CVE-2020-29583 (Zyxel Multiple Products Use of Hard-Coded Credentials)

Why hard-coded credentials are dangerous in connected products

Hard-coded credentials are embedded secrets that cannot be changed easily by operators, which makes them especially dangerous when they appear in networked appliances and other products exposed to real-world traffic. If one device, firmware branch or deployment is compromised, the same credential can become a reusable access path across many installations.

This class of flaw matters because the credential is not just present, it is baked into the product’s trust model. That can let an attacker bypass normal authentication, reach administrative functions, and use the product as a foothold for deeper intrusion, especially when the same secret is reused across customers or environments.

For a broader view of why these flaws remain so damaging, see the Guide to the Secret Sprawl Challenge and the CVE Program, which together help frame how embedded secrets become exploitable product vulnerabilities.

How organisations check exposure to a CVE like this

The first step is to confirm whether the affected product is present in the estate, including appliances, embedded deployments, test rigs and shadow instances that may sit outside normal software inventory. For internet-facing products, exposure is higher because hard-coded credentials are often easiest to abuse remotely before defenders have time to notice.

Exposure review should also include vendor advisories, because the authoritative remediation path is usually product-specific and may involve a fix, configuration change or compensating control. When the flaw is listed in CISA’s Known Exploited Vulnerabilities catalogue, it should be treated as actively abused rather than theoretical.

Use the NIST National Vulnerability Database to cross-check the record, and the vendor’s advisory for exact product guidance and remediation details.

Remediation and compensating controls when patching is delayed

Because hard-coded credentials are embedded into the product, remediation usually means removing the product’s dependence on that secret, replacing it with vendor-supported authentication, or isolating the system until a fix is available. Where patching cannot happen immediately, compensating controls should reduce reachability and reduce the chance that the secret can be reused.

That typically means restricting network exposure, segmenting access, limiting administrative paths, and watching for any interfaces that accept the built-in credential. If the product participates in automated workflows, credential replacement must be coordinated carefully so a rushed change does not break service while leaving the vulnerable path intact.

For practitioner-level handling of embedded secret problems, the Secrets Management Guide and the API Key Management Guide are useful references for rotation, revocation and reducing secret exposure.

Detection, incident review and identity impact after exploitation

Once exploitation is suspected, the question is not only whether the product was accessed, but what the attacker could do after that access. In many cases, a hard-coded credential is an entry point into administrative interfaces, logs, configuration stores or downstream systems, so the compromise review should look beyond the vulnerable product itself.

That review should include access logs, configuration changes, outbound connections, new accounts, and any evidence that the same secret was reused elsewhere. If the credential granted privileged access, organisations should assume the affected account or token path may need to be replaced, not just the device repaired.

Helpful background on credential abuse and post-compromise cleanup is available in the State of NHI & AI Agent Breach Report 2026 and the Guide to NHI Rotation Challenges, both of which address how credential compromise can persist when secrets are difficult to rotate.

Risk and Threat Considerations

Hard-coded credentials create a direct abuse path because the secret is usually recoverable from the product, firmware or traffic and may work unchanged across many deployments. That makes the flaw attractive to attackers seeking simple, repeatable access and increases the chance that compromise spreads beyond a single device.

Failure mechanism: The embedded credential bypasses normal lifecycle controls, so defenders cannot reliably rotate, revoke or scope it, and an attacker who learns it can keep using the same access path until the product is replaced or isolated.

Impact: Exposure can lead to authentication bypass, administrative compromise, lateral movement and, in some environments, a long-lived foothold that survives routine password changes elsewhere.

Practitioner Guidance

Why practitioners should care: Treat any product that ships with hard-coded credentials as a design-level security problem, not just a patching issue. The operational question is whether the product can be trusted to enforce its own authentication model without hidden secrets that operators cannot control.

What to watch for: Prioritise assets that are internet-facing, widely deployed, or used in privileged operational workflows, because those conditions magnify the blast radius of a built-in secret. If the product cannot be patched immediately, reduce exposure first and verify that no dependent process still relies on the vulnerable credential path.

Related resources from NHI Mgmt Group

• CVE-2020-8657 (EyesOfNetwork Use of Hard-Coded Credentials)
• CVE-2024-3272 (D-Link Multiple NAS Devices Use of Hard-Coded Credentials)
• CVE-2026-22769 (Dell RecoverPoint for Virtual Machines (RP4VMs) Use of Hard-coded Credentials)
• CVE-2019-6693 (Fortinet FortiOS Use of Hard-Coded Credentials)