CVE-2021-20090
📋 TL;DR
This CVE describes an unauthenticated path traversal vulnerability in Buffalo router web interfaces that allows attackers to bypass authentication mechanisms. It affects Buffalo WSR-2533DHPL2 and WSR-2533DHP3 routers with vulnerable firmware versions. Attackers can gain administrative access without credentials.
💻 Affected Systems
- Buffalo WSR-2533DHPL2
- Buffalo WSR-2533DHP3
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of router with ability to modify configurations, intercept traffic, install malware, and pivot to internal networks.
Likely Case
Unauthorized administrative access to router settings, enabling network reconnaissance, DNS hijacking, and credential theft.
If Mitigated
Limited impact if routers are behind firewalls with restricted WAN access and strong network segmentation.
🎯 Exploit Status
Exploitation is trivial with publicly available proof-of-concept code. Actively exploited in the wild according to security advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: WSR-2533DHPL2 > 1.02, WSR-2533DHP3 > 1.24
Vendor Advisory: https://www.buffalo.jp/support/security/2021/2102.html
Restart Required: Yes
Instructions:
1. Log into router admin interface. 2. Navigate to firmware update section. 3. Download latest firmware from Buffalo support site. 4. Upload and apply firmware update. 5. Reboot router after update completes.
🔧 Temporary Workarounds
Disable WAN Management Access
allPrevent external access to router web interface by disabling remote administration.
Network Segmentation
allPlace routers in isolated network segments with strict firewall rules limiting access.
🧯 If You Can't Patch
- Replace vulnerable routers with supported models from different vendors
- Implement network-based intrusion prevention systems (IPS) with rules blocking exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface. If version is WSR-2533DHPL2 <= 1.02 or WSR-2533DHP3 <= 1.24, device is vulnerable.Check Version:
Login to router web interface and check System Information or Firmware Version page.Verify Fix Applied:
Confirm firmware version is updated to patched versions: WSR-2533DHPL2 > 1.02 or WSR-2533DHP3 > 1.24.📡 Detection & Monitoring
Log Indicators:
- Unauthorized access attempts to admin pages
- Multiple failed login attempts followed by successful access without credentials
- Unusual configuration changes
Network Indicators:
- HTTP requests with path traversal patterns to router management interface
- Traffic from unexpected sources to router admin ports
SIEM Query:
source="router_logs" AND (uri="*..*" OR uri="*/../*") AND dest_port=80 OR dest_port=443🔗 References
- https://www.kb.cert.org/vuls/id/914124
- https://www.secpod.com/blog/arcadyan-based-routers-and-modems-under-active-exploitation/
- https://www.tenable.com/security/research/tra-2021-13
- https://www.kb.cert.org/vuls/id/914124
- https://www.secpod.com/blog/arcadyan-based-routers-and-modems-under-active-exploitation/
- https://www.tenable.com/security/research/tra-2021-13
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-20090
