CVE-2021-3128: ASUS Rt-AC1750 b1 Firmware

High severity, CVSS 7.5. EPSS: 2.2% chance of exploitation in the next 30 days.

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 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC1900 Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC1900P Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC1900U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC2900 Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC3100 Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC5300 Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC58U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC65U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC66U b1 Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC68P Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC68R Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC68RW Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC68U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC68W Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC85U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC86U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AC88U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AX3000 Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS RT-AX55 Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AX56U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AX58U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AX68U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AX82U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
ASUS Rt-AX86U Firmware: before 3.0.0.4.386.42095 (fixed in 3.0.0.4.386.42095); before 9.0.0.4.386.41994 (fixed in 9.0.0.4.386.41994)
and 2 more

Published 2021-04-12. Last modified 2026-06-17.