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CVE-2026-51723 – Unauthenticated Malicious CGI Module Installation – TOTOLINK T6 4.1.5cu.748_B202110

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Summary

CVE-2026-51723 is an incorrect access control vulnerability in the UploadCustomModule function of TOTOLINK T6 routers running firmware 4.1.5cu.748_B20211015. The flaw allows an unauthenticated attacker to install a custom CGI module on the device by sending a crafted POST request to /cgi-bin/cstecgi.cgi. It carries a CVSS v3.1 base score of 9.1 (Critical).

Technical details

  • Root cause: the UploadCustomModule function does not properly enforce access control, so it accepts and processes module-upload requests without requiring authentication.
  • Trigger condition: an attacker sends a specially crafted POST request to the /cgi-bin/cstecgi.cgi endpoint to invoke UploadCustomModule.
  • Attack vector: network-based, no privileges or user interaction required.
  • Impact: unauthenticated installation of a custom CGI module on the router, resulting in high confidentiality and integrity impact (per the CVSS vector, availability is not impacted).

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 page for T6 firmware updates and apply the latest available release for this model; no vendor-confirmed patched version is documented in the public CVE record as of this writing.
  • If no patch is available: Restrict access to the router’s management/CGI interface (/cgi-bin/cstecgi.cgi) from untrusted networks, disable remote/WAN-side administration, and place the device behind a firewall or VPN so only trusted internal hosts can reach the CGI endpoint. Consider replacing end-of-life TOTOLINK T6 devices that no longer receive security updates.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Raw response body: /cgi-bin/cstecgi.cgi
  • Page title: TOTOLINK

References

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