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CVE-2026-51728 – Unauthenticated Firmware Upload / RCE – TOTOLINK T6 4.1.5cu.748_B20211015

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Summary

CVE-2026-51728 is a critical improper access control vulnerability (CWE-284) in the UploadFirmwareFile function of TOTOLINK T6 routers running firmware 4.1.5cu.748_B20211015. The flaw allows an unauthenticated, remote attacker to upload an arbitrary crafted firmware image to the device via the CGI management interface. Given a CVSS v3.1 base score of 9.8 (Critical), successful exploitation can fully compromise device confidentiality, integrity, and availability.

Technical details

  • Root cause: the UploadFirmwareFile function does not enforce authentication or authorization before accepting firmware upload requests.
  • Trigger condition: an attacker sends a crafted HTTP POST request containing a firmware image to the /cgi-bin/cstecgi.cgi endpoint.
  • Attack vector: Network, requiring no privileges and no user interaction (AV:N/AC:L/PR:N/UI:N).
  • Impact: a malicious firmware image can be flashed onto the device, potentially resulting in full device takeover, persistent backdoor installation, network pivoting, or denial of service.

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-confirmed patched firmware version has been identified for this CVE at time of writing. Check the TOTOLINK download center for updates to the T6 product line and apply any newer firmware release as soon as it becomes available.
  • If no patch is available: Restrict management/CGI interface access (/cgi-bin/cstecgi.cgi) to trusted internal networks only; disable remote/WAN administration; place the device behind a firewall or VPN so the interface is not internet-exposed; monitor for unexpected firmware update activity; consider replacing end-of-life devices that do not 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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How IONIX’s External Exposure Management Platform Detects and Validates
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1

Map your entire attack surface (continously)

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2

Monitor for new CVEs

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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.

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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

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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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