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.

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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