Contact Us Today 01642 716680

Replay Attack

Definition: A replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed by an attacker who intercepts the data and retransmits it.

A replay attack poses a significant threat because they can lead to unauthorised access and execution of operations within a network. The attacker essentially eavesdrops on a secure network communication, captures authentication protocols, and reuses them to trick the system into thinking that a new request is legitimate. This type of attack exploits the fact that some authentication systems do not have measures in place to distinguish between legitimate and fraudulently reused credentials.

To combat a replay attack, security mechanisms such as the use of nonces (random numbers used only once), timestamps, session tokens, and challenge-response authentication are employed, which can help detect and prevent replayed data packets. Secure communication protocols, such as Transport Layer Security (TLS), also include measures to protect against replay attacks.

Tools such as ‘ntlmrelayx’ from the Impacket suite can be used to perform relay attacks, which are a variation of replay attacks. In a relay attack, the attacker intercepts credentials and relays them to another service to authenticate as if they were the victim. Impacket’s ‘ntlmrelayx’ tool is particularly effective against networks using NTLM (NT LAN Manager) authentication. An attacker can relay NTLM authentication sessions and execute arbitrary code, gain elevated privileges, or access restricted resources.

The continuous development of penetration testing tools such as Impacket and the ever-evolving threat landscape mean that network and security professionals must remain vigilant, implementing comprehensive measures against such attacks.

Key Characteristics:

  • Unauthorised retransmission or delay of valid data transmission
  • Often targets authentication communications
  • Defended against with unique session tokens, timestamps, and nonce values
  • Can be executed using tools like ‘ntlmrelayx’ part of the Impacket suite


  • Real-World Example: An attacker intercepts a user’s authentication token on an unsecured wireless network and uses it to gain access to a secured resource without the user’s knowledge, hours after the original token was sent.
  • Hypothetical Scenario: Using ‘ntlmrelayx’, an attacker relays NTLM credentials sent by a user to authenticate to a file server, providing access to sensitive corporate documents without having to crack or guess a password.

Related Terms:

  • Nonce: A random value that is generated for a specific session or transaction to prevent replay attacks.
  • NTLM Authentication: A challenge-response authentication protocol used by Microsoft, which can be vulnerable to relay and replay attacks if not properly secured.
  • Challenge-Response Authentication: An authentication mechanism where the user presents a response to a challenge (e.g., a password), with safeguards against replay attacks.
  • Impacket: A collection of Python classes and tools for working with network protocols, often used in penetration testing and network attacks.

Learn better by watching a video? Here is a YouTube video explaining the concept.

What is the OWASP Top 10: Download our flash cards to find out.

Inside you will find a description of the most common web vulnerabilities.

Contact us

Get a free, no obligation quote from one of our expert staff.