After obtaining the secret key, he can communicate with Y similarly as we have discussed in the case of X. If the client may communicate again with X, he can use the same previous key; there is no need to generate a ticket every time.
Only for the first time, he needs to obtain the ticket. In this article, we have seen What Kerberos is, how it works, and its advantages and disadvantages. I hope you will find this article helpful. This is a guide to Kerberos. You can also go through our other suggested articles to learn more—. Submit Next Question. By signing up, you agree to our Terms of Use and Privacy Policy. Forgot Password? This website or its third-party tools use cookies, which are necessary to its functioning and required to achieve the purposes illustrated in the cookie policy.
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Using Kerberos authentication within a domain or in a forest allows the user or service access to resources permitted by administrators without multiple requests for credentials. After initial domain sign on through Winlogon, Kerberos manages the credentials throughout the forest whenever access to resources is attempted.
As a result, in Windows operating systems, the Kerberos protocol lays a foundation for interoperability with other networks in which the Kerberos protocol is used for authentication. In addition, Microsoft publishes Windows Protocols documentation for implementing the Kerberos protocol. The documentation contains the technical requirements, limitations, dependencies, and Windows-specific protocol behavior for Microsoft's implementation of the Kerberos protocol.
Before Kerberos, NTLM authentication could be used, which requires an application server to connect to a domain controller to authenticate every client computer or service. With the Kerberos protocol, renewable session tickets replace pass-through authentication. The server is not required to go to a domain controller unless it needs to validate a Privilege Attribute Certificate PAC. Instead, the server can authenticate the client computer by examining credentials presented by the client.
Client computers can obtain credentials for a particular server once and then reuse those credentials throughout a network logon session. By using the Kerberos protocol, a party at either end of a network connection can verify that the party on the other end is the entity it claims to be.
NTLM does not enable clients to verify a server's identity or enable one server to verify the identity of another. It contains the following components:. When a user requests access to a service through the authentication service, they enter their username and password locally, and send the following information:. Authentication service issues a ticket granting ticket TGT if the user exists in the database.
The first message sent back to the user contains:. Another message is sent containing the "Authenticator", which is composed of the User ID and timestamp, encrypted with the user's session key. The TGS will respond to the user with two messages if it finds the user's information within the Kerberos database. The first message will contain the following information, encrypted with the server's secret service key:. A second message, encrypted with the user's session key for example a locked box within a locked box, where the user can only unlock the first box , will contain the service session key.
The user sends the service ticket to the requested service along with the service request in two messages. The first message will be the first message from the previous step encrypted with the server's secret service key. The second message will contain a new Authenticator with an updated timestamp, encrypted with the user's session key. The service server decrypts the ticket using its own secret key to retrieve the user's session key, which is used to decrypt the authenticator.
If the user's ID from previous messages matches, it will send a message encrypted with the user's session key to the user with the timestamp found in the new authenticator to confirm the service's identity. When creating a new account on an Active Directory Domain Controller, you get a username and password. The Kerberos client then adds a string known as a salt - a unique string used to improve the randomness of a credential - along with the Kerberos version number.
In most configurations, the salt is the user's username. It then runs these two values through a string2Key function which will return the shared secret. On a workstation, the user will request access to a service such as logging in to the machine by providing their username and password.
The local Kerberos client will perform the same steps as the DC to arrive at a shared secret. If this secret matches the secret stored on the DC, the user can log in. Now that we know how Kerberos works, it's important to understand the potential vulnerabilities inherent in its implementation, especially in Microsoft's proprietary extension to Kerberos.
You can detect the majority of these attacks using native tools to monitor logs, but it is important to know what to look for. This section will provide a high level overview of the various attacks you'll find against Kerberos systems. A golden ticket is a forged Kerberos key distribution center. You can create usable Kerberos tickets for accounts that do not exist in the Active Directory.
If you believe that someone created an unauthorized golden ticket, you would need to reset the Kerberos service account, krbtgt. While this isn't difficult, there are several critical steps to the process.
Because Active Directory stores the old and current passwords for all accounts, you must reset the krbtgt account twice. But the second reset should occur only after waiting the maximum user ticket lifetime after the first password reset. Microsoft provides a handy script to assist with this here. A silver ticket is similar to a Golden Ticket, but does not have the broad administrative privileges of the golden ticket. An attacker would typically only gain access to a single service on an application, and an attacker must have compromised legitimate user credentials from a computer's SAM or local service account.
What makes these attacks very difficult to detect is that forging a silver ticket for example using the service account password hash does not require any communication with a DC. In a backdoor skeleton key malware attack, the attacker typically has compromised the Domain Controller and executed a successful Golden Ticket attack. When the account authenticates, the malware will check the injected master password hash, and if it's a match will authenticate the user, regardless of the user's true password.
Legitimate users will still be able to log in with their normal credentials.
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