Mercusys AC12G (EU) V1 router with firmware AC12G(EU)_V1_200909 contains a critical security vulnerability where configuration backups are encrypted using a hardcoded DES key with single DES in ECB mode. This weak encryption implementation allows attackers who obtain backup files to decrypt them and recover sensitive credentials. The vulnerability exposes admin passwords, WiFi pre-shared keys (PSK), and DDNS credentials. The use of hardcoded encryption keys represents a fundamental security design flaw that compromises the confidentiality of router configuration data.