Technical analysis of CVE-2026-6837, an authenticated command-injection vulnerability in Zyxel’s PKCS#12 certificate export flow.

The post covers the vulnerable execution path, root cause, affected firmware scope, and the firmware-emulation methodology used during analysis.

I published my technical write-up for CVE-2026-6837, an authenticated command-injection issue in Zyxel’s certificate export functionality.

The analysis is based on the WAX650S, while Zyxel’s advisory expanded the affected scope to 18 AP models. The post includes root cause, affected versions, remediation, and the reproduction environment.

Insufficiently Protected Credentials vulnerability in Zyxel Networks WAH7601 allows Retrieve Embedded Sensitive Data.

This issue affects WAH7601: through 20072026.

A path traversal vulnerability in the CLI command used to execute configuration files in Zyxel ATP series firmware versions from V4.32 through V5.42 Patch 1, USG FLEX series firmware versions from V4.50 through V5.42 Patch 1, USG FLEX 50(W) series firmware versions from V4.16 through V5.42 Patch 1, and USG20(W)-VPN series firmware versions from V4.16 through V5.42 Patch 1 could allow an authenticated attacker with administrator privileges to execute a crafted malicious configuration file on an affected device.

An improper authentication vulnerability in the "sociallogin.cgi" CGI program in Zyxel WAX650S firmware versions through 7.10(ABRM.4)C0 could allow an attacker on the WLAN to bypass captive portal authentication.

A post-authentication command injection vulnerability in the "export-cgi" CGI program in Zyxel WAX650S firmware versions through 7.10(ABRM.4)C0 could allow an authenticated attacker with administrator privileges to execute OS commands on an affected device.

A stack-based buffer overflow vulnerability in the CGI program of Zyxel GS1900-48HPv2 firmware versions through 2.90(ABTQ.1)C0 could allow a LAN-based, unauthenticated attacker to exploit the flaw and potentially execute OS commands via a crafted HTTP request.

A buffer overflow vulnerability in the UPnP DeletePortMapping() command in Zyxel VMG4005-B50B firmware versions through 5.13(ABRL.5.4)C0 could allow an adjacent attacker to trigger a temporary denial-of-service (DoS) condition affecting the UPnP function of the affected device.

A buffer overflow vulnerability in the UPnP AddPortMapping() command in Zyxel VMG4005-B50B firmware versions through 5.13(ABRL.5.4)C0 could allow an adjacent attacker to trigger a temporary denial-of-service (DoS) condition affecting the UPnP function of the affected device.

A missing authorization vulnerability in Zyxel GS1200-5v3 firmware versions through 1.00(ACPS.2)C0, GS1200-8v3 firmware versions through 1.00(ACPT.2)C0,  GS1200-5HPv3 firmware versions through 1.00(ACPU.2)C0, GS1200-8HPv3 firmware versions through 1.00(ACPV.2)C0, and GS1200-10v3 firmware versions through 1.00(ACPW.2)C0 could allow a LAN-based, unauthenticated attacker to read the system configuration from a log file via a crafted HTTP request.

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.

UNSUPPORTED WHEN ASSIGNED A buffer overflow vulnerability in the formWep(), formWlAc(), formPasswordSetup(), formUpgradeCert(), and formDelcert() functions of the “webs” binary in Zyxel NWA1100-N customized firmware version 1.00(AACE.1)C0 could allow an attacker to trigger a denial-of-service (DoS) condition by sending a crafted HTTP request to a vulnerable device.

UNSUPPORTED WHEN ASSIGNED An insecure storage of sensitive information vulnerability in the configuration file of Zyxel WRE6505 v2 firmware version V1.00(ABDV.3)C0 could allow a local attacker with administrator privileges to download and decrypt a backup configuration file.

UNSUPPORTED WHEN ASSIGNED A command injection vulnerability in the CGI program of Zyxel WRE6505 v2 firmware version V1.00(ABDV.3)C0 could allow an adjacent attacker on the LAN to execute operating system (OS) commands on a vulnerable device by sending a crafted HTTP request.

UNSUPPORTED WHEN ASSIGNED An improper restriction of excessive authentication attempts vulnerability in the web management interface of Zyxel WRE6505 v2 firmware version V1.00(ABDV.3)C0 could allow an adjacent attacker on the LAN to brute-force the password and bypass authentication.

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.

UNSUPPORTED WHEN ASSIGNED An improper encoding or escaping vulnerability in the CGI program of Zyxel WRE6505 v2 firmware version V1.00(ABDV.3)C0 could allow an adjacent attacker on the WLAN to cause a denial-of-service (DoS) condition in the web management interface by convincing an authenticated administrator to visit the “AP Select” page while a malformed SSID is present.

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 command injection vulnerability in the UPnP function of the Zyxel EX3510-B0 firmware versions through 5.17(ABUP.15.1)C0 could allow a remote attacker to execute operating system (OS) commands on an affected device by sending specially crafted UPnP SOAP requests.

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 Dynamic DNS (DDNS) configuration CLI command in Zyxel ATP series firmware versions from V5.35 through V5.41, USG FLEX series firmware versions from V5.35 through V5.41, USG FLEX 50(W) series firmware versions from V5.35 through V5.41, and USG20(W)-VPN series firmware versions from V5.35 through V5.41 could allow an authenticated attacker with administrator privileges to execute operating system (OS) commands on an affected device by supplying a specially crafted string as an argument to the CLI command.