The analysis of vulnerabilities and threats in biometrics-based personal authentication systems is indispensable for the development and promotion of wide spread utilization of biometric technologies in accordance with international standardization of information security technologies. In this paper, we discuss the vulnerabilities and threats in a writer verification system which is an application of biometric technologies using handwriting information. We propose some attack methods that use verification results or user templates of a writer verification system. From some simulation results, it is clear that an attacker can estimate an authorized user’s biometric information (handwriting information) and produce some forged handwritings which are accepted by the verification system.