CVE-2025-7673: Zyxel EMG3525-T50B Firmware RCE Vulnerability

CVE-2025-7673 Overview

A buffer overflow vulnerability exists in the URL parser of the zhttpd web server embedded in multiple Zyxel residential gateway and router firmware. The vulnerability allows an unauthenticated attacker to cause denial-of-service (DoS) conditions and potentially achieve remote code execution by sending specially crafted HTTP requests to the affected devices.

Critical Impact

This vulnerability enables unauthenticated remote attackers to crash affected Zyxel devices or potentially execute arbitrary code, compromising network security at the edge.

Affected Products

• Zyxel VMG8825-T50K firmware versions prior to V5.50(ABOM.5)C0
• Zyxel VMG series devices (VMG1312-T20B, VMG3625-T50B, VMG3925-B10B/C, VMG3927 variants, VMG4005-B50B, VMG4927-B50A, VMG8623-T50B, VMG8825 variants, VMG8924-B10D)
• Zyxel EMG series devices (EMG3525-T50B, EMG5523-T50B, EMG5723-T50K, EMG6726-B10A)
• Zyxel EX series devices (EX3510-B0, EX5510-B0)
• Zyxel XMG series devices (XMG3927-B50A, XMG8825-B50A)

Discovery Timeline

• July 16, 2025 - CVE-2025-7673 published to NVD
• January 14, 2026 - Last updated in NVD database

Technical Details for CVE-2025-7673

Vulnerability Analysis

The vulnerability resides in the zhttpd web server component that provides the administrative web interface for affected Zyxel residential gateways and routers. The URL parser fails to properly validate the length of incoming URL data before copying it into a fixed-size buffer, creating a classic buffer overflow condition (CWE-120: Buffer Copy without Checking Size of Input).

When processing HTTP requests, the zhttpd service parses the URL path to determine the requested resource. Due to insufficient bounds checking, an attacker can supply an excessively long URL that overflows the allocated buffer. This memory corruption can overwrite adjacent stack or heap memory, leading to application crashes (denial of service) or, in more sophisticated attacks, control flow hijacking that enables arbitrary code execution.

The attack requires no authentication, as the vulnerability is triggered during the initial HTTP request parsing phase before any authentication checks occur. This makes the vulnerability particularly dangerous for devices with management interfaces exposed to the network.

Root Cause

The root cause is a buffer copy operation in the zhttpd URL parsing code that does not verify the input length against the destination buffer size (CWE-120). This is a fundamental input validation failure where user-controlled data from HTTP requests is copied into a fixed-size memory buffer without proper length restrictions.

Attack Vector

The attack can be executed remotely over the network by any unauthenticated attacker who can reach the device's web management interface. The attacker crafts an HTTP request containing an oversized URL path designed to overflow the vulnerable buffer. Successful exploitation results in denial of service at minimum, with potential for remote code execution depending on the memory layout and any existing exploit mitigations.

The attack scenario typically involves:

1. Identifying a vulnerable Zyxel device with an accessible web interface
2. Sending a malformed HTTP request with an excessively long URL
3. The zhttpd process crashes or the attacker gains code execution

Given the network-accessible nature of this vulnerability and the lack of authentication requirements, devices with management interfaces exposed to untrusted networks are at significant risk.

Detection Methods for CVE-2025-7673

Indicators of Compromise

• Unexpected crashes or restarts of Zyxel device web management services
• Presence of abnormally long HTTP request URLs in device logs targeting the web interface
• Network traffic showing repeated connection attempts to the device's HTTP/HTTPS management ports with large payloads
• Device unresponsiveness or availability issues correlated with external HTTP access attempts

Detection Strategies

• Monitor network traffic for HTTP requests to Zyxel devices containing URLs exceeding normal length thresholds (typically >2048 characters)
• Implement IDS/IPS rules to detect buffer overflow attack patterns targeting embedded web servers
• Enable and review device logs for zhttpd service crashes or abnormal termination events
• Deploy network monitoring to track unusual connection patterns to device management ports (commonly ports 80 and 443)

Monitoring Recommendations

• Establish baseline metrics for device management interface traffic and alert on anomalies
• Configure SIEM systems to correlate device availability events with external connection attempts
• Regularly audit which networks and hosts have access to device management interfaces
• Implement network segmentation to isolate management plane traffic from general network traffic

How to Mitigate CVE-2025-7673

Immediate Actions Required

• Apply firmware updates from Zyxel immediately for all affected devices
• Restrict access to device management interfaces to trusted administrative networks only
• If immediate patching is not possible, disable the web management interface and use alternative management methods
• Audit network configurations to ensure management interfaces are not exposed to the internet or untrusted networks

Patch Information

Zyxel has released firmware updates to address this vulnerability. For the VMG8825-T50K, update to firmware version V5.50(ABOM.5)C0 or later. Consult the Zyxel Security Advisory for the specific patched firmware versions for each affected device model.

Organizations should prioritize patching based on device exposure, giving highest priority to devices with management interfaces accessible from untrusted networks.

Workarounds

• Disable remote web management access and use local console or SSH management where possible
• Implement firewall rules to restrict HTTP/HTTPS access to the device management interface to specific trusted IP addresses
• Deploy a web application firewall (WAF) or reverse proxy with URL length validation in front of management interfaces
• Enable access control lists (ACLs) on the device to limit management interface access to authorized administrator workstations only

# Example firewall rule to restrict management access (adjust interface and IP as needed)
# On upstream firewall or router, block external access to device management ports
iptables -A FORWARD -d <DEVICE_IP> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <DEVICE_IP> -p tcp --dport 443 -j DROP

# Allow only from trusted management network
iptables -I FORWARD -s <TRUSTED_MGMT_NETWORK> -d <DEVICE_IP> -p tcp --dport 80 -j ACCEPT
iptables -I FORWARD -s <TRUSTED_MGMT_NETWORK> -d <DEVICE_IP> -p tcp --dport 443 -j ACCEPT