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CVE-2026-51731 – Unauthenticated VLAN Configuration Tampering – TOTOLINK T6 (firmware 4.1.5cu.748_B2

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Summary

CVE-2026-51731 is an improper access control vulnerability (CWE-284) in the delVlanCfg function of the TOTOLINK T6 router’s web management CGI interface, running firmware version 4.1.5cu.748_B20211015. The flaw allows an unauthenticated, remote attacker to delete VLAN configuration entries by sending a crafted POST request to /cgi-bin/cstecgi.cgi. The vulnerability carries a CRITICAL CVSS v3.1 base score of 9.1.

Technical details

  • Root cause: the delVlanCfg function exposed by /cgi-bin/cstecgi.cgi does not enforce authentication or authorization checks before executing the requested action.
  • Trigger condition: sending a specially crafted HTTP POST request to /cgi-bin/cstecgi.cgi invoking the delVlanCfg operation.
  • Attack vector: network-based, requiring no privileges and no user interaction (AV:N/AC:L/PR:N/UI:N).
  • Impact: an unauthenticated attacker can remove existing VLAN entries from the device configuration, altering network segmentation and device configuration integrity; CISA’s SSVC assessment rates the technical impact as "total" and the flaw as automatable.

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: no vendor-confirmed patched firmware version has been identified in public sources at the time of writing; check TOTOLINK’s official download page for the T6 model for an updated firmware release addressing this issue.
  • 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 administration, and place management interfaces behind a VPN or network access control.
  • Monitor for unexpected changes to VLAN configuration entries as a possible indicator of exploitation attempts.

References

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