How do you patch or mitigate CVE-2021-40655 (D-Link DIR-605 Router Information Disclosure)?

What patching means for CVE-2021-40655 on a legacy router

This issue is not a normal in-service patch-and-forget event. The affected D-Link DIR-605 family is legacy hardware, so the real decision is whether you can still apply the vendor fix or mitigation, or whether retirement is the only safe outcome. Start by confirming the exact router model and hardware revision, then check the vendor advisory and the CISA KEV listing for the required action.

For a device like this, patching is only useful if the router is still supported and reachable for a legitimate maintenance path. If the device is already beyond end-of-life or end-of-service, the practical mitigation becomes exposure reduction: remove it from internet-facing roles, isolate it, or replace it entirely rather than treating it as a permanent exception.

Why information disclosure on an edge router matters

An information disclosure flaw on a perimeter router can expose details that help an attacker map the device, understand its configuration, or prepare a follow-on compromise. Even when the initial issue is “only” disclosure, edge devices sit in a high-value position because they mediate traffic, remote administration, and trust boundaries.

The security question is therefore not just whether the router leaks data, but whether that leak can be used to identify the device, enumerate interfaces, or support broader network access. If the router is old enough that vendor support has ended, disclosure risk is amplified because there may be no durable corrective path beyond replacement.

How to mitigate when you cannot rely on a normal patch cycle

The first step is asset confirmation: locate every D-Link DIR-605 instance, validate its revision, and determine whether a vendor security update or documented workaround exists for that exact hardware. If a supported fix is available, apply it under change control and verify that the router is still functioning as intended after the update.

If patching is not possible, the mitigation should be operational, not hopeful. Remove direct internet exposure, restrict management access to trusted administrative networks, and replace the device on a defined timeline. Where the router carried any sensitive access paths, review adjacent systems for signs that the disclosure may have been used to support reconnaissance or later intrusion.

Risk and Threat Considerations

Legacy perimeter devices create disproportionate risk because they often remain deployed after their support window closes. That makes them hard to remediate quickly and easy to overlook, especially when they still appear to “work” in normal traffic.

Failure mechanism: An information disclosure flaw can leak device details or configuration cues that help an attacker fingerprint the router, target it more precisely, or chain the exposure into a broader network attack.

Impact: The practical consequence is increased exposure of the edge network, higher likelihood of follow-on compromise, and a longer window in which an unsupported device can be abused if it remains reachable.

Practitioner Guidance

What to prioritise: Treat the replacement decision as part of remediation, not as a separate procurement problem. If the router is end-of-life or end-of-service, do not let a “pending patch” status become an indefinite exception.

What to verify: Confirm the exact hardware revision, confirm whether the vendor still publishes a fix or mitigation for that revision, and confirm whether the device is exposed from the internet or only from an internal management path.

Decision rule: If the device cannot be patched to a supported state, reduce exposure immediately and schedule removal or replacement. If you cannot verify the device’s state, assume it is still a live risk until proven otherwise.

Practitioner takeaway: For legacy routers, the correct mitigation is often not “wait for a patch,” but “eliminate the unsupported exposure and prove the device is no longer part of the trust boundary.”

Related resources from NHI Mgmt Group

• Guide: IoT Security Guide
• CVE-2021-40655 (D-Link DIR-605 Router Information Disclosure)
• How do you patch or mitigate CVE-2011-4723 (D-Link DIR-300 Router Cleartext Storage of a Password)?
• How do you patch or mitigate CVE-2023-25280 (D-Link DIR-820 Router OS Command Injection)?
• How do you patch or mitigate CVE-2021-31196 (Microsoft Exchange Server Information Disclosure)?