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Credential Replay

A credential replay attack allows an adversary to gain unauthorized access to a system by intercepting and reusing authentication data — such as a hashed password — without ever knowing the actual credentials. Understanding this attack and its variants is essential for recognizing authentication vulnerabilities in modern systems.

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About this video

A credential replay attack exploits weaknesses in authentication processes by allowing an adversary to capture and reuse intercepted credentials — such as hashed passwords or authentication tokens — to gain unauthorized access to a system. The attack does not require the adversary to know or crack the actual password. Instead, replaying the captured credential is sufficient to authenticate with the target server, making this a particularly dangerous and stealthy attack vector. The vulnerability becomes clear in protocols like FTP, which transmit credentials in plaintext, giving any attacker monitoring network traffic direct access to user passwords. Hashing credentials before transmission is a common mitigation, but it is not sufficient on its own. If a system accepts a hash as proof of identity without additional verification steps — such as challenge-response mechanisms or session tokens — an attacker who captures the hash can simply replay it to authenticate as the legitimate user. Several well-documented attack techniques fall under the credential replay category, including pass the hash, pass the ticket, and pass the token. Each targets a specific type of authentication credential but shares the same underlying principle: the intercepted data can be reused to impersonate a legitimate user. Defending against these attacks requires authentication systems that incorporate replay-resistant mechanisms, ensuring that captured credentials cannot be reused outside their original session context.

What you'll learn

Explain how a credential replay attack exploits intercepted authentication data to gain unauthorized access without knowing the original credentials
Distinguish between common credential replay attack variants, including pass the hash, pass the ticket, and pass the token

What's covered

Credential Replay Attack

How Attack Works
- Intercept Credential Data
  - Clear Text Example (FTP)
  - Hashed Credentials
- Replay to Server
  - No Password Needed
Why Hashing Is Vulnerable
- Hash Sent Without Verification
- Adversary Replays Hash
Attack Variations
- Pass the Hash
- Pass the Ticket
- Pass the Token
Mitigation
- Additional Verification Process

Key terms

Authentication: The process of verifying the identity of a user, device, or system.
Hash Function: A mathematical algorithm that converts input data of any size into a fixed-size output value used to verify data integrity.
Man-in-the-Middle Attack: An attack where an adversary secretly intercepts and potentially alters communications between two parties.
Credential Replay Attack: An attack where an adversary intercepts authentication data such as a password hash and retransmits it to a server to gain unauthorized access without knowing the original credentials.
Pass the Hash: A credential replay attack variant where an attacker captures a password hash and uses it directly to authenticate to a system without cracking it.
Pass the Ticket: A credential replay attack variant where an attacker steals a Kerberos authentication ticket and reuses it to access network resources without knowing the user's password.
Pass the Token: A credential replay attack variant where an attacker intercepts and reuses an authentication token to gain unauthorized access to systems or applications.

Topics

Credential Replay Pass The Hash Pass The Ticket Pass The Token Authentication Cybersecurity

Transcript

We may be able to get into a system with a username and password without ever knowing the actual username and password. We could do this by doing something like a credential replay attack.

The Scenario

Let's develop a little scenario here. Let's say we have a machine right here and it's trying to FTP into the server. What it's going to do is send the password over to the server so the server can verify that this user can get in and access the files on this FTP server. What's happening is this gets sent with FTP, gets sent over in clear text. So anybody that sees that traffic now has captured that password. This can obviously be really problematic, because now the adversary has the password.

Sending a Hash Instead

In order to protect this password, one thing that can happen is we could hash the password, and then during the authentication process we send the hash over rather than the actual password. Of course, this stops the adversary from being able to see the password and keeps it more secure.

Now, the way this technology works, if it just sends the hash over and doesn't have any other type of verification process that this goes through, then it's susceptible to something called a credential replay attack. This adversary then sees what the hash is. The problem with this is that they might not know what the password is, but what they can do is replay the hash in the communication to the server and then gain access to that server. That's all they need to do, is just replay this in order to gain access to the server.

There are mechanisms that can stop this from happening, but if you just send the hash over, it's still susceptible to this credential replay attack.

Variations

There are many different variations of this credential replay attack. The pass the hash, pass the ticket, and pass the token are a few examples of that.

Here's the attack card on the credential replay attack. That's where an adversary can take the data that's flowing and replay it to the server in order to gain access to the server. They're replaying the credentials here to gain access into the server.

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