CVE-2026-42376

CriticalPublic PoCUpdated

Published: 04 May 2026

Published

04 May 2026

Modified

11 May 2026

KEV Added

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Patch

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CVSS Score 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Score 0.0017 37.3th percentile

Risk Priority 20 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-42376 is a critical-severity Use of Hard-coded Credentials (CWE-798) vulnerability in Dlink Dir-456U Firmware. Its CVSS base score is 9.8 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique External Remote Services (T1133); ranked at the 37.3th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

This vulnerability is AI-related — categorised as Other AI Platforms.

The strongest mitigations our analysis identified are NIST 800-53 IA-5 (Authenticator Management) and SA-22 (Unsupported System Components).

Threat & Defense at a Glance

What attackers do: exploitation maps to External Remote Services (T1133) and 1 other technique. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SA-22 Unsupported System Components good match

prevent

SA-22 directly requires organizations to identify, assess risks of, and retire or isolate unsupported EOL system components like the D-Link DIR-456U router that cannot receive patches for the hardcoded backdoor.

SC-7 Boundary Protection good match

prevent

SC-7 enforces boundary protections such as firewalls to monitor and control communications, blocking local network access to the telnet daemon exploited by the backdoor.

IA-5 Authenticator Management good match

prevent

IA-5 mandates proper management of authenticators, prohibiting hardcoded static credentials like the 'Alphanetworks:whdrv01_dlob_dir456U' used in the telnet backdoor.

MITRE ATT&CK Enterprise TechniquesAI

T1133 External Remote Services Persistence

Adversaries may leverage external-facing remote services to initially access and/or persist within a network.

attack.mitre.org →

T1078.001 Default Accounts Stealth

Adversaries may obtain and abuse credentials of a default account as a means of gaining Initial Access, Persistence, Privilege Escalation, or Defense Evasion.

attack.mitre.org →

Why these techniques?

Hardcoded telnet backdoor with static credentials directly enables initial access via external remote services using a vendor backdoor/default account on a publicly reachable network device.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

D-Link DIR-456U Hardware Revision A1 (End-of-Life, EOL) contains a hardcoded telnet backdoor. The device starts a telnet daemon at boot via /etc/init0.d/S80telnetd.sh with 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 uses strcmp() to validate credentials. Successful authentication grants an unauthenticated attacker on the local network a root shell with full administrative control. The device has reached End-of-Life (EOL) and will not receive patches.

Deeper analysisAI

CVE-2026-42376 is a hardcoded telnet backdoor vulnerability (CWE-798) affecting the D-Link DIR-456U Hardware Revision A1 router, which has reached end-of-life (EOL) status. The flaw stems from a telnet daemon launched at boot via the script /etc/init0.d/S80telnetd.sh, configured with static credentials—username "Alphanetworks" and password "whdrv01_dlob_dir456U"—sourced from /etc/config/image_sign. A custom telnetd binary supports a -u user:password flag, while the custom login binary performs credential validation using strcmp(). The vulnerability carries a CVSS v3.1 base score of 9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H).

An unauthenticated attacker with network access to the device can exploit this backdoor by connecting to the telnet service using the known hardcoded credentials, thereby obtaining a root shell with full administrative control over the router.

Advisories, including those published by Securin.io, confirm the device is EOL and will not receive patches or firmware updates. Mitigation relies on isolating affected devices from untrusted networks or retiring them entirely, as no vendor remediation is available.