Hardcoded DES Key Exposes Router Configuration Backups

Mercusys AC12G (EU) V1 router with firmware AC12G(EU)_V1_200909 encrypts configuration backups with a hardcoded DES key using single DES in ECB mode. An attacker who obtains a backup file can decrypt it to recover all stored credentials including admin password, WiFi PSK, and DDNS credentials.

Casky was already ahead

This CVE exploits attack patterns that Casky's 215matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

CVE-2026-36606 affects Mercusys AC12G (EU) V1 routers running firmware AC12G(EU)_V1_200909, which encrypt configuration backups using a hardcoded DES key in the weakest possible mode—single DES with ECB (Electronic Codebook). This vulnerability is critical because configuration backups are often stored in accessible locations or transmitted insecurely, and an attacker who obtains one can trivially decrypt it using the publicly known hardcoded key. The decrypted backup exposes all credentials stored on the device: administrative passwords, WiFi pre-shared keys, and DDNS authentication credentials. This affects home and small office users who rely on these routers for network security, creating a cascading risk where compromised router credentials enable lateral movement, persistent access, and further network exploitation.

Casky's 215 matching skills leverage Claude AI's extended reasoning to detect the attack patterns and defense gaps mapped to MITRE ATT&CK technique TA0006 (Credential Access). Practitioners using Casky would surface findings related to weak cryptographic practices (CWE-798: Use of Hardcoded Credentials), including detection of backup file recovery attempts, suspicious decryption of router configuration files, and credential extraction from unencrypted or improperly encrypted backups. The platform would identify that attackers pursuing TA0006 techniques—such as OS Credential Dumping or Credentials from Password Stores—could easily pivot from a single router backup to multi-device compromise. Practitioners would see recommendations to audit firmware versions, enforce strong credential rotation independent of router backups, implement secure backup encryption with user-controlled keys, and monitor for unauthorized backup file access or exfiltration on their networks.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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215 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-36606.

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MITRE ATT&CK Techniques

TA0006

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