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CVE-2026-51701 – Unauthenticated Access Control Bypass – TOTOLINK T6 4.1.5cu.748_B20211015

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Summary

CVE-2026-51701 is an incorrect access control vulnerability (CWE-284) in the setMacFilterRules function of the TOTOLINK T6 router running firmware 4.1.5cu.748_B20211015. The flaw allows an unauthenticated, remote attacker to modify the device’s MAC address filtering/access control rules by sending a crafted POST request to the router’s CGI endpoint. The issue is rated critical with a CVSS v3.1 base score of 9.1.

Technical details

  • Root cause: the setMacFilterRules function, reachable via /cgi-bin/cstecgi.cgi, fails to enforce an authentication/session check before processing requests that alter access control rules.
  • Trigger condition: an attacker sends a specially crafted POST request to /cgi-bin/cstecgi.cgi invoking the setMacFilterRules action, without needing valid credentials.
  • Attack vector: network-based, no authentication or user interaction required (AV:N, PR:N, UI:N).
  • Impact: unauthenticated modification of device access control (MAC filter) settings, which can be leveraged to permit or block arbitrary devices on the network, undermining the router’s access control posture. The vector reflects high confidentiality and integrity impact with no availability impact.

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 official TOTOLINK advisory or patched firmware version addressing this specific issue has been identified at this time. Check TOTOLINK’s official firmware download page for updates to the T6 model and apply any newer release as soon as it becomes available.
  • If no patch: restrict access to the router’s web management interface to trusted internal networks only, disable remote/WAN administration, and place management interfaces behind a VPN or firewall ACL. Monitor for unexpected changes to MAC filter/access control rules as a sign of exploitation attempts.

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, N600R (Firmware version V, TOTOLINK

References

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How IONIX’s External Exposure Management Platform Detects and Validates
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1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

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By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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