Schule is open-source school management system software. The generateOTP() function generates a 4-digit numeric One-Time Password (OTP). Prior to version 1.0.1, even if a secure random number generator is used, the short length and limited range (1000–9999) results in only 9000 possible combinations. This small keyspace makes the OTP highly vulnerable to brute-force attacks, especially in the absence of strong rate-limiting or lockout mechanisms. Version 1.0.1 fixes the issue.
The product does not require that users should have strong passwords, which makes it easier for attackers to compromise user accounts.