CVE-2026-42376
Published: 04 May 2026
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 36.7th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 IA-5 (Authenticator Management) and SA-22 (Unsupported System Components).
Deeper analysis
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.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-27029
Vulnerability details
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…
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-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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
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.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
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 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 mandates proper management of authenticators, prohibiting hardcoded static credentials like the 'Alphanetworks:whdrv01_dlob_dir456U' used in the telnet backdoor.