The intricate security issues affecting cross-chain blockchain systems are covered in this chapter. A presentation on historical hacks that reveal system vulnerabilities in smart contracts and inter-chain bridges, including the DAO vulnerability, the Wormhole bridge attack, and the Poly Network exploit, opens up the discussion. This chapter describes how malicious actors exploit differences in consensus and contract enforcement across chains to explain replay vulnerabilities and double-spending attacks in decentralized systems. Particular attention is paid to Oracle manipulation and the shortcomings of existing cryptographic defenses. Using operational solutions like Thorchain, RenVM and AnySwap, as well as cryptographic techniques like trustworthy relays, threshold signatures, and authenticated data feeds, mitigation options are investigated. Along with providing strong design requirements for compatible smart contracts, this chapter also examines recent developments in AI-based security models and quantum-resistant encryption. The final section provides researchers and developers with a blueprint for safe, compatible systems by describing current risks in identity management, cross-chain governance and oracle trustworthiness.