A replay attack is a type of man-in-the-middle attack where a hacker intercepts a network session and reuses the legitimate user’s credentials to repeat the session. For example, if an attacker performs packet sniffing or another form of eavesdropping on a user’s Internet session and intercepts their login, the attacker can “replay” that session and log into the user’s account. The replayed session or request appears legitimate because it originally came from the real user. Even encrypted Internet sessions are susceptible to replay attacks because the attacks don’t require decryption: the hacker can simply replay the session, encrypted data and all. This is particularly dangerous for enterprises, which store a wealth of sensitive data within different accounts.
Preventing replay attacks
Because replay attacks depend on reusing the session credentials that an attacker has intercepted, preventing replay attacks often means generating a single-use encryption key or ID for an Internet session. Many network transmissions between two users now use a specific, single-use encryption key, which is only valid for one session and will not allow an attacker to replay the session.
Users may even log into an account with a single-use password, which will have to be reset for every subsequent login. This prevents a replay attacker from submitting another request with the intercepted password; it will no longer be usable.
A virtual private network may protect users from man-in-the-middle attacks: they set up a computer network separate from the standard network, which typically prevents attackers from eavesdropping on the Internet connection. However, VPNs are not perfect, and they’ve occasionally allowed attackers to access the user’s network through endpoint insecurities. Some VPNs actually have flaws that allow attackers to replay Internet sessions, having gained access to their network connection using cookies that weren’t dealt with properly. If you are using a VPN to avoid replay attacks, research different options carefully and watch for security bugs that have come to light in certain VPN products.