Critical NETGEAR Router Flaw Allows Full Admin Access by Attackers

Critical NETGEAR Router Flaw Allows Full Admin Access by Attackers

A severe authentication bypass vulnerability (CVE-2025-4978) has been uncovered in NETGEAR’s DGND3700v2 wireless routers, enabling unauthenticated attackers to gain full administrative control over affected devices.

The flaw, rated with a critical CVSSv4 score of 9.3, stems from a hidden backdoor mechanism in the router’s firmware and impacts versions V1.1.00.15_1.00.15NA.

Security researchers warn that exploitation could lead to complete network compromise, including credential theft, malware deployment, and traffic interception.

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Bypassing Authentication via Embedded Backdoor

The vulnerability resides in the router’s mini_http server, a lightweight HTTP daemon responsible for handling administrative interface requests.

Attackers can trigger the flaw by accessing the unauthenticated endpoint /BRS_top.html, which sets an internal flag start_in_blankstate to 1.

This flag disables HTTP Basic Authentication checks in the sub_404930 function, effectively bypassing login credentials.

Affected Components:

  • Firmware Version: V1.1.00.15_1.00.15NA
  • Vulnerable Endpoint: /BRS_top.html
  • Impacted Authentication Mechanism: HTTP Basic Authentication

Once exploited, attackers gain unrestricted access to the router’s administrative interface, including DNS settings, firewall configurations, and stored Wi-Fi credentials.

NETGEAR has released patched firmware (V1.1.00.26) to address the issue, available for download on their support page.

Exploiting the start_in_blankstate Flag

The vulnerability arises from improper handling of the start_in_blankstate variable within the router’s firmware.

Here’s a breakdown of the exploit chain:

  1. Triggering the Backdoor:
    Accessing /BRS_top.html sets start_in_blankstate = 1, bypassing authentication checks in the sub_404930 function. This function normally validates credentials via HTTP Basic Authentication, but skips verification when the flag is active.
  2. Privilege Escalation:
    With authentication disabled, attackers can navigate to privileged endpoints like /adv_index.htm or /WLG_wireless_tbl.htm to modify network settings, update firmware with malicious code, or extract sensitive data.
  3. Persistence Mechanisms:
    Successful exploitation allows attackers to implant persistent backdoors, reroute traffic through malicious DNS servers, or disable security features such as firewalls.

Proof of Concept (PoC):
Researchers have published a demonstrative GIF showing the exploit in action, highlighting how accessing /BRS_top.html grants immediate admin access without credentials.

Mitigation and Vendor Response

NETGEAR has acknowledged the vulnerability and released firmware version 1.1.00.26 to patch the flaw. Users are urged to:

  1. Update Firmware Immediately: Download the patch from NETGEAR’s official support portal.
  2. Disable Remote Management: Ensure the router’s administrative interface is not accessible from external networks.
  3. Monitor Network Traffic: Look for unauthorized DNS changes or unfamiliar devices connected to the network.

Long-Term Implications:
This vulnerability underscores systemic risks in embedded HTTP servers used by IoT devices.

Security experts recommend that enterprises adopt zero-trust frameworks for network devices and enforce regular firmware audits.

The discovery of CVE-2025-4978 highlights critical gaps in router security, particularly in legacy firmware.
With active exploitation likely, users must prioritize patching to prevent unauthorized access and potential network-wide breaches.
NETGEAR’s prompt response provides a remediation path, but the incident serves as a stark reminder of the persistent vulnerabilities in consumer-grade networking hardware.

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