This paper presents a fault attack on side-channel secure masked implementations of LWE-based key-encapsulation mechanisms (KEMs), exploiting fault propagation through the Arithmetic-to-Boolean (A2B) conversion. The attack targets the decapsulation module of Kyber and uses Belief Propagation (BP) for key recovery. The A2B conversion is an artifact of masking and is used to generate Boolean shares from arithmetic shares. The attack exploits the data dependency of the adder carry chain in A2B and extracts sensitive information, even with masking. The attack is demonstrated on Kyber, although it also applies to other schemes like Saber. The attack is practical, with a real-world implementation on an STM32 platform using electromagnetic (EM) faults. The paper also discusses the robustness of masking against fault attacks and highlights the importance of understanding the interaction between different countermeasures. The attack is based on the fault-induced information-leakage channel through the carry propagation logic of a masked adder. The paper shows that the attack can be used to recover the secret key by solving a system of inequalities derived from the fault-induced decryption failures. The attack is effective even with multi-bit faults, and the paper provides a detailed analysis of the fault propagation through the A2B conversion and its impact on the decryption process. The paper also discusses the use of the Belief Propagation algorithm for solving the system of inequalities and highlights the importance of understanding the interaction between different countermeasures in the context of post-quantum cryptography.This paper presents a fault attack on side-channel secure masked implementations of LWE-based key-encapsulation mechanisms (KEMs), exploiting fault propagation through the Arithmetic-to-Boolean (A2B) conversion. The attack targets the decapsulation module of Kyber and uses Belief Propagation (BP) for key recovery. The A2B conversion is an artifact of masking and is used to generate Boolean shares from arithmetic shares. The attack exploits the data dependency of the adder carry chain in A2B and extracts sensitive information, even with masking. The attack is demonstrated on Kyber, although it also applies to other schemes like Saber. The attack is practical, with a real-world implementation on an STM32 platform using electromagnetic (EM) faults. The paper also discusses the robustness of masking against fault attacks and highlights the importance of understanding the interaction between different countermeasures. The attack is based on the fault-induced information-leakage channel through the carry propagation logic of a masked adder. The paper shows that the attack can be used to recover the secret key by solving a system of inequalities derived from the fault-induced decryption failures. The attack is effective even with multi-bit faults, and the paper provides a detailed analysis of the fault propagation through the A2B conversion and its impact on the decryption process. The paper also discusses the use of the Belief Propagation algorithm for solving the system of inequalities and highlights the importance of understanding the interaction between different countermeasures in the context of post-quantum cryptography.