CVE-2025-67186: TOTOLINK A950RG Buffer Overflow Flaw

CVE-2025-67186 Overview

CVE-2025-67186 is a critical buffer overflow vulnerability affecting the TOTOLINK A950RG router firmware version V4.1.2cu.5204_B20210112. The vulnerability exists in the setUrlFilterRules interface located within /lib/cste_modules/firewall.so. Due to improper validation of the url parameter length, remote attackers can exploit this flaw to trigger a buffer overflow condition, potentially leading to arbitrary code execution or denial of service on affected devices.

Critical Impact

This network-accessible vulnerability requires no authentication or user interaction, allowing remote attackers to potentially execute arbitrary code or crash affected TOTOLINK A950RG routers.

Affected Products

• TOTOLINK A950RG V4.1.2cu.5204_B20210112

Discovery Timeline

• 2026-02-03 - CVE-2025-67186 published to NVD
• 2026-02-04 - Last updated in NVD database

Technical Details for CVE-2025-67186

Vulnerability Analysis

This buffer overflow vulnerability (CWE-120: Buffer Copy without Checking Size of Input) occurs in the firewall module of the TOTOLINK A950RG router. The setUrlFilterRules interface within the firewall.so shared library fails to properly validate the length of user-supplied input passed through the url parameter before copying it to a fixed-size buffer. When an attacker submits a specially crafted request with an excessively long url value, the data overflows the destination buffer's boundaries, corrupting adjacent memory regions.

The network-accessible nature of this vulnerability, combined with the lack of authentication requirements, makes it particularly dangerous for deployed devices. Successful exploitation could allow attackers to overwrite critical memory structures such as return addresses or function pointers, enabling arbitrary code execution with the privileges of the router's firmware process. Alternatively, exploitation could cause the service or entire device to crash, resulting in denial of service.

Root Cause

The root cause of this vulnerability is the absence of proper input length validation in the setUrlFilterRules function. The code accepts the url parameter from network requests and copies it directly into a stack or heap buffer without verifying that the input length does not exceed the buffer's allocated size. This classic buffer overflow pattern violates secure coding practices for handling untrusted input in memory-constrained environments like embedded router firmware.

Attack Vector

The attack vector for CVE-2025-67186 is network-based. An attacker can remotely send malicious HTTP requests to the router's web management interface targeting the setUrlFilterRules endpoint. By crafting a request with an oversized url parameter value, the attacker can trigger the buffer overflow condition. Since no authentication is required and no user interaction is needed, this vulnerability can be exploited directly against any exposed TOTOLINK A950RG device running the vulnerable firmware version.

The exploitation mechanism involves sending a crafted payload containing a url parameter that exceeds the expected buffer size. Technical details and proof-of-concept documentation are available in the GitHub PoC Documentation.

Detection Methods for CVE-2025-67186

Indicators of Compromise

• Unexpected router reboots or service crashes without apparent cause
• Anomalous HTTP requests to the router's management interface containing unusually long URL parameters
• Suspicious outbound network connections originating from the router to unknown destinations
• Modified firewall rules or configuration changes not initiated by administrators

Detection Strategies

• Deploy network intrusion detection systems (IDS) with rules to flag HTTP requests to TOTOLINK router interfaces containing oversized parameters
• Monitor for HTTP POST requests targeting /cgi-bin/ endpoints with url parameters exceeding typical lengths
• Implement deep packet inspection to detect buffer overflow payload patterns targeting embedded device interfaces
• Enable logging on network perimeter devices to capture and analyze traffic destined for router management ports

Monitoring Recommendations

• Continuously monitor TOTOLINK router device logs for unexpected crashes or service restarts
• Set up alerts for HTTP requests with parameter lengths exceeding established baselines for the setUrlFilterRules interface
• Monitor network traffic patterns for indicators of exploitation attempts against IoT and embedded devices
• Implement network segmentation monitoring to detect lateral movement if a router is compromised

How to Mitigate CVE-2025-67186

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Disable remote management features if not explicitly required for operations
• Place affected TOTOLINK A950RG devices behind a firewall that blocks external access to management ports
• Monitor vendor communications for firmware updates addressing this vulnerability

Patch Information

As of the last update on 2026-02-04, no official patch has been released by TOTOLINK for this vulnerability. Organizations should monitor the TOTOLINK support website and security advisories for firmware updates. Until a patch is available, implement the recommended workarounds to reduce exposure risk.

Workarounds

• Disable the web management interface entirely if remote administration is not required
• Configure access control lists (ACLs) to restrict management interface access to specific trusted internal IP addresses
• Deploy the router behind a properly configured firewall that blocks inbound connections to management ports from untrusted networks
• Consider replacing affected devices with alternative hardware if critical operations depend on the router and no patch is forthcoming

# Example: Restrict management access via firewall rules (external firewall)
# Block external access to router management port (adjust IP and port as needed)
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 443 -j DROP

# Allow only trusted management subnet
iptables -A FORWARD -s 10.0.0.0/24 -d 192.168.1.1 -p tcp --dport 80 -j ACCEPT