CVE-2021-39615
📋 TL;DR
CVE-2021-39615 is a critical vulnerability in D-Link DSR-500N routers where hard-coded credentials for undocumented accounts exist in the /etc/passwd file. If attackers recover the cleartext passwords from the hashes, they can gain SSH/Telnet access to the embedded Linux operating system. This affects DSR-500N version 1.02 devices that are no longer supported by the vendor.
💻 Affected Systems
- D-Link DSR-500N
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Full compromise of the router with administrative access to the underlying Linux OS, enabling network pivoting, traffic interception, and persistent backdoor installation.
Likely Case
Unauthorized access to router configuration and network traffic monitoring capabilities.
If Mitigated
Limited impact if SSH/Telnet services are disabled or network access is restricted.
🎯 Exploit Status
Exploitation requires password hash cracking and SSH/Telnet access. Public advisory includes hash details.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version 2.12/2
Vendor Advisory: https://supportannouncement.us.dlink.com/announcement/publication.aspx?name=SAP10235
Restart Required: Yes
Instructions:
1. Download firmware version 2.12/2 from D-Link support site. 2. Log into router web interface. 3. Navigate to System > Firmware Update. 4. Upload and apply the new firmware. 5. Reboot the router.
🔧 Temporary Workarounds
Disable SSH and Telnet Services
allPrevent remote access by disabling SSH and Telnet management interfaces.
# Via web interface: System > Management > Services > Disable SSH and Telnet
Restrict Management Interface Access
allLimit SSH/Telnet access to specific trusted IP addresses only.
# Configure firewall rules to restrict access to management IP/port
🧯 If You Can't Patch
- Replace affected DSR-500N routers with supported models or alternative vendors
- Implement network segmentation to isolate vulnerable routers from critical assets
🔍 How to Verify
Check if Vulnerable:
Check router firmware version via web interface (System > Status) or SSH/Telnet with 'cat /etc/passwd' to look for hard-coded accounts.Check Version:
cat /proc/version or check web interface System > StatusVerify Fix Applied:
Verify firmware version is 2.12/2 or later and check /etc/passwd file no longer contains undocumented accounts with hard-coded credentials.📡 Detection & Monitoring
Log Indicators:
- Failed SSH/Telnet login attempts for undocumented usernames
- Successful logins from unusual IP addresses
Network Indicators:
- SSH/Telnet connections to router from unexpected sources
- Unusual outbound traffic from router
SIEM Query:
source="router_logs" AND (event="ssh_login" OR event="telnet_login") AND (user="undocumented_account" OR src_ip NOT IN trusted_ips)🔗 References
- https://supportannouncement.us.dlink.com/announcement/publication.aspx?name=SAP10235
- https://www.dlink.com/en/security-bulletin/
- https://www.nussko.com/advisories/advisory-2021-08-02.txt
- https://supportannouncement.us.dlink.com/announcement/publication.aspx?name=SAP10235
- https://www.dlink.com/en/security-bulletin/
- https://www.nussko.com/advisories/advisory-2021-08-02.txt
