This security vulnerability arises when an Amazon Cognito user pool has an explicitly configured password policy that lacks sufficient complexity requirements. When a passwordPolicy object is provided in CDK, it overrides the strong defaults. Incomplete policies may allow weak passwords that are susceptible to brute force attacks, dictionary attacks, and credential stuffing. Ensure the passwordPolicy sets minLength to at least 8 and enables requireUppercase, requireLowercase, requireDigits, and requireSymbols to true.