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CVE-2026-51720 – Unauthenticated Firewall Rule Tampering – TOTOLINK T6 4.1.5cu.748_B20211015

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Summary

CVE-2026-51720 is an incorrect access control vulnerability (CWE-284) in the delIpPortFilterRules function of the TOTOLINK T6 router’s web management CGI handler. It allows a remote, unauthenticated attacker to delete configured firewall/port-filter rules by sending a crafted POST request, with no login or user interaction required. The flaw carries a CVSS v3.1 base score of 9.1 (Critical).

Technical details

  • Root cause: The delIpPortFilterRules function, exposed through the device’s CGI handler, fails to enforce authentication or authorization checks before processing requests that remove firewall filter rules.
  • Trigger conditions: An attacker sends a crafted HTTP POST request to /cgi-bin/cstecgi.cgi invoking the vulnerable function.
  • Attack vector: Network-based, low attack complexity, no privileges required, no user interaction required (AV:N/AC:L/PR:N/UI:N).
  • Impact: High confidentiality and integrity impact — an attacker can remove firewall filter rules that protect the device/network, weakening or disabling filtering protections. No direct availability impact is scored.

Affected software

  • TOTOLINK T6, firmware version 4.1.5cu.748_B20211015

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: Check the TOTOLINK support/download portal for a firmware update addressing this issue for the T6 model and apply it as soon as it is available; no fixed version is currently documented in the CVE record.
  • If no patch is available: Restrict access to the router’s web management interface (/cgi-bin/cstecgi.cgi) to trusted internal networks only, disable remote/WAN-side management, place the device behind a firewall/VPN, and monitor for unexpected changes to firewall/port-filter rule configurations.

References

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