# What is CVE-2023-6399?

> CVE-2023-6399 is a medium-severity flaw (CVSS 6.5) in Atp100 Firmware from Zyxel, published on 20 February 2024. It is not on the list of flaws known to be exploited today. The latest forecast puts its chance of use in the next 30 days at 0.6%. If you run an affected version, plan in the vendor’s fix.

Canonical page: https://www.edgeprotect.io/what-is/cve-2023-6399

## At a glance

- **Verdict:** Plan it in. No sign of exploitation and a low forecast. Patch it in your normal update cycle.
- **EdgeProtect risk score:** 24 out of 100 (Low); action: Monitor. Kept up to date as the evidence changes.
- **Severity:** Medium (CVSS 6.5 out of 10)
- **Known to be exploited:** No, not on the list of flaws known to be exploited today
- **Chance of use in the next 30 days:** 0.6% (latest exploit-likelihood forecast)
- **Published:** 20 February 2024
- **Record last updated:** 17 June 2026
- **Main product:** Atp100 Firmware from Zyxel
- **Affected products:** 22

## What is CVE-2023-6399? In plain terms

It can be attacked from anywhere on the internet with an ordinary user account, and nobody has to click anything. If it works, the attacker can take it offline.

### The official description

A format string vulnerability in Zyxel ATP series firmware versions from 4.32 through 5.37 Patch 1, USG FLEX series firmware versions from 4.50 through 5.37 Patch 1, USG FLEX 50(W) series firmware versions from 4.16 through 5.37 Patch 1, USG20(W)-VPN series firmware versions from 4.16 through 5.37 Patch 1, and USG FLEX H series firmware versions from 1.10 through 1.10 Patch 1 could allow an authenticated IPSec VPN user to cause DoS conditions against the “deviceid” daemon by sending a crafted hostname to an affected device if it has the “Device Insight” feature enabled.

### What kind of flaw is it?

- CWE-134: Use of Externally-Controlled Format String

### 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: An ordinary user account
- Does someone have to click: No, nobody has to do anything
- What an attacker gets: Can take it offline

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

## Is CVE-2023-6399 being exploited? Not that we know of

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 0.6%. That can change at any time, which is why we keep watching.

Exploit likelihood over the last 7 days: 0.6%, 0.6%, 0.6%, 0.6%, 0.6%, 0.6%, 0.6%.

### Timeline

- 20 February 2024: Published
- 17 June 2026: Record last updated

### How EdgeProtect ranks it

CVE-2023-6399 scores **24 out of 100 (Low)**, with the action "Monitor".

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: key context
- Real threat activity around it: key context

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-2023-6399?

The published record lists 22 products. Versions are as published: "5.10 to before 5.37" means from 5.10 up to, but not including, 5.37.

- [Atp100 Firmware](https://www.edgeprotect.io/product/zyxel/atp100_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Atp100w Firmware](https://www.edgeprotect.io/product/zyxel/atp100w_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Atp200 Firmware](https://www.edgeprotect.io/product/zyxel/atp200_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Atp500 Firmware](https://www.edgeprotect.io/product/zyxel/atp500_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Atp700 Firmware](https://www.edgeprotect.io/product/zyxel/atp700_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Atp800 Firmware](https://www.edgeprotect.io/product/zyxel/atp800_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 100 Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_100_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 100ax Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_100ax_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 100h Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_100h_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 100w Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_100w_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 200 Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_200_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 200h Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_200h_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 200hp Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_200hp_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 50 Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_50_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 500 Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_500_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 500h Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_500h_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 50w Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_50w_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 700 Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_700_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg Flex 700h Firmware](https://www.edgeprotect.io/product/zyxel/usg_flex_700h_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg20-VPN Firmware](https://www.edgeprotect.io/product/zyxel/usg20-vpn_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Usg20w-VPN Firmware](https://www.edgeprotect.io/product/zyxel/usg20w-vpn_firmware) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 5.10 to before 5.37, 5.37, 5.37-patch1
- [Uos](https://www.edgeprotect.io/product/zyxel/uos) by [Zyxel](https://www.edgeprotect.io/vendors/zyxel) (Operating system): 1.10, 1.10-patch1

## How do I fix CVE-2023-6399?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? EdgeProtect 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. **Keep an eye on it.** Track the product and we tell you as soon as we spot this flaw on the list of exploited flaws, or a jump in its exploit likelihood.

### References

- [zyxel.com](https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-vulnerabilities-in-firewalls-and-aps-02-20-2024) (Vendor advisory)

## Questions

### What is CVE-2023-6399?

CVE-2023-6399 is a medium-severity flaw in Atp100 Firmware from Zyxel, published on 20 February 2024. It can be attacked from anywhere on the internet with an ordinary user account, and nobody has to click anything. If it works, the attacker can take it offline.

### How serious is CVE-2023-6399?

It scores 6.5 out of 10 on the CVSS severity scale, which is medium. Our risk score for CVE-2023-6399 is 24 out of 100, which is Low. Its action is "Monitor". No sign of exploitation and a low forecast. Patch it in your normal update cycle.

### Is CVE-2023-6399 being exploited?

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 0.6%. That can change at any time, which is why we keep watching.

### Which versions does CVE-2023-6399 affect?

For Atp100 Firmware from Zyxel: 5.10 to before 5.37, 5.37, 5.37-patch1. The record lists 22 products in all, each with its versions, under "Who is affected" on this page.

### How do I fix CVE-2023-6399?

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.

## Does CVE-2023-6399 affect you?

EdgeProtect 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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