Zyxel Firewall OS Command Injection
An OS command-injection vulnerability in multiple Zyxel firewall product lines allows an unauthenticated attacker to execute arbitrary commands by sending a crafted packet to the device. Exploited by Mirai-variant botnets shortly after disclosure.
- Vendor
- Zyxel
- Product
- ZyWALL/USG, USG FLEX, ATP, and VPN series firewalls
- CVSS
- 9.8
- EPSS (exploit probability)
- 99.3%
- Status
- kev
- CISA patch-by (BOD 22-01)
- Published
Is CVE-2023-28771 being exploited?
Yes. CISA added CVE-2023-28771 to its Known Exploited Vulnerabilities catalog on . CISA only lists a CVE when it has reliable evidence of exploitation in the wild. It is also listed in CIRCL's KEV catalog.
FIRST's EPSS model puts the probability of exploitation activity in the next 30 days at 99.3% (scored October 14, 2026).
- CISA KEV[ LISTED ]
- ENISA EUVD (exploited)[ — ]
- CIRCL KEV[ LISTED ]
Catalog snapshot 2026-11-13. Full cross-catalog view: /kev-comparison
CVE-2023-28771 timeline
Every event links to its source. Catalog dates are each catalog's own; day offsets count from NVD publication. Data: JSON
What to do
US federal civilian agencies must remediate by (21 days after listing) under BOD 22-01. Everyone else: that deadline is a sensible upper bound, not a target. Vendor guidance and the NVD record are linked under Source above.
CVE-2023-28771 is an OS command-injection vulnerability affecting multiple Zyxel firewall product lines, including the ZyWALL/USG, USG FLEX, ATP, and VPN series. Improper error-message handling lets an unauthenticated, remote attacker execute arbitrary commands by sending a specially crafted packet to an affected device.
Zyxel patched the vulnerability in late April 2023. Within weeks, security researchers observed multiple Mirai-variant botnets — including ones tracked as “Mirai-derived” and “Dark.IoT” — actively scanning for and exploiting unpatched Zyxel firewalls to expand DDoS-for-hire botnets, prompting CISA to add the CVE to its KEV catalog.
Why it mattered
Consumer- and SMB-grade firewalls and VPN appliances like Zyxel’s are a perennial botnet target precisely because they’re numerous, often unmanaged, and rarely patched promptly by their owners. A pre-auth command-injection bug with no special preconditions is close to the easiest possible bar for mass, automated exploitation — exactly the profile that botnet operators look for.
Zyxel’s security advisory and the NVD record above list affected models, firmware versions, and patch availability.
Other Zyxel CVEs we track
| CVE / Vulnerability | Severity | CVSS | EPSS | Added to KEV |
|---|---|---|---|---|
| CVE-2026-7273 Zyxel GS1900 Series Switches Stack-Based Buffer Overflowwrite-up | [ HIGH ] | 8.8 | 2.7% | |
| CVE-2024-40891 Zyxel DSL CPE OS Command Injection Vulnerability | [ HIGH ] | 8.8 | 22.7% | |
| CVE-2024-40890 Zyxel DSL CPE OS Command Injection Vulnerability | [ HIGH ] | 8.8 | 20.6% | |
| CVE-2024-11667 Zyxel Multiple Firewalls Path Traversal Vulnerabilityransomware use | [ HIGH ] | 7.5 | 3.0% | |
| CVE-2017-6884 Zyxel EMG2926 Routers Command Injection Vulnerabilityransomware use | [ HIGH ] | 8.8 | 33.0% | |
| CVE-2017-18368 Zyxel P660HN-T1A Routers Command Injection Vulnerability | [ CRITICAL ] | 9.8 | 94.5% | |
| CVE-2023-27992 Zyxel Multiple NAS Devices Command Injection Vulnerability | [ CRITICAL ] | 9.8 | 82.3% | |
| CVE-2023-33010 Zyxel Multiple Firewalls Buffer Overflow Vulnerability | [ CRITICAL ] | 9.8 | 29.0% |
