A replay attack, also known as a "Playback" attack, is a form of cyberattack in which an attacker intercepts a victim's communications and then retransmits them over the network.

Network security protocols, based on the authenticity of the original data (usually provided by an authorized user), may mistakenly treat such an attack as a standard data transfer. Because intercepted messages are transmitted unchanged, attackers using this method typically do not require messages to be decrypted.
What can hackers do with such an attack?
Replay attacks pose a security risk by allowing attackers to access protected information using real credentials.

They can also be used to defraud financial institutions by duplicating transactions, allowing hackers to extract funds from victims' accounts. In some cases, attackers can combine pieces of different encrypted messages and then transmit the resulting ciphertext to the network using cut-and-paste tactics. The network's response to such attacks often provides attackers with valuable information that they can use to further infiltrate the system.

Despite the serious risks associated with replay attacks, there are limits to what hackers can do with them. Attackers cannot modify the data they send unless the network rejects it, reducing the effectiveness of the attack by limiting it to repeating previous actions. It is relatively easy to defend against such attacks. For example, adding a timestamp to transmitted data can help prevent simple hacking attempts. Servers can also cache duplicate messages and stop processing them after a certain number of repetitions, limiting the number of times an attacker can try to repeat messages at a rapid pace.
Replay Attacks in the Cryptocurrency World
While replay attacks are not unique, they are particularly relevant in the context of cryptocurrency transactions and blockchain ledgers. This is because blockchain's distributed ledgers are periodically subject to protocol changes and updates, known as hard forks. In a hard fork, the existing blockchain ledger is split into two parts: one continues to use an outdated version of the software, while the other moves to a new, updated version. Some hard forks are intended to simply update the main ledger, while others result in the creation of entirely new cryptocurrencies. One of the most famous examples of the latter type of hard fork is the split of Bitcoin Cash from the Bitcoin main ledger, which occurred on August 1, 2017.

When hard forks occur, attackers could theoretically use replay attacks on the blockchain ledger. Transactions made on one distributed ledger before the hard fork remain valid on the other. This means that the recipient of cryptocurrency through one ledger can repeat the transaction on another ledger, fraudulently transferring an identical amount of funds into their account a second time. However, new wallets created after a hard fork do not have access to the previous transaction history on the blockchain and are therefore not susceptible to such attacks. This creates some level of protection for users joining the blockchain after a hard fork.
How can you protect yourself from such attacks?
Although the vulnerability of forked blockchain ledgers to replay attacks poses a serious threat, most hard forks include special security protocols designed to prevent such attacks from succeeding. There are two main categories of effective measures against replay attacks on a blockchain: strong protection and additional protection against replay attacks. In the case of strong security, a special token is added to the new distributed ledger formed as a result of a hard fork, which ensures that transactions carried out on it will not be recognized as valid in the legacy ledger, and vice versa. This type of protection was implemented, for example, when separating Bitcoin Cash from Bitcoin.

When using strong replay protection, it is launched automatically immediately after a hard fork occurs. On the other hand, Opt-in protection against replay attacks involves the user manually making changes to their transactions so that they cannot be replayed. Opt-in protection can be useful in cases where the hard fork is intended to update the main cryptocurrency ledger, rather than completely split it.

In addition to distributed ledger solutions, individual users can take their own measures to protect against replay attacks. One way is to block the transfer of coins until the distributed ledger reaches a certain number of blocks. This will help prevent any replay attacks that use these coins on the network. However, it should be noted that not all wallets or blockchains provide this feature.
Replay attacks pose a real threat to network security. Unlike many other types of attacks, they do not require data to be decrypted, making them an effective way to bypass encryption-based security. Blockchain ledgers are particularly vulnerable to such cyberattacks, especially in the context of hard forks used to update or split blockchain ledgers. However, there are reliable ways to protect registries from replay attacks. In particular, using strong replay protection can ensure that attackers will not be able to carry out replay transactions after a hard fork occurs.

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