CVE-2026-4377: D-Link DWR-X1820 Auth Bypass Vulnerability

CVE-2026-4377 Overview

CVE-2026-4377 affects the D-Link DWR-X1820 router, which generates its default Wi-Fi password from the device's International Mobile Equipment Identity (IMEI) number. The router does not prompt users to change the default password during initial setup. An attacker who knows the password generation algorithm and the device IMEI can derive the default credentials and authenticate to the wireless network. The flaw is categorized under [CWE-1391]: Use of Weak Credentials. D-Link has addressed the issue in firmware version 1.00B16CP.

Critical Impact

Adjacent-network attackers who learn or guess a DWR-X1820 IMEI can reconstruct the default Wi-Fi password and gain unauthorized access to the wireless network.

Affected Products

• D-Link DWR-X1820 router firmware versions prior to 1.00B16CP
• Deployments using factory-default Wi-Fi credentials
• Devices where administrators have not manually rotated the default password

Discovery Timeline

• 2026-05-28 - CVE-2026-4377 published to NVD
• 2026-05-28 - Last updated in NVD database

Technical Details for CVE-2026-4377

Vulnerability Analysis

The D-Link DWR-X1820 is a 5G LTE router that ships with a factory-set Wi-Fi password. The password is deterministically derived from the device IMEI, a value that is not secret. Cellular network operators, repair technicians, and anyone with physical proximity to the device label can read the IMEI. Because the derivation function is reversible by anyone who reconstructs the algorithm, the default password offers no meaningful entropy. The router additionally fails to enforce a credential change at first login, so deployments that skip manual hardening remain exposed for the lifetime of the device.

Root Cause

The weakness stems from two design decisions. First, the firmware uses a low-entropy input — the IMEI — as the seed for the default Wi-Fi key generation routine. Second, the device provisioning workflow treats the default credential as acceptable for long-term use rather than as a one-time bootstrap secret. Together, these choices map directly to [CWE-1391] (Use of Weak Credentials) and contradict standard guidance that requires unique, high-entropy default credentials with mandatory rotation.

Attack Vector

Exploitation requires adjacent-network access, meaning the attacker must be within wireless range of the target router. An attacker who obtains the IMEI — through visual inspection of the device label, social engineering, leaked logistics data, or operator-side records — applies the known derivation algorithm to compute the default password. The attacker then associates with the Wi-Fi network and operates as an authenticated client, enabling lateral movement, traffic interception, and abuse of LAN-exposed services.

No public proof-of-concept code is available. The CERT Polska advisory describes the mechanism without publishing the derivation algorithm. See the CERT Polska advisory for CVE-2026-4377 for the disclosure details.

Detection Methods for CVE-2026-4377

Indicators of Compromise

• Unrecognized client MAC addresses associated with the DWR-X1820 wireless interface
• DHCP leases issued to devices not on the asset inventory
• Outbound traffic from the LAN segment to hosts or geographies not consistent with normal usage
• Repeated authentication events from new clients shortly after device deployment or relocation

Detection Strategies

• Audit the firmware version of every DWR-X1820 in the fleet and flag any unit below 1.00B16CP
• Compare the configured Wi-Fi pre-shared key against the factory-printed value to identify devices still using defaults
• Correlate wireless association logs with the asset inventory to surface unknown clients

Monitoring Recommendations

• Forward router syslog and DHCP events to a central log platform for retention and correlation
• Alert on first-seen MAC addresses joining the SSID outside of provisioning windows
• Track the firmware inventory of edge routers and trigger remediation workflows when versions lag the vendor advisory

How to Mitigate CVE-2026-4377

Immediate Actions Required

• Upgrade DWR-X1820 firmware to version 1.00B16CP or later, available from the D-Link DWR-1820 support page
• Change the Wi-Fi pre-shared key on every deployed unit to a high-entropy value unrelated to the IMEI
• Restrict physical access to devices so the IMEI label cannot be photographed by unauthorized parties

Patch Information

D-Link released firmware 1.00B16CP, which addresses the default credential weakness. Administrators should download the firmware from the official D-Link support portal, verify the file, and apply it through the router management interface. After flashing, confirm the running version and rotate the Wi-Fi password as part of the post-upgrade checklist.

Workarounds

• Manually set a unique, long Wi-Fi password on each router before connecting it to production networks
• Disable broadcast of the default SSID and replace it with a custom value to reduce targeting signals
• Segment the DWR-X1820 LAN from sensitive internal resources using VLANs or firewall rules until firmware is updated

# Post-upgrade verification checklist
# 1. Confirm firmware version
show version
# Expected output should reference 1.00B16CP or later

# 2. Rotate the Wi-Fi pre-shared key from the admin UI
#    Wireless > Security > set WPA2/WPA3 passphrase (>= 20 random chars)

# 3. Force re-association of all clients to validate the new key