I did some restyling and cleanup on Zyxel CVE-2021-35036 writeup and wanted to re-share it here. A Zyxel credential leak that started with one VMG3625-T50B firmware image later expanded across a much wider set of CPE, ONT, LTE, and 5G devices.
A low-privileged router session could reach backend DAL endpoints that returned supervisor/admin account data, FTPS credentials, and TR-069 management secrets. So the practical impact was closer to post-login privilege escalation and remote-management exposure than a boring “passwords exist in config” bug.
The writeup also includes a firmware lab where I ran Zyxel’s own password generator under QEMU and traced the deterministic supervisor password routines.
This is the longer technical writeup behind CVE-2021-35036. The short CVE summary makes it sound like simple cleartext storage, but the useful part is the access path.
A low-privileged Zyxel router session could query DAL handlers like login\privilege and tr69 and receive password-bearing backend objects in the response. That included higher-privilege local account data, FTPS credentials, and TR-069 management secrets. Zyxel’s advisory later expanded the scope from the original VMG3625-T50B report into broader CPE, ONT, LTE, and 5G product lines.
I also included the password-generation side: QEMU runtime, LD\PRELOAD serial hook, getpassword analysis, and the Method2 / Method3 supervisor password logic.
A post-authentication command injection vulnerability in the “DomainName” parameter of the DHCP configuration file in Zyxel DX3301-T0 and EX3301-T0 firmware versions through 5.50(ABVY.7.1)C0 could allow an authenticated attacker with administrator privileges to execute OS commands on an affected device.
A post-authentication command injection vulnerability in the EasyMesh-related APIs of Zyxel DX3300-T0 firmware versions through 5.50(ABVY.7.1)C0 could allow an authenticated, adjacent attacker with administrator privileges to execute OS commands on an affected device.
A post-authentication command injection vulnerability in the TR-369 certificate download CGI program of the Zyxel VMG3625-T50B firmware versions through 5.50(ABPM.9.7)C0 could allow an authenticated attacker with administrator privileges to execute operating system (OS) commands on an affected device.
A post-authentication command injection vulnerability in the log file download function of the Zyxel EX3301-T0 firmware versions through 5.50(ABVY.7)C0 could allow an authenticated attacker to execute operating system (OS) commands on an affected device.
A null pointer dereference vulnerability in the Wake-on-LAN CGI program of the Zyxel VMG3625-T50B firmware version through 5.50(ABPM.9.6)C0 and the Zyxel WX3100-T0 firmware versions through 5.50(ABVL.4.8)C0 could allow an authenticated attacker with administrator privileges to trigger a denial-of-service (DoS) condition by sending a crafted HTTP request.
A null pointer dereference vulnerability in the IP settings CGI program of the Zyxel VMG3625-T50B firmware versions through 5.50(ABPM.9.6)C0 and the Zyxel WX3100-T0 firmware versions through 5.50(ABVL.4.8)C0 could allow an authenticated attacker with administrator privileges to trigger a denial-of-service (DoS) condition by sending a crafted HTTP request.
A null pointer dereference vulnerability in the account settings CGI program of the Zyxel VMG3625-T50B firmware versions through 5.50(ABPM.9.6)C0 and the Zyxel WX3100-T0 firmware versions through 5.50(ABVL.4.8)C0 could allow an authenticated attacker with administrator privileges to trigger a denial-of-service (DoS) condition by sending a crafted HTTP request.
A null pointer dereference vulnerability in the certificate downloader CGI program of the Zyxel VMG3625-T50B firmware versions through 5.50(ABPM.9.6)C0 and the Zyxel WX3100-T0 firmware versions through 5.50(ABVL.4.8)C0 could allow an authenticated attacker with administrator privileges to trigger a denial-of-service (DoS) condition by sending a crafted HTTP request.
A post-authentication command injection vulnerability in the "priv" parameter of Zyxel DX3300-T0 firmware version 5.50(ABVY.6.3)C0 and earlier could allow an authenticated attacker to execute operating system (OS) commands on an affected device.
An uncontrolled resource consumption vulnerability in the web server of Zyxel DX3301-T0 firmware version 5.50(ABVY.6.3)C0 and earlier could allow an attacker to perform Slowloris‑style denial‑of‑service (DoS) attacks. Such attacks may temporarily block legitimate HTTP requests and partially disrupt access to the web management interface, while other networking services remain unaffected.
