CVE-2020-9054 remediation guide
Zyxel Multiple Network-Attached Storage (NAS) Devices
Multiple ZyXEL network-attached storage (NAS) devices running firmware version 5.21 contain a pre-authentication command injection vulnerability, which may allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable device. ZyXEL NAS devices achieve authentication by using the weblogin.cgi CGI executable. This program fails to properly sanitize the username parameter that is passed to it. If the username parameter contains certain characters, it can allow command injection with the privileges of the web server that runs on the ZyXEL device. Although the web server doe...
Identify the affected asset
Confirm whether Zyxel Multiple Network-Attached Storage (NAS) Devices is installed, record the deployed version and installation path, and identify exposed interfaces.
- Record the product name, package or appliance identifier, and installed version.
- Identify internet-facing, administrative, API, and internal access paths.
- Preserve the pre-change configuration and relevant service logs.
Validate the affected-version match
Compare the deployed version with the affected range: V5.21(AAZF.7)C0, V5.21(AASZ.3)C0, V5.21(AATB.4)C0, V5.21(ABAG.4)C0, all, V4.75(AALH.2)C0, V4.75(AANV.2)C0, V4.81(AAAJ.1)C0, V4.81(AALS.1)C0, < 5.21\(aazf.7\)c0, < 5.21\(aasz.3\)c0, < 5.21\(aatb.4\)c0, < 5.21\(abag.4\)c0, >= 4.35 < 4.35\(abps.3\)c0, >= 4.35 < 4.35\(abfw.3\)c0, >= 4.35 < 4.35\(abfu.3\)c0, >= 4.35 < 4.35\(abiq.3\)c0, >= 4.35 < 4.35\(abaq.3\)c0, >= 4.35 < 4.35\(abar.3\)c0, >= 4.35 < 4.35\(aala.3\)c0. Treat incomplete inventory results as unresolved.
Prioritize with independent signals
CISA has listed this vulnerability in the Known Exploited Vulnerabilities catalog. Combine this signal with asset criticality, exposure, compensating controls, and business impact.
Apply supported remediation
Apply updates per vendor instructions.
Verify the result
Confirm the resulting version, service health, authentication and access paths, monitoring coverage, and relevant security logs.
- Confirm the expected version and package state.
- Run a functional health check for critical service paths.
- Review authentication, error, application, and security monitoring logs.
- Reassess exposure if the supported fix cannot be applied.
Operational boundary
This page does not replace the vendor advisory, support contract, change-management process, or recovery plan. Do not infer that an asset is unaffected from an incomplete inventory query or a missing fixed-version field.