Expert found a secret backdoor in Zyxel firewall and VPN

Zyxel addressed a critical flaw in its firmware, tracked as CVE-2020-29583, related to the presence of a hardcoded undocumented secret account.

The Taiwanese vendor Zyxel has addressed a critical vulnerability in its firmware related to the presence of a hardcoded undocumented secret account. The vulnerability, tracked as CVE-2020-29583 received a CVSS score of 7.8, it could be exploited by an attacker to login with administrative privileges and take over the networking devices.

“Firmware version 4.60 of Zyxel USG devices contains an undocumented account (zyfwp) with an unchangeable password. The password for this account can be found in cleartext in the firmware.” reads the advisory published by NIST. “This account can be used by someone to login to the ssh server or web interface with admin privileges.”

The CVE-2020-29583 flaw affects the firmware version 4.60 that is used by multiple Zyxel devices.

The vendor removed all vulnerable firmware versions from its cloud and website, except for USG FLEX 100W/700 due to base FW upgrade. 

Impacted devices include Unified Security Gateway (USG), ATP, USG FLEX and VPN firewalls products.

Affected product series	Patch available in
Firewalls
ATP series running firmware ZLD V4.60	ZLD V4.60 Patch1 in Dec. 2020
USG series running firmware ZLD V4.60	ZLD V4.60 Patch1 in Dec. 2020
USG FLEX series running firmware ZLD V4.60	ZLD V4.60 Patch1 in Dec. 2020
VPN series running firmware ZLD V4.60	ZLD V4.60 Patch1 in Dec. 2020
AP controllers
NXC2500	V6.10 Patch1 in April 2021
NXC5500	V6.10 Patch1 in April 2021

The vulnerability was discovered by the security researcher Niels Teusink from EYE.

The expert discovered an undocumented account (“zyfwp”) with the password “PrOw!aN_fXp” stored in plaintext. The credentials could be also used by a malicious third-party to login to the SSH server or web interface with admin privileges.

“When doing some research (rooting) on my Zyxel USG40, I was surprised to find a user account ‘zyfwp’ with a password hash in the latest firmware version (4.60 patch 0). The plaintext password was visible in one of the binaries on the system.” reads the post published by Teusink. “I was even more surprised that this account seemed to work on both the SSH and web interface.”

$ ssh zyfwp@192.168.1.252
Password: Pr*******Xp
Router> show users current
No: 1
  Name: zyfwp
  Type: admin
(...)
Router>

The expert pointed out that the user is not visible in the device’s interface and its password cannot be changed.

Teusink reported the flaw to Zyxel on November 29 and the company addressed the flaw with the release of a firmware patch (ZLD V4.60 Patch1) on December 18.

According to the vendor, the hidden account was used to deliver automatic firmware updates to connected access points through FTP.

The expert added that around 10% of 1000 devices in the Netherlands run a vulnerable version of the firmware.

“As the zyfwp user has admin privileges, this is a serious vulnerability. 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.” concludes the expert.

“Because of the seriousness of the vulnerability and it being so easy to exploit, we have decided not to release the password for this account at this time. We do expect others to find and release it, which is why we suggest you install the updated firmware as soon as possible.”

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Pierluigi Paganini

(SecurityAffairs – hacking, Zyxel)

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