What are Replay Attacks and How to Stop Them Forever?

Attention Honda and Acura car owners! Your precious ride may be at risk of theft, all thanks to a tiny open sesame device. Criminals can capture the unlock signals your keyfob emits using open sesame, and replay them later to unlock and steal your car. 

Replay attacks use the same concept we see in the folk tale Ali Baba and the Forty Thieves. Ali Baba overhears and later repeats the magic phrase open sesame to enter the treasure cave. That’s essentially a replay attack in action. 

Replay attacks are commonplace in the cyber world. Cybercriminals can capture the credit card information you enter while shopping online. They can then resend or “replay” it to make fraudulent transactions. 

Replay attacks can be much more sophisticated and damaging than the super-basic example above. Cybercriminals can even extract or manipulate confidential data before retransmitting it. 

What is a Replay Attack?

A replay attack, a.k.a session replay attack, is a network-based cyberattack in which an attacker eavesdrops on your network communications and captures and resends your authenticated data packets to a website or service. Since the replayed message in fact came from you, the website gets tricked into believing it’s a legitimate communication from you.

Replay attacks are a specific type of man-in-the-middle (MitM) attack in which the attacker is essentially sitting in the middle, between you and the destination server you’re trying to reach. Replay attacks are especially bad because they can work even if your data traffic is encrypted. The attacker can simply retransmit the data packet as it is without knowing the details of what’s inside. 

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The Rise of Cyberattacks and Cyber Warfare

Individuals, businesses, and governments alike rely more and more heavily on digital technologies. That’s the way modern life is, and COVID has only increased our dependence on tech, with more people working remotely and other innovations to keep economies going. It’s only natural that cyberattacks — motivated by political agendas, financial gains, or some other nefarious purpose — would also increase. 

Today, even nation-state actors have turned to cyber warfare to complement on-field combat and espionage. Here’s the thing though: cyberattacks know no boundaries. Thanks to the internet’s global connectivity, they can easily escalate beyond the target’s systems. That means no one is safe. 

As the Russia-Ukraine war unfolds, untrained or unaware people from either side can become part of the ongoing conflict. Unlike sophisticated cybercriminals or state-sponsored actors, nonprofessionals are more likely to launch attacks that get out of control. 

Any business or individual, including you, can get caught in the crossfire. A clumsily-crafted cyberattack can hit your home or office network. That’s why it’s important to take your cybersecurity seriously and take all the precautions necessary to navigate the internet safely and privately. 

How Does a Replay Attack Work?

The first step to stopping a cyberattack is to understand how it works, at least on a basic level. Replay attacks in particular are common because they don’t require much technical expertise or sophisticated tools. 

Here’s what a replay attack might look like in action: 

1. You enter your credentials to log in to a website. Your browser sends a data packet with your credentials to the corresponding server.

2. A malicious intruder eavesdrops on your network communication using packet sniffers or network analyzer software. The intruder now has the data packet with your credentials. 

3. You don’t suspect anything and enjoy your online session. 

4. The intruder later resends the captured data packet to the website’s server. The server authenticates the intruder as you because the packet has valid credentials. 

5. The cybercriminal has access to your web account. 

This is just one example of how malicious actors can carry out replay attacks. It’s not hard to imagine all the ways cybercriminals use this kind of MiTM scenario to steal from or generally wreak havoc on people, businesses, and even governments.

How Replay Attacks Harm Your Security

The basic definition of replay attacks could make it seem like you don’t have much at stake except perhaps placing an e-order twice or sending an email multiple times. Replay attacks, however, can be a lot more sinister.

Here’s how three different session replays can play out for you in real life:

⚠️ Impersonation

Cybercriminals usually replay authentication sessions, which give attackers full control of your accounts and all the privileges you enjoy on specific websites or apps. They can impersonate you online, send and receive messages on your behalf, and access confidential data or documents. 

⚠️ Fraudulent Transactions

Cybercriminals can capture your financial transaction requests post-authentication. The receiving server will accept the replayed request and make another transaction since it’s already authenticated. 

