Fraudulent blocks are malicious blocks that attempt to alter the blockchain's state in ways that benefit the attacker, such as:

1. _Double-spending_: Including transactions that spend the same coins twice, potentially allowing the attacker to spend funds they don't own.

2. _Transaction tampering_: Modifying or deleting transactions to alter the blockchain's state.

3. _Block reorganization_: Reordering or replacing blocks to change the blockchain's history.

Fraudulent blocks can be created by:

1. _Malicious miners_: Miners who manipulate the block creation process to include fraudulent transactions.

2. _51% Attackers_: Attackers who control a majority of the network's mining power to force fraudulent blocks onto the blockchain.

3. _Smart contract exploits_: Exploiting vulnerabilities in smart contracts to manipulate the blockchain state.

To prevent fraudulent blocks, blockchain networks employ various security measures, such as:

1. _Cryptographic algorithms_: Secure hashing and digital signatures to ensure block integrity.

2. _Consensus mechanisms_: Algorithms like PoW or PoS that require significant resources to create fraudulent blocks.

3. _Node validation_: Nodes verifying blocks and transactions to ensure their validity.

4. _Smart contract auditing_: Regular audits and testing to identify and fix vulnerabilities.

By preventing fraudulent blocks, blockchain networks can maintain their integrity, security, and trustworthiness, ensuring a reliable and decentralized ledger.