CVE-2026-51655 Overview
CVE-2026-51655 is an incorrect access control vulnerability [CWE-284] affecting the getMacFilterRules function in TOTOLINK T6 firmware version 4.1.5cu.748_B20211015. The flaw allows unauthenticated attackers on an adjacent network to retrieve MAC filter rules by sending a crafted POST request to the /cgi-bin/cstecgi.cgi endpoint. Because the function omits authentication checks, any client with access to the router's local network segment can query filter configuration data intended for administrators.
Critical Impact
Unauthenticated adjacent-network attackers can enumerate MAC filter rules on affected TOTOLINK T6 devices, exposing network access control configuration and connected client identifiers.
Affected Products
- TOTOLINK T6 router, firmware 4.1.5cu.748_B20211015
- Vulnerable endpoint: /cgi-bin/cstecgi.cgi
- Vulnerable function: getMacFilterRules
Discovery Timeline
- 2026-08-28 - CVE-2026-51655 published to the National Vulnerability Database (NVD)
- 2026-09-01 - Last updated in NVD database
Technical Details for CVE-2026-51655
Vulnerability Analysis
The vulnerability resides in the getMacFilterRules handler exposed through the CGI binary cstecgi.cgi on the TOTOLINK T6 router. The function processes POST requests without validating the caller's authentication state or session token. An attacker on the wireless or wired LAN segment can invoke the function and receive the router's MAC address filter rule set in the response.
MAC filter rules identify allowed or denied client hardware addresses. Disclosure of these rules reveals which devices are permitted to access the network and can be used to spoof legitimate clients or plan follow-on attacks against connected hosts. The vulnerability is limited to confidentiality impact; the flaw does not, on its own, permit modification of rules or disruption of service.
Root Cause
The root cause is a missing authorization check in the request dispatcher for cstecgi.cgi. The getMacFilterRules action is reachable through the standard POST interface without a valid administrative session cookie. This mirrors a broader pattern in the TOTOLINK T6 CGI handler where multiple get* functions omit consistent access control enforcement.
Attack Vector
Exploitation requires network adjacency to the target router, typically LAN or Wi-Fi access. The attacker sends a crafted HTTP POST request to /cgi-bin/cstecgi.cgi specifying getMacFilterRules as the requested topicurl or action parameter. The device returns the configured MAC filter rules in the HTTP response without authenticating the request. No user interaction is required, and the operation leaves minimal traces in default logging configurations.
Technical details, request formats, and coordination notes are documented in the DarkBoulder CVE coordination repository and the ShengWu00 CVE coordination repository.
Detection Methods for CVE-2026-51655
Indicators of Compromise
- HTTP POST requests to /cgi-bin/cstecgi.cgi containing the getMacFilterRules action from LAN-side clients that are not authenticated administrative sessions.
- Repeated unauthenticated requests to cstecgi.cgi targeting multiple get* functions, indicating enumeration of exposed handlers.
- Outbound queries from unknown or unauthorized LAN hosts to the router management interface outside expected administrative windows.
Detection Strategies
- Inspect router access logs, where available, for POST requests to cstecgi.cgi that lack a valid session context.
- Deploy network sensors on the LAN segment to flag HTTP traffic to the router's management port from non-administrative endpoints.
- Correlate management-interface traffic with known administrator source addresses to identify anomalous requesters.
Monitoring Recommendations
- Baseline expected administrative traffic to the router and alert on deviations, particularly requests from wireless guest segments.
- Monitor DHCP and ARP tables for unexpected clients that could reach the router management interface.
- Forward router syslog output to a centralized logging platform for retention and analysis beyond the device's limited local storage.
How to Mitigate CVE-2026-51655
Immediate Actions Required
- Restrict access to the router's management interface to a dedicated administrative VLAN or trusted host list.
- Disable wireless access to the management interface where the router firmware supports that option.
- Audit MAC filter rule contents and rotate any client identifiers that must remain confidential.
Patch Information
No vendor patch is referenced in the NVD entry at the time of publication. Consult the TOTOLINK official website and the TOTOLINK download portal for firmware updates addressing the T6 platform. Apply any updated firmware after verifying integrity against vendor-published hashes.
Workarounds
- Segment the router onto a management VLAN and block LAN client traffic to the management IP except from designated administrator hosts.
- Enforce Wi-Fi client isolation to prevent wireless clients from reaching the router web interface.
- Replace affected TOTOLINK T6 devices with supported hardware if no patched firmware is available and network segmentation is not feasible.
# Example: block LAN client access to the router management interface
# from an upstream firewall or managed switch ACL
iptables -A FORWARD -s 192.0.2.0/24 -d 192.0.2.1 -p tcp --dport 80 -j DROP
iptables -A FORWARD -s 192.0.2.0/24 -d 192.0.2.1 -p tcp --dport 443 -j DROP
iptables -I FORWARD -s 192.0.2.10 -d 192.0.2.1 -j ACCEPT # admin host allowlist
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.