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CVE-2026-51741 – Unauthenticated Diagnostic Log Erasure (Broken Access Control) – TOTOLINK T6 4.1.5c

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Summary

CVE-2026-51741 is an incorrect (missing) access control vulnerability in the clearDiagnosisLog function of TOTOLINK T6 routers running firmware 4.1.5cu.748_B20211015. The flaw allows a remote, unauthenticated attacker to send a crafted HTTP POST request to the device’s CGI interface and erase diagnostic logs without providing any credentials. The vulnerability has been assigned a CVSS v3.1 base score of 9.8 (Critical) due to its network-exploitable, no-privilege, no-interaction attack profile.

Technical details

  • Root cause: The clearDiagnosisLog function, exposed through the router’s /cgi-bin/cstecgi.cgi endpoint, fails to enforce authentication or session validation before executing the log-clearing action (CWE-284: Improper Access Control).
  • Trigger conditions: An attacker sends a crafted POST request to /cgi-bin/cstecgi.cgi invoking the clearDiagnosisLog action (e.g., via a topicurl=clearDiagnosisLog style parameter used by other functions on this same CGI endpoint), with no prior authentication required.
  • Attack vector: Network — the endpoint is reachable over HTTP/HTTPS to any host that can route to the device’s management interface, including the public internet if the interface is exposed or port-forwarded.
  • Impact: Successful exploitation lets an unauthenticated attacker erase diagnostic/log data, which undermines the integrity and availability of device audit trails. This can be used to conceal prior or ongoing malicious activity on the device (e.g., masking evidence of a separate compromise) and is part of a broader pattern of unauthenticated administrative functions exposed on this firmware’s CGI interface.

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 official vendor patch has been referenced for this CVE at time of publication. Check TOTOLINK’s official support/download portal for a firmware update addressing this issue and apply it as soon as it becomes available.
  • If no patch is available:
    • Do not expose the TOTOLINK T6 management/CGI interface (/cgi-bin/cstecgi.cgi) to the public internet; disable any WAN-side port forwarding, remote management, or UPnP rules that grant external access to it.
    • Restrict access to the device’s web management interface to trusted internal network segments only, using firewall/ACL rules.
    • Monitor devices for unexpected log-clearing activity as a potential indicator of exploitation attempts.
    • Where feasible, plan migration to actively maintained networking hardware, as this firmware line has multiple other unauthenticated access-control and command-injection issues disclosed around the same time.

References

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