CVE-2026-38577: Tenda HG21 Privilege Escalation Vulnerability

CVE-2026-38577 Overview

CVE-2026-38577 is a hardcoded credentials vulnerability in the Tenda HG21 V4.0.0-260302 router. The device ships with static administrative credentials embedded in the firmware. Attackers who know or extract these credentials can authenticate to the Admin account remotely and obtain root access on the device. The flaw is classified under CWE-798: Use of Hard-coded Credentials.

Critical Impact
Remote, unauthenticated attackers can gain root-level control of affected Tenda HG21 devices using embedded credentials, enabling full device compromise, traffic interception, and pivoting into the internal network.

Affected Products

Tenda HG21 firmware version V4.0.0-260302
Tenda HG21 series gateway (V4 hardware revision)
Deployments exposing the device management interface to untrusted networks

Discovery Timeline

2026-08-31 - CVE-2026-38577 published to NVD
2026-09-01 - Last updated in NVD database

Technical Details for CVE-2026-38577

Vulnerability Analysis

The Tenda HG21 V4.0.0-260302 firmware embeds static credentials for the Admin account directly in the device image. Because the credentials are identical across all devices running this firmware build, any attacker who recovers them once can reuse them against every deployed unit. Successful authentication grants root privileges on the underlying Linux system, not just administrative access to the web UI.

The vulnerability requires no user interaction and no prior privileges. An attacker only needs network reachability to the device's management interface. Exploitation results in complete loss of confidentiality, integrity, and availability of the device.

Root access on a gateway device allows adversaries to modify firewall rules, capture LAN traffic, alter DNS resolution, install persistent implants in firmware, and use the device as a foothold to reach internal hosts.

Root Cause

The root cause is the inclusion of static administrator credentials inside the shipped firmware image [CWE-798]. Hardcoded credentials cannot be revoked by the end user through normal configuration and typically survive factory resets. Any binary analysis of the firmware, such as extracting the filesystem with binwalk and inspecting configuration files or authentication binaries, exposes the secret to attackers.

Attack Vector

The attack vector is network-based. An attacker sends authentication requests to the device's exposed management service (typically HTTP/HTTPS on the WAN or LAN interface) using the hardcoded Admin credentials. Once authenticated, the attacker gains a root shell or equivalent administrative control. Refer to the GitHub Gist proof-of-concept for technical details on reproducing the issue.

Detection Methods for CVE-2026-38577

Indicators of Compromise

Successful logins to the Admin account from unexpected source IP addresses, particularly external addresses reaching the device management interface.
Configuration changes such as new port forwards, altered DNS settings, or modified firewall rules that were not initiated by administrators.
Unexpected outbound connections from the router to unknown hosts, indicating command-and-control activity.
Presence of the Tenda HG21 firmware version V4.0.0-260302 in asset inventory.

Detection Strategies

Inventory all Tenda HG21 devices and identify units running firmware V4.0.0-260302 or earlier.
Monitor authentication logs on the device for repeated logins to the Admin account, especially from non-management subnets.
Inspect network flow data for administrative protocol traffic (HTTP, HTTPS, Telnet, SSH) reaching the device from untrusted zones.

Monitoring Recommendations

Forward device syslog output to a centralized log platform and alert on authentication events for the Admin user.
Baseline outbound traffic from gateway devices and alert on deviations that suggest tunneling or C2 activity.
Track configuration changes on network appliances using periodic snapshots and diff-based alerting.

How to Mitigate CVE-2026-38577

Immediate Actions Required

Remove affected Tenda HG21 devices from any exposure to untrusted or WAN-side networks until a vendor patch is applied.
Restrict access to the management interface using upstream firewall rules that permit only trusted administrative source IPs.
Disable remote administration features on the device where supported.
Rotate any credentials, keys, or secrets that may have transited a compromised device.

Patch Information

No vendor advisory or patched firmware version is referenced in the NVD entry at the time of publication. Administrators should monitor the Tenda official support portal for firmware updates addressing CVE-2026-38577 and apply them as soon as they are released. Hardcoded credential issues cannot be mitigated by password changes in the UI; a firmware update from the vendor is required.

Workarounds

Place affected devices behind a segmentation firewall and block inbound access to management ports (HTTP/80, HTTPS/443, Telnet/23, SSH/22) from untrusted networks.
Replace the affected device with a supported model if no firmware fix becomes available within an acceptable timeframe.
Deploy network intrusion detection signatures to alert on authentication attempts against the device using known default or hardcoded credential patterns.

bash

# Example upstream firewall rule to restrict management access
# Only allow the trusted admin subnet 10.10.0.0/24 to reach the device management interface
iptables -A FORWARD -s 10.10.0.0/24 -d <device_ip> -p tcp -m multiport --dports 22,23,80,443 -j ACCEPT
iptables -A FORWARD -d <device_ip> -p tcp -m multiport --dports 22,23,80,443 -j DROP