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Dir 456u FirmwareOperating system · Dlink

CVE-2026-42376

CRITICAL · 9.8 CVSS v3.1 Published 2026-05-04
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
Public exploit Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The D-Link DIR-456U router contains a hardcoded telnet backdoor with credentials (username: Alphanetworks, password: whdrv01_dlob_dir456u) that are compiled into the firmware and read from /etc/config/image_sign. The telnet daemon starts automatically at boot via an init script, and the custom login binary uses insecure strcmp() for authentication. Any attacker with local network access can authenticate and obtain a root shell, granting full administrative control over the device.

MitigationSince this device is End-of-Life and will not receive patches, the primary remediation is to replace the affected hardware with a supported model. If replacement is not immediately feasible, implement compensating controls such as network segmentation to isolate the device and firewall rules to block inbound telnet traffic (port 23) from untrusted networks.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Dir 456u FirmwareOperating system
Affected:all versions

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm device model is DIR-456U
    Identify the device by checking the product label on the physical device or by logging into the router web interface and viewing the device info/status page
    Affected if Device model is D-Link DIR-456U
  2. Check if telnet port 23 is open
    From a machine on the same network, run 'nmap -p 23 <router_ip>' or 'telnet <router_ip>' to test connectivity to port 23
    Affected if Telnet service is listening on port 23 and accepts connections
  3. Test backdoor credentials
    Connect to the telnet service and attempt login with username 'Alphanetworks' and password 'whdrv01_dlob_dir456u'
    Affected if Authentication succeeds with these hardcoded credentials and provides root shell access
  4. Verify telnet auto-start mechanism
    If firmware extraction is possible, inspect init scripts for a service that automatically launches telnetd at boot time
    Affected if An init script exists that starts telnet daemon automatically on device boot

If the device is a D-Link DIR-456U with telnet port 23 accessible and the hardcoded credentials (Alphanetworks/whdrv01_dlob_dir456u) successfully authenticate to provide root access, the device is vulnerable to CVE-2026-42376.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Since this device is End-of-Life and will not receive patches, the primary remediation is to replace the affected hardware with a supported model. If replacement is not immediately feasible, implement compensating controls such as network segmentation to isolate the device and firewall rules to block inbound telnet traffic (port 23) from untrusted networks.

Recommended fix High confidence
  1. Replace the D-Link DIR-456U device with a currently supported router from a vendor that provides regular security updates.
  2. If immediate replacement is not possible, disable the telnet service on the device if the firmware provides such an option.
  3. Isolate the affected device on a separate network segment (VLAN) to limit exposure to local attackers.
  4. Block inbound TCP port 23 (telnet) at the network perimeter firewall.
  5. Monitor network traffic for connections to the telnet service on the affected device.
  6. Implement network intrusion detection to alert on unauthorized access attempts to the device.
Caveat Device is End-of-Life with no vendor support; replacement with a currently supported device is the only reliable remediation.

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Dir 456u Firmware Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing3.0 h
  • Review / QA2.0 h
17.0 hours of engineering $3,050
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-42376 in production — separate from our analysis above.

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