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Replay Attacks

A replay attack occurs when an adversary intercepts network traffic and retransmits it to gain unauthorized access or facilitate a man-in-the-middle attack. Even hashed or encrypted credentials can be vulnerable if the authentication protocol lacks proper replay protection.

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About this video

A replay attack is a broad category of network attack in which an adversary captures legitimate data in transit and retransmits it, either to gain unauthorized access or to manipulate communications between two parties. The attack does not require the adversary to fully understand or decrypt the intercepted data — simply replaying it can be enough to achieve the attacker's objective, depending on how the target system handles incoming requests. A credential replay attack illustrates this concept clearly. When a client authenticates with a server, the credentials may be hashed before transmission, preventing the attacker from reading the actual password. However, if the authentication protocol lacks replay protection, the attacker can capture the hash and retransmit it verbatim, gaining the same level of access as the legitimate user. This means that hashing or encrypting credentials alone is not sufficient to prevent replay attacks — the protocol itself must be designed to ensure each authentication exchange is unique. Replay attacks also appear in adversary-in-the-middle scenarios, where the attacker inserts themselves into the communication path between two systems. In this position, the attacker receives traffic from one party and forwards it to the other, with the ability to replay, delay, or alter messages in either direction. Defending against replay attacks typically involves protocol-level controls such as session tokens, timestamps, or nonces — values that make each exchange unique and cause retransmitted traffic to be rejected as invalid.

What you'll learn

Explain how replay attacks exploit intercepted network traffic, including credential replay and adversary-in-the-middle scenarios

What's covered

Replay Attacks

What Is a Replay Attack
- Adversary Overhears Traffic
- Traffic Replayed to Server
Credential Replay Attack
- Client Authenticates to Server
- Hashed Credentials Intercepted
- Hash Replayed for Access
Adversary in the Middle
- Traffic Routed Through Adversary
- Traffic Replayed Both Directions
- Traffic May Be Altered
Key Takeaways
- Replays Credentials or Traffic
- Works Against Weak Auth Protocols

Key terms

Replay Attack: An attack in which a threat actor captures valid network data — such as an authentication token or session credential — and retransmits it later to impersonate the original user or device without needing to know the underlying password.
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: A type of replay attack in which intercepted authentication data, such as a hashed password, is retransmitted to a server to gain unauthorized access without knowing the original credential.

Topics

Replay Attacks Network Security Authentication Adversary In The Middle Credential Theft Cryptography

Transcript

What a Replay Attack Is

Replay attacks are just kind of a general category of attacks that can be applied in a few different scenarios. A replay attack is quite simply an attack that replays certain traffic. So if there is a client machine talking to a server, but an adversary overhears that message, they can replay that attack and send it to the server.

Credential Replay

There's some advantages to doing this. One great example is a credential replay attack, where an adversary is going to replay credentials. So what would that look like?

Well, we have right here a client machine, and maybe they're authenticating with this server right here. Now, their information could be sent through clear text, so their password could be sent through clear text. But maybe not — let's say it's more secure than that, and so it hashes that information before it sends it across, so no one can listen in on that conversation. Well, even though this adversary never sees what the password actually is, because it's hashed, they can see what the hash is, and maybe they replay that hash, replay that communication, therefore getting access to that server.

So that's just an example of how that same information could be replayed, whether it's hashed or encrypted or whatever the case. If it's not a correct protocol for authentication, then what could happen is it could be susceptible to a replay attack and get replayed, therefore the adversary gaining access to that server.

Adversary in the Middle

An adversary in the middle is also another example of that. So here we have an adversary, and maybe traffic is just coming to this adversary, and then that adversary is then sending it to where it needs to go, and then the reverse traffic comes back through. So the adversary is sitting in the middle. What's happening is this adversary is just replaying that traffic back and forth. Now, they could be altering it, but maybe it's exactly the same, and sending it back and forth between these two devices.

So here's that attack card on the replay attack. It just means that the adversary is replaying network traffic, and it could be replaying credentials to gain access, or it could be replaying network traffic for some sort of adversary in the middle attack.

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