Hackers are attempting to exploit a recently discovered backdoor built into multiple Zyxel device models that hundreds of thousands of individuals and businesses use as VPNs, firewalls, and wireless access points.
The backdoor comes in the form of an undocumented user account with full administrative rights that’s hardcoded into the device firmware, a researcher from Netherlands-based security firm Eye Control recently reported. The account, which uses the username zyfwp, can be accessed over either SSH or through a Web interface.
A serious vulnerability
The researcher warned that the account put users at considerable risk, particularly if it were used to exploit other vulnerabilities such as Zerologon, a critical Windows flaw that allows attackers to instantly become all-powerful network administrators.
“As the zyfwp user has admin privileges, this is a serious vulnerability,” Eye Control researcher Niels Teusink wrote. “An attacker could completely compromise the confidentiality, integrity and availability of the device. Someone could for example change firewall settings to allow or block certain traffic. They could also intercept traffic or create VPN accounts to gain access to the network behind the device. Combined with a vulnerability like Zerologon this could be devastating to small and medium businesses.”
Andrew Morris, founder and CEO of security firm GreyNoise, said on Monday that his company’s sensors have detected automated attacks that are using the account credentials in an attempt to log in to vulnerable devices. In most or all of the login attempts, the attackers have simply added the credentials to existing lists of default username/password combinations used to hack into unsecured routers and other types of devices.
“By definition, anything we’re seeing has to be opportunistic,” Morris said, meaning the attackers are using the credentials against IP addresses in a pseudorandom manner in hopes of finding connected devices that are susceptible to takeover. GreyNoise deploys collection sensors in hundreds of data centers worldwide to monitor Internetwide scanning and exploitation attempts.
The login attempts GreyNoise is seeing are happening over SSH connections, but Eye Control researcher Teusink said the undocumented account can also be accessed using a Web interface. The researcher said that a recent scan showed that more than 100,000 Zyxel devices have exposed the Web interface to the Internet.
Teusink said the backdoor appears to have been introduced in firmware version 4.39, which was released a few weeks ago. A scan of Zyxel devices in the Netherlands showed that about 10 percent of them were running that vulnerable version. Zyxel has issued a security advisory noting the specific device models that are affected. They include:
- ATP series running firmware ZLD V4.60
- USG series running firmware ZLD V4.60 ZLD
- USG FLEX series running firmware ZLD V4.60
- VPN series running firmware ZLD V4.60
- NXC2500 running firmware V6.00 through V6.10
- NXC5500 running firmware V6.00 through V6.10
For firewall models, a fix is already available. AP controllers, meanwhile, are scheduled to receive a fix on Friday. Zyxel said it designed the backdoor to deliver automatic firmware updates to connected access points over FTP.
People who use one of these affected devices should be sure to install a security fix as soon as it becomes available. Even when devices are running a version predating 4.6, users should still install the update, since it fixes separate vulnerabilities found in earlier releases. Disabling remote administration is also a good idea unless there is a good reason for allowing it.