What’s Vulnerable › Vulnerabilities

CVE-2021-3128

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-04-12 · updated 2026-06-17
• Weakness: CWE-834

Description

In ASUS RT-AX3000, ZenWiFi AX (XT8), RT-AX88U, and other ASUS routers with firmware < 3.0.0.4.386.42095 or < 9.0.0.4.386.41994, when IPv6 is used, a routing loop can occur that generates excessive network traffic between an affected device and its upstream ISP's router. This occurs when a link prefix route points to a point-to-point link, a destination IPv6 address belongs to the prefix and is not a local IPv6 address, and a router advertisement is received with at least one global unique IPv6 prefix for which the on-link flag is set.

Affected products

• ASUS Rt-Ac1750 B1 Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac1900 Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac1900P Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac1900U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac2900 Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac3100 Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac5300 Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac58U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac65U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac66U B1 Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac68P Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac68R Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac68Rw Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Japan Inc. RT-AC68U: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac68W Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac85U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS RT-AC86U: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ac88U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUSTeK COMPUTER INC. ASUS Router RT-AX3000: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS RT-AX55: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS RT-AX56U: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ax58U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ax68U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• Asus RT-AX82U: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Rt-Ax86U Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS RT-AX88U: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994
• ASUS Zenwifi Ax (Xt8) Firmware: before 3.0.0.4.386.42095; before 9.0.0.4.386.41994 — fixed in 3.0.0.4.386.42095, 9.0.0.4.386.41994

How to fix

Upgrade to a fixed version: ASUS Rt-Ac1750 B1 Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac1900 Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac1900P Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac1900U Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac2900 Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac3100 Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac5300 Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac58U Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac65U Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac66U B1 Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac68P Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994; ASUS Rt-Ac68R Firmware 3.0.0.4.386.42095, 9.0.0.4.386.41994. Check the vendor advisory for mitigations.

References

• https://www.asus.com/Networking-IoT-Servers/Whole-Home-Mesh-WiFi-System/ZenWiFi-WiFi-Systems/ASUS-ZenWiFi-AX-XT8-/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX3000/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX55/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX56U/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX58U/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX68U/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX82U/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX86U/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-6/All-series/RT-AX88U/HelpDesk_BIOS/
• https://www.asus.com/Networking-IoT-Servers/WiFi-Routers/ASUS-WiFi-Routers/RT-AC66U-B1/HelpDesk_Download/
• https://www.asus.com/supportonly/RT-AC1750_B1/HelpDesk_download/
• https://www.asus.com/supportonly/RT-AC1900/HelpDesk_download/

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