As technology advances, the integrity and security of society’s online transactions, data, and identities are is of paramount importance.

From online banking to social media and various forms of digital identity, we are increasingly entrusting digital systems with mission-critical societal functions. With this progress, the sophistication, frequency, and attack surface of cyberattacks continues to increase.

Replay Attacks Explained

Replay attacks occur when an attacker intercepts an existing message—often encrypted—and maliciously retransmits the valid message to the receiver to gain authentication or initiate fraudulent actions on a network. Simply put, in a replay attack, an attacker effectively masquerades as a valid message sender by intercepting and then “replaying” the same valid message repeatedly to the receiver.

Replay attacks are known for their simplicity. They don’t require complicated tasks such as cracking encryption codes or exploiting software vulnerabilities. Instead, replay attacks solely require an attacker to capture, store, and reissue valid messages sent by valid network participants without being detected by the network. They’re often used to perform unauthorized actions, duplicate transactions fraudulently, or impersonate users.

Replay Attack Examples

Replay attacks are a fairly universal concept in cybersecurity. From online banking transactions to keyless car entry, replay attacks are a security concern whenever an authenticated message authorizes a specific action. This action can be unlocking a car, sending a banking transaction, or any other number of security-sensitive actions.

Online Banking

A simple example of replay attacks can be seen in online banking. When a user initiates a transaction such as transferring funds to another user, the validity of the transaction is often authenticated using a digital token or signature.

In a replay attack, an attacker captures a transaction message, which includes an encrypted digital token or signature, and then replays the exact transaction in a repeated manner to potentially transfer funds multiple times without the user’s consent by using the same message repeatedly.

Without specific protections in place, the online banking network might assume these duplicated transactions are valid because they are being sent using an accepted digital token or signature.

Keyless Car Entry

Keyless car entry often works using specific radio waves that, when transmitted in close vicinity to the car, unlock the vehicle.

In a replay attack, an attacker can place a device near a keyless-entry car to capture the specific radio frequency used to unlock a car and store it for later use. Again, without the proper protections in place, this would give the attacker the ability to unlock the car in a repeated manner because they have captured the particular radio frequency that acts as authentication for entry.

Network Authentication

Businesses often house sensitive information within networks, with key security measures such as authentication processes set in place to ensure only valid participants can access particular information.

A replay attack in a network communications setting involves intercepting a successful authentication process—often using a valid session token that gives a particular user access to the network—and replaying that authentication to the network to gain access.

Again, this does not require any decryption or software vulnerabilities. If the attacker can sneak into the middle of the transmission and then replay it later for the recipient exactly as it has been sent, the network can be fooled into giving the attacker access to the network.