CVE-2022-25219
📋 TL;DR
This vulnerability allows unauthenticated attackers on the local network to predict ephemeral telnet passwords on affected routers with 1-in-94 odds, enabling unauthorized telnet access. Attackers can leverage a null byte interaction error in telnetd_startup daemon combined with an unpadded RSA cipher vulnerability (CVE-2022-25218) to make passwords predictable. This affects routers running vulnerable firmware with telnet services enabled.
💻 Affected Systems
- DrayTek Vigor routers
📦 What is this software?
K2 Firmware by Phicomm
K3 Firmware by Phicomm
⚠️ Risk & Real-World Impact
Worst Case
Complete router compromise leading to persistent backdoor access, network traffic interception, credential theft, and lateral movement to other devices on the network.
Likely Case
Unauthorized telnet access to router allowing configuration changes, service disruption, and potential credential harvesting from connected devices.
If Mitigated
Limited impact due to network segmentation, disabled telnet services, or proper access controls preventing local network access.
🎯 Exploit Status
Exploitation requires crafting UDP packets and leveraging two vulnerabilities together (CVE-2022-25219 and CVE-2022-25218). Attackers need local network access and understanding of the specific packet exchange.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: DrayTek firmware updates addressing both CVE-2022-25219 and CVE-2022-25218
Vendor Advisory: https://www.draytek.com/support/security-advisory/
Restart Required: Yes
Instructions:
1. Check DrayTek security advisory for affected models. 2. Download latest firmware from vendor portal. 3. Backup router configuration. 4. Upload and apply firmware update via web interface. 5. Reboot router. 6. Verify firmware version.
🔧 Temporary Workarounds
Disable Telnet Service
allCompletely disable telnet service on the router to prevent exploitation of this vulnerability
Login to router admin interface
Navigate to Management > Access Control > Telnet
Disable Telnet service
Save configuration
Network Segmentation
allIsolate router management interface to trusted VLAN/subnet to limit attack surface
Configure VLAN segmentation
Restrict management interface access to specific IPs
Implement firewall rules to block unauthorized access to router management ports
🧯 If You Can't Patch
- Implement strict network access controls to limit who can reach router management interfaces
- Monitor network traffic for UDP packets targeting router ports and unusual telnet connection attempts
🔍 How to Verify
Check if Vulnerable:
Check router firmware version against DrayTek security advisory. If telnet is enabled and firmware is outdated, assume vulnerable.Check Version:
telnet [router_ip] or check web interface under System Status > Firmware InformationVerify Fix Applied:
Verify firmware version is updated to patched version from vendor advisory and test telnet service functionality.📡 Detection & Monitoring
Log Indicators:
- Failed telnet authentication attempts
- Successful telnet logins from unusual IPs
- UDP packets to router on unusual ports
Network Indicators:
- Unusual UDP traffic patterns to router
- Telnet connections from unauthorized IPs
- Multiple failed telnet attempts followed by success
SIEM Query:
source="router_logs" AND (event="telnet_auth_failed" OR event="telnet_login") | stats count by src_ip