Forged tickets
Silver, Golden, Diamond and Sapphire tickets are forged or modified Kerberos tickets that can be used with pass-the-ticket to access services in an Active Directory domain.
Glossary
PAC (Privileged Authentication Certificate): a special set of data contained in the ticket (TGT or Service Ticket) that give information about the requesting user (username, groups, UserAccountControl, etc.).
Long-term key: the long-term key of an account refers to its NT hash (when the RC4 etype is not disabled in the domain) or another Kerberos key (DES, AES128, AES256).
Techniques
Silver tickets
The long-term key of a service account can be used to forge a Service ticket that can later be used with Pass-the-ticket to access that service. In a Silver Ticket scenario, an attacker will forge a Service Ticket containing a PAC that features arbitrary information about the requesting user, effectively granting lots of access.
Read the Silver ticket article for more insight
Golden ticket
The long-term key of the krbtgt account can be used to forge a special TGT (Ticket Granting Ticket) that can later be used with Pass-the-ticket to access any resource within the AD domain. The krbtgt's key is used to encrypt the PAC. In a Golden Ticket scenario, an attacker that has knowledge of the krbtgt long-term key, will usually forge a PAC indicating that the user belongs to privileged groups. This PAC will be embedded in a forged TGT. The TGT will be used to request Service Tickets than will then feature the PAC presented in the TGT, hence granting lots of access to the attacker.
Read the Golden ticket article for more insight
Diamond ticket
Golden and Silver tickets can usually be detected by probes that monitor the service ticket requests (KRB_TGS_REQ) that have no corresponding TGT requests (KRB_AS_REQ). Those types of tickets also feature forged PACs that sometimes fail at mimicking real ones, thus increasing their detection rates. Diamond tickets can be a useful alternative in the way they simply request a normal ticket, decrypt the PAC, modify it, recalculate the signatures and encrypt it again. It requires knowledge of the target service long-term key (can be the krbtgt for a TGT, or a target service for a Service Ticket).
Read the Diamond ticket article for more insight
Sapphire ticket
Sapphire tickets are similar to Diamond tickets in the way the ticket is not forged, but instead based on a legitimate one obtained after a request. The difference lays in how the PAC is modified. The Diamond ticket approach modifies the legitimate PAC to add some privileged groups (or replace it with a fully-forged one). In the Sapphire ticket approach, the PAC of another powerful user is obtained through an S4U2self+u2u trick. This PAC then replaces the one featured in the legitimate ticket. The resulting ticket is an assembly of legitimate elements, and follows a standard ticket request, which makes it then most difficult silver/golden ticket variant to detect.
Read the Sapphire ticket article for more insight
The bronze bit vuln
The Bronze bit vulnerability (CVE-2020-17049) introduced the possibility of forwarding service tickets when it shouldn't normally be possible (protected users, unconstrained delegation, constrained delegation configured with protocol transition).
Kerberos delegations mindmap