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CVE-2026-51691 – Unauthenticated Access Control Bypass (Firmware Upload/Flash Manipulation) – TOTOLI

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Summary

CVE-2026-51691 is an incorrect access control vulnerability (CWE-284) in the setUploadSetting function of TOTOLINK T6 routers running firmware 4.1.5cu.748_B20211015. The flaw allows a remote, unauthenticated attacker to manipulate the device’s upload or firmware flash workflow by sending a crafted POST request to the device’s management CGI interface. It carries a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: The setUploadSetting function fails to enforce proper access control/authentication checks before processing requests that control the device’s upload or firmware flashing workflow.
  • Trigger condition: An attacker sends a specially crafted HTTP POST request to the /cgi-bin/cstecgi.cgi endpoint, invoking the setUploadSetting action without providing valid credentials.
  • Attack vector: Network-based, no authentication or user interaction required (AV:N/AC:L/PR:N/UI:N).
  • Impact: Full compromise of confidentiality, integrity, and availability — an attacker can manipulate the router’s firmware upload/flash process, potentially leading to unauthorized firmware modification or device takeover.

Affected software

  • TOTOLINK T6 — firmware version 4.1.5cu.748_B20211015

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: No vendor patch has been confirmed as publicly available for this specific CVE at time of writing. Check TOTOLINK’s official support/download page for firmware updates for the T6 model beyond 4.1.5cu.748_B20211015 and apply any available update immediately.
  • If no patch is available:
    • Disable remote/WAN access to the router’s web management interface (/cgi-bin/cstecgi.cgi).
    • Restrict access to the management interface to trusted internal network segments only, via firewall rules or network segmentation.
    • Disable UPnP and remote administration features if not required.
    • Monitor for unexpected POST requests to /cgi-bin/cstecgi.cgi referencing upload/flash-related actions.
    • Consider replacing end-of-life or unsupported devices with actively maintained hardware if no fix becomes available.

References

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