⚠️ DDoS Attacks

Attackers may replay your request multiple times to bombard a server with more requests than it can handle, causing denial of service (DoS). You won’t be able to use a service or complete your transactions. Since many websites use mechanisms to prevent DDoS (distributed denial of service) attacks, attackers can even get you blocked from certain websites or services.

How Can Replay Attacks Bypass Encryption?

Passwords are usually hashed before your browser sends them to the corresponding server. That means your password is converted into a random string of characters. The process is irreversible so intruders can’t use the hash to extract your passwords in plain text. 

Unfortunately, intruders don’t need plain-text passwords to execute replay attacks. Here’s how replay attacks render password hashing useless:

• Attackers can capture and pass the password hash instead of the actual password in a pass-the-hash (PtH) attack.
• They may capture and reuse your session ID that websites allot you after you’ve authenticated yourself. Your session ID will give them all the user privileges you have on a website or a service after authentication. 

Prevention Mechanisms for Replay Attacks

Cybersecurity has always been a cat-and-mouse game. Malicious actors manipulate a vulnerability to launch some kind of cyberattack. The cybersecurity community patches the vulnerabilities or comes up with security measures to combat that form of cyberattack. On and on it goes.

Here are some mechanisms websites use to detect and prevent replay attacks:

1. One-time passwords (OTP): This mechanism is often used in banking transactions. When you request a transaction, the server sends you an OTP via email, SMS, or a mobile application. The password is valid for exactly one transaction. The OTP won’t work if someone tries to replay it. 

2. Time-bound session keys: Since attackers can replay encrypted messages, it makes sense to encrypt messages using time-bound session keys that expire after one transaction. 

For instance, you send your credit card information to buy something online. The server side decrypts the information using the random, time-bound session key. If someone resends the encrypted credit card information, the session key would’ve already expired. The fraudulent transaction will fail. 

HTTPS websites use such advanced encryption mechanisms (SSL/TLS encryption) to avoid session replay attacks. 

3. Timestamps: This method involves attaching a timestamp to each request and setting up a short time frame on the server side. The server ignores any request that falls outside the time frame. This is one way secure VPN protocols provide replay protection. 

4. Device intelligence and geolocation review: Certain websites and applications, especially corporate apps and emails, won’t grant access or process other requests if they seem to be coming from an unknown device or unexpected geolocation. Such intelligence-based mechanisms can prevent the misuse of stolen credentials and replay attacks. 

What Can You Do to Stop Replay Attacks?

The methods above are all server-side prevention strategies, meaning the websites or services you visit need to implement them. However, you can follow some security best practices to improve your digital security.

• • 🔒 Avoid unsecured public networks: Most replay attacks occur on unsecured public networks because your IP address and plain-text data are exposed. Malicious users can easily capture your data traffic. If you have to use public Wi-Fi, use a reliable VPN like CyberGhost to get top-notch public Wi-Fi security
  • 🔒Use only HTTPS websites: HTTPS websites use SSL/TLS encryption to protect your communication with the website’s servers. They use time-bound session keys to prevent replay attacks. If you must visit an HTTP website, never enter any credentials, credit card information, or any other sensitive data.
  • 🔒Enable 2FA (two-factor authentication): 2FA ensures that no one can access your accounts with stolen credentials alone. You need to provide another authentication factor that only you can possess, like a one-time code sent to your mobile device or email. 2FA isn’t fool-proof, but will prevent basic replay attacks.
  • 🔒Beware of phishing attacks: All of your security measures will go to waste if you click on a phishing link or email attachment. Cybercriminals use phishing tactics to install packet sniffers or network analyzers on victims’ devices, making it a breeze for them to pull off MitM attacks, including replay attacks.
    Never click on a link or attachment from an unknown sender. Always verify the sender’s email address to ensure it isn’t a spoofed email. Vigilance and caution are the only way you can protect yourself from advanced phishing and spear-phishing attacks.
  • 🔒Use a reliable VPN: Add an extra layer of replay protection even on unsecured, public Wi-Fi networks and HTTP websites with a reliable VPN. CyberGhost VPN reroutes and encrypts your digital data using state-of-the-art VPN protocols with replay protection. 

Can VPNs protect against replay attacks?

