Expert found a secret backdoor in Zyxel firewall and VPN
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2020-29583 | Hard-Coded 'zyfwp' Admin Backdoor in Zyxel USG Firewall Firmware 4.60 Zyxel USG-series firewall/VPN gateway firmware version 4.60 ships with an undocumented built-in account named 'zyfwp' whose password is unchangeable and stored in cleartext in the firmware image, making the credentials effectively public once the firmware is examined (use of hard-coded credentials, CWE-522). An unauthenticated attacker who can reach the device's SSH server or web management interface can log in with these embedded credentials and gain full administrator privileges, enabling configuration changes, theft of credentials or logs, and pivoting into the networks the firewall protects; the flaw scores 9.8 (CVSS 3.1) because it is network-exploitable with no privileges or user interaction required. Affected products are the twelve Zyxel USG models listed by CISA (USG20-VPN through USG2200), widely deployed in small/medium-business, branch-office, and ISP/MSP-managed networks. Exploitation is confirmed in the wild: CISA added the CVE to its Known Exploited Vulnerabilities catalog on 2021-11-03 (ransomware use unknown), EPSS assigns a 90.2% probability of exploitation within 30 days (100th percentile), and a public PoC write-up plus news reports of active attacks against Zyxel firewalls and VPNs are available. Do: Upgrade affected USG devices to a firmware release later than 4.60 per Zyxel's advisory (CISA's required action is to apply vendor updates). Until patched, restrict SSH and web management access to trusted networks and review device logs for logins by the 'zyfwp' account, since its password is public and cannot be changed on vulnerable firmware; check for signs of compromise when upgrading. | 9.8 | 90% | KEV PoC |
| largetens of thousands of internet-exposed Zyxel USG firewalls (order 10^4-10^5 devices; total installed base likely higher) |
Full article625 words · extracted from securityaffairs.com · click to collapse
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 [email protected] 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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(SecurityAffairs – hacking, Zyxel)
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Text extracted automatically; images, tables and formatting may be missing. Original: https://securityaffairs.com/112877/iot/secret-backdoor-zyxel-devices.html