A buffer overflow vulnerability in the URL parser of the zhttpd web server in Zyxel VMG8825-T50K firmware versions prior to V5.50(ABOM.5)C0 could allow an unauthenticated attacker to cause denial-of-service (DoS) conditions and potentially execute arbitrary code by sending a specially crafted HTTP request.
A post-authentication command injection vulnerability in the ”zyUtilMailSend” function of the Zyxel AX7501-B1 firmware version V5.17(ABPC.5.3)C0 and earlier could allow an authenticated attacker with administrator privileges to execute operating system (OS) commands on a vulnerable device.
A post-authentication command injection vulnerability in the "ZyEE" function of the Zyxel EX5601-T1 firmware version V5.70(ACDZ.3.6)C0 and earlier could allow an authenticated attacker with administrator privileges to execute operating system (OS) commands on a vulnerable device.
A post-authentication buffer overflow vulnerability in the parameter "action" of the CGI program in Zyxel VMG3625-T50B firmware versions through V5.50(ABPM.9.2)C0 could allow an authenticated attacker with administrator privileges to cause a temporary denial of service (DoS) condition against the web management interface by sending a crafted HTTP GET request to a vulnerable device if the function ZyEE is enabled.
A buffer overflow vulnerability in the packet parser of the third-party library "libclinkc" in Zyxel VMG8825-T50K firmware versions through V5.50(ABOM.8.4)C0 could allow an attacker to cause a temporary denial of service (DoS) condition against the web management interface by sending a crafted HTTP POST request to a vulnerable device.
An improper restriction of operations within the bounds of a memory buffer in the USB file-sharing handler of the Zyxel VMG8825-T50K firmware versions through 5.50(ABOM.8)C0 could allow an authenticated attacker with administrator privileges to cause potential memory corruptions, resulting in a thread crash on an affected device.
An improper restriction of operations within the bounds of a memory buffer in the MAC address parser of the Zyxel VMG8825-T50K firmware versions through 5.50(ABOM.8)C0 could allow an authenticated attacker with administrator privileges to cause potential memory corruptions, resulting in a thread crash on an affected device.
An improper restriction of operations within the bounds of a memory buffer in the IPv6 address parser of the Zyxel VMG8825-T50K firmware versions through 5.50(ABOM.8)C0 could allow an authenticated attacker with administrator privileges to cause potential memory corruptions, resulting in a thread crash on an affected device.
An improper restriction of operations within the bounds of a memory buffer in the parameter type parser of the Zyxel VMG8825-T50K firmware versions through 5.50(ABOM.8)C0 could allow an authenticated attacker with administrator privileges to cause potential memory corruptions, resulting in a thread crash on an affected device.
A buffer overflow vulnerability in the library "libclinkc" of the Zyxel VMG8825-T50K firmware version 5.50(ABOM.8)C0 could allow an unauthenticated attacker to cause denial of service (DoS) conditions by sending a crafted HTTP request to a vulnerable device.
The buffer overflow vulnerability in the DX3300-T1 firmware version V5.50(ABVY.4)C0 could allow an authenticated local attacker to cause denial of service (DoS) conditions by executing the CLI command with crafted strings on an affected device.
The buffer overflow vulnerability in the CGI program of the VMG3625-T50B firmware version V5.50(ABPM.8)C0 could allow an authenticated remote attacker to cause denial of service (DoS) conditions by sending a crafted HTTP request to a vulnerable device.
A potential buffer overflow vulnerability was identified in some internal functions of Zyxel VMG3312-T20A firmware version 5.30(ABFX.5)C0, which could be exploited by a local authenticated attacker to cause a denial of service.
A command injection vulnerability in the CGI program of Zyxel VMG3312-T20A firmware version 5.30(ABFX.5)C0 could allow a local authenticated attacker to execute arbitrary OS commands on a vulnerable device via a LAN interface.
A cleartext storage of information vulnerability in the Zyxel VMG3625-T50B firmware version V5.50(ABTL.0)b2k could allow an authenticated attacker to obtain sensitive information from the configuration file.