Zyxel VMG4325-B10A Legacy DSL CPE Telnet Command Injection Vulnerability (CVE-2024-40891) Analysis and Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTLegacy and End-of-Life (EOL) Zyxel VMG4325-B10A DSL CPE devices running firmware version 1.00(AAFR.4)C0_20170615.
An authenticated attacker can execute arbitrary operating system (OS) commands with root privileges via the Telnet management interface.
Since the device is unsupported (EOL), immediate replacement of the hardware is highly recommended. Temporarily disable external Telnet access and enforce strong credentials.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
This product is End-of-Life (EOL) and unsupported by the vendor; therefore, no official security patches will be released. The primary recommended action is to decommission the device and replace it with a supported model. Prior to replacement, ensure all default passwords are changed immediately.
2. Temporary Workaround & Hardening Configuration
Disable external WAN-side Telnet (TCP port 23) and Web management access. Use firewall rules to restrict Telnet access exclusively to trusted internal management IPs.
# Block all incoming Telnet traffic from the WAN interface (e.g., eth0)
iptables -A INPUT -i eth0 -p tcp --dport 23 -j DROP
# Allow Telnet only from a specific trusted management IP (e.g., 192.168.1.100)
iptables -A INPUT -p tcp -s 192.168.1.100 --dport 23 -j ACCEPT
iptables -A INPUT -p tcp --dport 23 -j DROP3. Log Audit & IoC Verification Commands
Run the following commands on the device's shell to verify the firmware version, inspect active connections, and detect potential unauthorized processes.
# 1. Check the running firmware version and system details
cat /etc/release 2>/dev/null || uname -a
# 2. Audit active Telnet connections and listening ports
netstat -antp | grep :23
# 3. Scan for suspicious active processes or reverse shells
ps w | grep -E 'sh|bash|nc|wget|curl' | grep -v grepStop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2024-40891) is a post-authentication command injection vulnerability located in the management command parser of the legacy Zyxel VMG4325-B10A DSL CPE firmware. The root cause lies in the lack of proper input validation and sanitization of user-supplied arguments passed to administrative commands via the Telnet interface (CWE-78). This allows authenticated users to escape the restricted CLI environment and execute arbitrary system commands.
2. Exploitation Mechanics & Attack Scenario
An attacker first establishes an authenticated session via Telnet using valid credentials (which may be obtained via default credentials or credential stuffing). Once authenticated, the attacker injects shell metacharacters (such as ';', '&', or '|') followed by malicious OS commands into the management command parameters. Because the input is directly concatenated into a system shell call without sanitization, the device executes the injected commands under the context of the administrative user (typically root).
3. Infrastructure & Business Impact
This vulnerability is actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog. Successful exploitation grants full control over the CPE device, enabling attackers to intercept network traffic, perform DNS hijacking, pivot into the internal local area network (LAN), or enlist the compromised device into a malicious botnet.
Official Vendor Advisories & References
**UNSUPPORTED WHEN ASSIGNED** A post-authentication command injection vulnerability in the management commands of the legacy DSL CPE Zyxel VMG4325-B10A firmware version 1.00(AAFR.4)C0_20170615 could allow an authenticated attacker to execute operating system (OS) commands on an affected device via Telnet.
Frequently Asked SecOps Questions (CVE-2024-40891)
FAQQ1. How critical is CVE-2024-40891 and is it actively exploited in the wild?
CVE-2024-40891 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 22.9%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2024-40891?
Primary impacted vendors include zyxel across vmg4325-b10a. Scope summary: Zyxel VMG4325-B10A firmware version 1.00(AAFR.4)C0_20170615 (End-of-Life)
Q3. What temporary mitigations or workarounds exist for CVE-2024-40891 before patching?
[Mitigation] Since the device is unsupported (EOL), immediate replacement of the hardware is highly recommended. Temporarily disable external Telnet access and enforce strong credentials. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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