# What is CVE-2023-28771?

> CVE-2023-28771 is a critical-severity flaw (CVSS 9.8) in Atp100 Firmware from Zyxel, published on 25 April 2023. It is on the list of flaws known to be exploited, so criminals are already using it. The latest forecast puts its chance of use in the next 30 days at over 99%. If you run an affected version, patch it first.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2023-28771

## At a glance

- **Verdict:** Fix this now. Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.
- **Precursor Intelligence risk score:** 89 out of 100 (Critical); action: Emergency patch. Kept up to date as the evidence changes.
- **Severity:** Critical (CVSS 9.8 out of 10)
- **Known to be exploited:** Yes, it is on the government list of flaws known to be exploited
- **Chance of use in the next 30 days:** over 99% (latest exploit-likelihood forecast)
- **Published:** 25 April 2023
- **Record last updated:** 27 October 2025
- **Main product:** Atp100 Firmware from Zyxel
- **Affected products:** 19

## What is CVE-2023-28771? In plain terms

It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can read data, change data and take it offline.

### The official description

Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.35, USG FLEX series firmware versions 4.60 through 5.35, and ATP series firmware versions 4.60 through 5.35, which could allow an unauthenticated attacker to execute some OS commands remotely by sending crafted packets to an affected device.

### What kind of flaw is it?

- CWE-78: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

### How an attack on it works

- Where an attacker can be: Anywhere on the internet
- How hard it is to pull off: Easy: no special conditions
- Login needed first: No login needed
- Does someone have to click: No, nobody has to do anything
- What an attacker gets: Can read data, change data and take it offline

Severity vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`

## Is CVE-2023-28771 being exploited? Yes

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at over 99%.

Exploit likelihood over the last 7 days: >99%, >99%, >99%, >99%, >99%, >99%, >99%.

### Timeline

- 25 April 2023: Published
- 27 October 2025: Record last updated

### How Precursor Intelligence ranks it

CVE-2023-28771 scores **89 out of 100 (Critical)**, with the action "Emergency patch".

Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less:

- Evidence it is being exploited: biggest factor
- How severe it is: strong factor
- How widely exposed it is: smaller factor
- Threat activity around it: smaller factor

85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it.

## Who is affected by CVE-2023-28771?

The published record lists 19 products. Versions are as published: "4.60 to before 5.36" means from 4.60 up to, but not including, 5.36.

- [Atp100 Firmware](https://www.precursorintelligence.com/product/zyxel/atp100_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Zywall Usg 100 Firmware](https://www.precursorintelligence.com/product/zyxel/zywall_usg_100_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 4.73, 4.73
- [Zywall Usg 310 Firmware](https://www.precursorintelligence.com/product/zyxel/zywall_usg_310_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 4.73, 4.73
- [Atp100w Firmware](https://www.precursorintelligence.com/product/zyxel/atp100w_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.35
- [Atp200 Firmware](https://www.precursorintelligence.com/product/zyxel/atp200_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Atp500 Firmware](https://www.precursorintelligence.com/product/zyxel/atp500_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Atp700 Firmware](https://www.precursorintelligence.com/product/zyxel/atp700_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Atp800 Firmware](https://www.precursorintelligence.com/product/zyxel/atp800_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Usg Flex 100 Firmware](https://www.precursorintelligence.com/product/zyxel/usg_flex_100_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Usg Flex 100w Firmware](https://www.precursorintelligence.com/product/zyxel/usg_flex_100w_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Usg Flex 200 Firmware](https://www.precursorintelligence.com/product/zyxel/usg_flex_200_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Usg Flex 50 Firmware](https://www.precursorintelligence.com/product/zyxel/usg_flex_50_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Usg Flex 500 Firmware](https://www.precursorintelligence.com/product/zyxel/usg_flex_500_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Usg Flex 50w Firmware](https://www.precursorintelligence.com/product/zyxel/usg_flex_50w_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Usg Flex 700 Firmware](https://www.precursorintelligence.com/product/zyxel/usg_flex_700_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Vpn100 Firmware](https://www.precursorintelligence.com/product/zyxel/vpn100_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Vpn1000 Firmware](https://www.precursorintelligence.com/product/zyxel/vpn1000_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Vpn300 Firmware](https://www.precursorintelligence.com/product/zyxel/vpn300_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36
- [Vpn50 Firmware](https://www.precursorintelligence.com/product/zyxel/vpn50_firmware) by [Zyxel](https://www.precursorintelligence.com/vendors/zyxel) (Operating system): 4.60 to before 5.36

## How do I fix CVE-2023-28771?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list.
2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisory first in the list.
3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in.
4. **Check it wasn’t used before you patched.** Because criminals are already using this flaw, look for signs of a break-in on affected systems using the vendor’s guidance.

### References

- [zyxel.com](https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls) (Vendor advisory)
- [packetstormsecurity.com](http://packetstormsecurity.com/files/172820/Zyxel-IKE-Packet-Decoder-Unauthenticated-Remote-Code-Execution.html) (Exploit, Third-party advisory)
- [cisa.gov](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-28771) (Government resource)

## Questions

### What is CVE-2023-28771?

CVE-2023-28771 is a critical-severity flaw in Atp100 Firmware from Zyxel, published on 25 April 2023. It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can read data, change data and take it offline.

### How serious is CVE-2023-28771?

It scores 9.8 out of 10 on the CVSS severity scale, which is critical. Our risk score for CVE-2023-28771 is 89 out of 100, which is Critical. Its action is "Emergency patch". Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.

### Is CVE-2023-28771 being exploited?

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at over 99%.

### Which versions does CVE-2023-28771 affect?

For Atp100 Firmware from Zyxel: 4.60 to before 5.36. The record lists 19 products in all, each with its versions, under "Who is affected" on this page.

### How do I fix CVE-2023-28771?

Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet. Because criminals are already using it, also check affected systems for signs of a break-in.

## Does CVE-2023-28771 affect you?

Precursor Intelligence keeps checking for you:

- **Tell us what you run.** Pick your software from a list, or let us work it out from your website.
- **We match it to every flaw.** Over 400,000 published flaws, checked against your list.
- **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it.
- **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools.

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