGT ticket gets saved to that computer's memory;
The reason TGTs get cached in memory is so the computer (with delegation rights) can impersonate the authenticated user as and when required for accessing any other services on that user's behalf.
Essentially this looks like so:
User --- authenticates to ---> IIS server ---> authenticates on behalf of the user ---> DB server
Any user authentication (i.e CIFS) to the computer with unconstrained delegation enabled on it, will cache that user's TGT in memory, which can later be dumped and reused by an adversary.
Setup
Let's give one of our domain computers/our victim computer IIS01 unrestricted kerberos delegation privilege:
To confirm/find computers on a domain that have unrestricted kerberos delegation property set:
Get-ADComputer -Filter {TrustedForDelegation -eq $true -and primarygroupid -eq 515} -Properties trustedfordelegation,serviceprincipalname,description
We can see our victim computer IIS01 with TrustedForDelegation field set to $true - we are good to attack:
Execution
On the computer IIS01 with kerberos delegation rights, let's do a base run of mimikatz to see what we can find in memory:
sekurlsa::tickets
Note that we do not have a TGT for offense\administrator (Domain Admin) just yet.
Let's now send an HTTP request to IIS01 from a DC01 host from the context of offense\administrator:
Invoke-WebRequest http://iis01.offense.local -UseDefaultCredentials -UseBasicParsing
We see the request got a HTTP 200 OK response:
Let's check the victim host IIS01 for new kerberos tickets in memory:
mimikatz # sekurlsa::tickets
We can see that the IIS01 has now got a TGT for offense\administrator - this means that we have effectively compromised the entire offense.local domain. We will get back to this in a  moment.
First, let's export all kerberos tickets from IIS01 memory, so we can load offense\administrator ticket TGT into the current session and assume its privileges:
mimikatz::tickets /export
..but before we proceed with pass-the-ticket attack and become a DA, let's try PSRemoting to the DC01 from IIS01 and check currently available kerberos tickets in a current logon session - just to make sure we currently do not have DA rights:
Above screenshow shows that there are no tickets and PSSession could not be established - as expected.
Let's now proceed and import the previously dumped offense\administrator TGT into our current logon session on the IIS01 host:
mimikatz # kerberos::ptt C:\Users\Administrator\Desktop\mimikatz\[0;3c785]-2-0-40e10000-[email protected]-OFFENSE.LOCAL.kirbi
Once the TGT is imported on IIS01, let's check available tickets and try connecting to the DC01 again:
As you can see from the above screengrab, the IIS01 system now contains a krbtgt for offense\administrator, which enables this session to access DC01 C$ share and establish a PSSession with an interactive shell with Domain admin privileges.
Reminder
Note that successful authentication to ANY service on the IIS01 will cache the authenticated user's TGT. Below is an example of a user offense\delegate accessing a share on IIS01 - the TGT gets cached:
Mitigation
Some of the available mitigations:
Disable kerberos delegation where possible
Be cautious of whom you give privilege Enable computer and user accounts to be trusted for delegation - these are users who can enable unrestricted kerberos delagation
Enable Account is sensitive and cannot be delegated for high privileged accounts
References
Active Directory Security Risk #101: Kerberos Unconstrained Delegation (or How Compromise of a Single Server Can Compromise the Domain)
Active Directory Security
@_xpn_ - Kerberos AD Attacks - Kerberoasting
XPN InfoSec Blog
Kerberos for the Busy Admin
docsmsft

Kerberoasting: Requesting RC4 Encrypted TGS when AES is Enabled

Kerberos Constrained Delegation

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