Yes, VPNs can protect you against replay attacks, but it really depends on the VPN you’re using. 

Malicious actors usually capture your network communications using easily available software tools. A VPN hides your IP address and encrypts all of your data in a secure tunnel, making it impossible for intruders to identify you or see what you’re up to. This way, a VPN can protect you from targeted replay attacks. 

That said, whether or not VPN encryption works against random replay attacks depends on the VPN protocol a specific VPN service uses. Not all VPN protocols will protect your data traffic against replay attacks. 

Replay Protection: Does the VPN Protocol Matter?

VPN protocols determine how your data is encrypted and rerouted through the VPN tunnel. Let’s take a look at some common VPN protocols, so you can choose a service that uses protocols with replay protection.

• Point-to-Point Tunneling Protocol (PPTP): PPTP is one of the oldest VPN protocols and it’s notoriously prone to all kinds of cyberattacks, including replay attacks. Although it’s outdated, many commercial VPNs, especially the free ones, still use it for its speed-over-security approach. 

• IPSec: IPSec uses an anti-replay window to detect and discard replay data packets. In an IPSec tunnel, each data packet has its own unique sequence number. Data packets with duplicate sequence numbers are automatically discarded. Many VPNs pair Layer 2 Tunnel Protocol (L2TP) with IPSec — L2TP/IPSec — for better speed and security. 

• OpenVPN: OpenVPN is a widely popular, open-source VPN protocol that uses virtually unbreakable 256-bit AES encryption. It also uses timestamps, and sometimes a counter, to identify and discard out-of-sequence and replayed data packets. Being open-source, OpenVPN is always under high scrutiny, and its committed open-source community quickly resolves minor security vulnerabilities as soon as they surface. That makes it one of the strongest VPN protocols around. 

• WireGuard®: WireGuard® is a newer addition to the VPN protocol family. It’s rapidly gained popularity for its fast speed and excellent security. WireGuard® uses timestamping primitive (TAI64N) to detect replay attacks. The protocol drops data packets with older timestamps. 

Why multiple protocols? 

Different kinds of online activities require different levels of speed and security. That’s why CyberGhost lets you choose the one that suits you the most. For instance, you could use OpenVPN to make highly secure banking transactions and switch to WireGuard® for the ultimate speed and DDoS protection while gaming. Either way, you’ll automatically benefit from our replay protection. 

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What to Look For in a VPN for Superior Replay Protection

Here’s a checklist of VPN features that’ll help you find one that can protect your digital identity and prevent replay attacks:

• • ✅ Anti-replay protection: This goes without saying – get a VPN that supports secure VPN protocols with in-built replay protection mechanisms. PPTP and L2TP by themselves don’t qualify.
  • ✅ Device and Platform Support: Replay attacks don’t discriminate against devices or platforms, and neither should your VPN. Get one that supports all your devices, regardless of the OS.
  • ✅ Automatic Kill Switch: You don’t want to accidentally expose yourself if your VPN connection drops for a second. A Kill Switch will block your traffic instantly if your VPN connection falters.
  • ✅ No Logs Policy: What’s the point of hiding your data from your ISP and cybercriminals only to hand it over to your VPN provider? Make sure the VPN you use has a strict No Logs policy, which means it doesn’t keep a record of your activity.
  • ✅ Price: Most free VPNs simply can’t afford to offer state-of-the-art server infrastructure and sophisticated VPN protocols. Remember, if it’s free, it’s probably fishy.

If you’re unsure about committing to a VPN right away, CyberGhost VPN checks all of the boxes above and comes with a generous 45-day money-back guarantee. We let you test the waters before taking the plunge.

Stay Safe Online with CyberGhost VPN

If your life had a replay button, you wouldn’t want it in someone else’s hands. Unlike your life, your online sessions can actually be replayed if you don’t take steps to secure your connections. Replay attacks are just one of the many kinds of cyberattacks your data is vulnerable to. 

Protect your identity, data, and money from prying cybercriminals with a little vigilance and basic security measures, like using a secure VPN and sticking to HTTPS websites. Keep your digital communications to yourself, and put an end to all kinds of MitM attacks with CyberGhost VPN!

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