ZyXEL NAS products running firmware version 5.21 and earlier are vulnerable to pre-authentication command injection in weblogin.cgi

published 2020-03-04 6 y ago modified 2026-10-08 31 d ago also EUVD-2020-29883

cvss: 9.8; version 3.1, CNA
cisa kev: Exploited; added 2022-03-25
epss: 100.0%; EPSS 100.0% chance of exploitation in 30 days, percentile 99.9%
public exploits: 0; none known, plus a Nuclei check

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 does not run as the root user, ZyXEL devices include a setuid utility that can be leveraged to run any command with root privileges. As such, it should be assumed that exploitation of this vulnerability can lead to remote code execution with root privileges. By sending a specially-crafted HTTP POST or GET request to a vulnerable ZyXEL device, a remote, unauthenticated attacker may be able to execute arbitrary code on the device. This may happen by directly connecting to a device if it is directly exposed to an attacker. However, there are ways to trigger such crafted requests even if an attacker does not have direct connectivity to a vulnerable devices. For example, simply visiting a website can result in the compromise of any ZyXEL device that is reachable from the client system. Affected products include: NAS326 before firmware V5.21(AAZF.7)C0 NAS520 before firmware V5.21(AASZ.3)C0 NAS540 before firmware V5.21(AATB.4)C0 NAS542 before firmware V5.21(ABAG.4)C0 ZyXEL has made firmware updates available for NAS326, NAS520, NAS540, and NAS542 devices. Affected models that are end-of-support: NSA210, NSA220, NSA220+, NSA221, NSA310, NSA310S, NSA320, NSA320S, NSA325 and NSA325v2

Patch / fix

cisa required action Apply updates per vendor instructions.

fixed in 20

zyxel nas326_firmware 5.21\(aazf.10\)c0
zyxel nas520_firmware 5.21\(aasz.3\)c0
zyxel nas540_firmware 5.21\(aatb.7\)c0
zyxel nas542_firmware 5.21\(abag.7\)c0
zyxel atp100_firmware 4.35\(abps.3\)c0
zyxel atp200_firmware 4.35\(abfw.3\)c0
zyxel atp500_firmware 4.35\(abfu.3\)c0
zyxel atp800_firmware 4.35\(abiq.3\)c0
zyxel usg20-vpn_firmware 4.35\(abaq.3\)c0
zyxel usg20w-vpn_firmware 4.35\(abar.3\)c0
zyxel usg40_firmware 4.35\(aala.3\)c0
zyxel usg40w_firmware 4.35\(aalb.3\)c0
zyxel usg60_firmware 4.35\(aaky.3\)c0
zyxel usg60w_firmware 4.35\(aakz.3\)c0
zyxel usg110_firmware 4.35\(aaph.3\)c0
zyxel usg210_firmware 4.35\(aapi.3\)c0
zyxel usg310_firmware 4.35\(aapj.3\)c0
zyxel usg1100_firmware 4.35\(aapk.3\)c0
zyxel usg1900_firmware 4.35\(aapl.3\)c0
zyxel usg2200_firmware 4.35\(abae.3\)c0

Timeline

2020-03-04 Published
2022-03-25 Added to CISA KEV
2022-04-15 CISA patch deadline
2025-02-07 CISA SSVC: exploitation active

Known exploited

Added: 4 y ago (2022-03-25)
Patch by: 2022-04-15 (4 y ago)
Ransomware: Not known
Required action: Apply updates per vendor instructions.
Vendor, product: Zyxel, Multiple Network-Attached Storage (NAS) Devices
CISA name: Zyxel Multiple NAS Devices OS Command Injection Vulnerability

CISA SSVC

Exploitation: Active, exploited in the wild
Automatable: no
Technical impact: total
Assessed: 21 mo ago (2025-02-07)

Exploit prediction

99.99% chance of exploitation in the next 30 days, higher than 99.9% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

Nuclei template Zyxel NAS Firmware 5.21- Remote Code Execution nuclei detection template
krebsonsecurity.com https://krebsonsecurity.com/2020/02/zyxel-fixes-0day-in-network-storage-devices/ referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CISA ADP	3.1	9.8	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD	3.1	9.8	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CNA	3.1	9.8	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD	2.0	10.0	AV:N/AC:L/Au:N/C:C/I:C/A:C

Affected products

ZyXEL NAS326
ZyXEL NAS520
ZyXEL NAS540
ZyXEL NAS542
ZyXEL NSA210
ZyXEL NSA220
ZyXEL NSA220+
ZyXEL NSA221
ZyXEL NSA310
ZyXEL NSA320
ZyXEL NSA320S
ZyXEL NSA325
ZyXEL NSA325v2

Classification

Weakness

CWE-78 Improper Neutralization of Special Elements used in an OS Command

References

krebsonsecurity.com /2020/02/zyxel-fixes-0day-in-network-storage-devices/
exploit, third-party-advisory
zyxel.com /support/remote-code-execution-vulnerability-of-NAS-products.shtml
vendor-advisory
cwe.mitre.org /data/definitions/78.html
third-party-advisory
kb.cert.org /vuls/id/498544/
third-party-advisory, us-government-resource
kb.cert.org /artifacts/cve-2020-9054.html
third-party-advisory, us-government-resource
cisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-9054
government-resource, us-government-resource

Sources

CVE List CVE-2020-9054
NVD CVE-2020-9054

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary