CVE-2026-42376: D-Link DIR-456U Auth Bypass Vulnerability

CVE-2026-42376 Overview

CVE-2026-42376 is a hardcoded credentials vulnerability [CWE-798] in the D-Link DIR-456U Hardware Revision A1 router. The device starts a telnet daemon at boot through /etc/init0.d/S80telnetd.sh using the username Alphanetworks and the static password whdrv01_dlob_dir456U read from /etc/config/image_sign. The custom telnetd binary accepts a -u user:password flag, and the custom login binary validates credentials using strcmp(). An unauthenticated attacker on the local network can authenticate against the backdoor and gain a root shell. The device has reached End-of-Life (EOL) status and will not receive a vendor patch.

Critical Impact

Successful exploitation grants an unauthenticated network attacker a root shell with full administrative control over the affected router.

Affected Products

• D-Link DIR-456U Hardware Revision A1 (End-of-Life)
• Devices running the factory firmware that ships /etc/init0.d/S80telnetd.sh
• Routers using the custom telnetd and login binaries that read from /etc/config/image_sign

Discovery Timeline

• 2026-05-04 - CVE-2026-42376 published to NVD
• 2026-05-05 - Last updated in NVD database

Technical Details for CVE-2026-42376

Vulnerability Analysis

The DIR-456U A1 ships with a backdoor telnet service enabled by default. At boot, the init script /etc/init0.d/S80telnetd.sh launches a vendor-modified telnetd binary that accepts a -u user:password argument. This argument hardcodes the credential pair used for authentication, bypassing the standard system authentication path.

The credentials consist of the username Alphanetworks and the password whdrv01_dlob_dir456U, which is read from the file /etc/config/image_sign. The device's custom login binary compares supplied credentials against this pair using strcmp(). A successful match drops the connection into a root shell.

Because the password is fixed in firmware and shared across all DIR-456U A1 units, any attacker who reaches TCP port 23 on the device can log in. Vulnerability classification falls under hardcoded credentials [CWE-798].

Root Cause

The root cause is the inclusion of static, factory-set credentials in production firmware. The vendor's telnetd binary is designed to consume credentials from a configuration file rather than the system password database. Combined with a strcmp()-based authentication check in the modified login binary, the device exposes a deterministic authentication path that cannot be disabled through configuration.

Attack Vector

Exploitation requires network reachability to the router's telnet service on TCP port 23. An attacker on the LAN, or on any network segment routed to the device's management interface, connects with the username Alphanetworks and password whdrv01_dlob_dir456U. Authentication succeeds without any prior privilege or user interaction, and the session is established with root privileges. From there, the attacker can modify configuration, pivot to internal hosts, intercept traffic, or persist malicious firmware changes. See the Securin Zero-Day Analysis for additional technical detail.

Detection Methods for CVE-2026-42376

Indicators of Compromise

• Successful telnet authentication events on port 23 against DIR-456U A1 devices using the username Alphanetworks.
• Outbound or lateral telnet connections to router management interfaces from non-administrative hosts.
• Unexpected configuration changes, new accounts, or DNS server modifications on the affected router.

Detection Strategies

• Monitor network flow data for TCP/23 sessions terminating on internal router IP addresses, especially from user VLANs.
• Inspect packet captures for the literal string Alphanetworks in telnet authentication exchanges.
• Compare DIR-456U firmware images against known vendor builds to identify the presence of S80telnetd.sh and the modified telnetd binary.

Monitoring Recommendations

• Alert on any telnet traffic crossing security zones, since modern administration should not rely on cleartext telnet.
• Track router configuration drift through periodic SNMP or scripted config snapshots and diff against a known-good baseline.
• Forward router syslog and authentication events to a centralized logging platform for correlation against host telemetry.

How to Mitigate CVE-2026-42376

Immediate Actions Required

• Replace DIR-456U A1 devices with a currently supported router model, since the product is End-of-Life and will not receive a patch.
• Block inbound TCP/23 to the router's WAN and LAN management interfaces at upstream firewalls and access control lists.
• Segment the affected device onto an isolated VLAN with no access to sensitive internal resources until decommissioned.

Patch Information

No patch is available. D-Link has designated the DIR-456U Hardware Revision A1 as End-of-Life, and the vendor will not issue firmware updates that remove the hardcoded telnet backdoor. Device replacement is the only complete remediation. Reference the Securin Zero-Day Analysis for vendor status confirmation.

Workarounds

• Apply LAN-side ACLs that restrict TCP/23 access to a single hardened administrative jump host, if removal is not yet possible.
• Disable any port-forwarding or remote management features that could expose telnet to untrusted networks.
• Treat the device as untrusted: rotate any shared secrets, Wi-Fi PSKs, and downstream credentials that may have transited the router.