CVE-2026-42376
Published: 04 May 2026
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…
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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.
Likely Mitigating ControlsAI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Enables users to notice when hard-coded credentials have been exploited for unauthorized access.
Security training explicitly warns against hard-coded credentials, lowering their use in systems.
Policy and procedures prohibit hard-coded credentials in favor of managed authentication.
External identity providers eliminate the need for hard-coded credentials in applications.
Changing default authenticators prior to first use and protecting content prevents use of hard-coded credentials.
Central credential stores and rotation policies remove the need for hard-coded credentials in configuration files or code.
Intelligence programs surface reports of campaigns that abuse hard-coded credentials in products, prompting removal or replacement and thereby reducing successful exploitation.
Planned investment enables secure credential storage and management systems instead of hard-coded credentials.
Security SummaryAI
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.
Details
- CWE(s)
AI Security AnalysisAI
- AI Category
- Other AI Platforms
- Risk Domain
- N/A
- OWASP Top 10 for LLMs 2025
- None mapped
- MITRE ATLAS Techniques
- None mapped
- Classification Reason
- Matched keywords: backdoor
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.