CVE-2025-67189

Executive Summary

CVE-2025-67189 is a critical buffer overflow vulnerability discovered in the TOTOLINK A950RG router, specifically in firmware version V4.1.2cu.5204_B20210112. The flaw resides in the setParentalRules interface, where the urlKeyword parameter is concatenated into a fixed-size stack buffer without proper boundary validation. This allows a remote attacker to overflow the buffer, leading to denial of service (DoS) or potentially arbitrary code execution. As of this report, no official patch has been released, and no public proof-of-concept (PoC) or exploit code is available. The vulnerability is classified under CWE-121 (Stack-based Buffer Overflow) and aligns with MITRE ATT&CK tactic T1190 (Exploit Public-Facing Application). The EPSS score is 0.0%, indicating low exploitation activity in the wild, but the severity remains high due to the potential for remote code execution.

Supply Chain & Package Security

This vulnerability is not related to any software package manager (NPM, PyPI, Go, Ruby). It affects embedded firmware in a networking device, making supply chain risks primarily associated with the hardware vendor's firmware distribution and update mechanisms.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0 in the NVD data, but based on the vulnerability description, a more realistic assessment is provided below:

• Attack Vector (AV): Network (N) – The vulnerability is exploitable remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions are required; the attacker can send a crafted request.
• Privileges Required (PR): None (N) – No authentication is needed to trigger the overflow.
• User Interaction (UI): None (N) – The victim does not need to perform any action.
• Scope (S): Unchanged (U) – The overflow affects the same component.
• Confidentiality (C): High (H) – Arbitrary code execution could lead to data exfiltration.
• Integrity (I): High (H) – Code execution allows modification of system state.
• Availability (A): High (H) – The overflow can cause a denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Estimated Base Score: 9.8 (Critical)

Threat Level: Critical. The lack of authentication and remote exploitability make this a high-priority vulnerability for any organization using the TOTOLINK A950RG router.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the setParentalRules CGI interface. The router's web server handles HTTP POST requests to this endpoint, parsing parameters such as urlKeyword. The firmware code likely uses a function like sprintf or strcpy to concatenate user-supplied input into a fixed-size stack buffer (e.g., 256 bytes) without checking the length of the input. When an attacker sends a payload larger than the buffer, it overwrites adjacent stack memory, including the return address and other critical data.

Key Technical Details:

• Affected Interface: /cgi-bin/luci/;stok=/locale or similar path handling setParentalRules.
• Vulnerable Parameter: urlKeyword (and potentially other parameters like domain, time, etc., as they are concatenated).
• Root Cause: Lack of input validation and use of unsafe string copy functions.
• Impact: Overwriting the return address can redirect execution to attacker-controlled shellcode, achieving remote code execution with root privileges (as the web server typically runs as root on embedded devices).

Attack Scenario:

1. Attacker identifies a TOTOLINK A950RG router exposed to the internet (e.g., via Shodan).
2. Attacker sends a crafted HTTP POST request to the setParentalRules endpoint with an overly long urlKeyword parameter.
3. The overflow corrupts the stack, causing either a crash (DoS) or, if carefully crafted, execution of arbitrary code.
4. Successful exploitation could allow the attacker to install malware, pivot to internal networks, or exfiltrate data.

Vulnerable vs. Patched Code

Since no patch is available, the following is a hypothetical comparison based on common remediation patterns.

Vulnerable Code (C pseudocode):

void setParentalRules(char *urlKeyword, char *domain, char *time) {
    char buffer[256];
    sprintf(buffer, "url=%s&domain=%s&time=%s", urlKeyword, domain, time);
    // Process buffer...
}

Patched Code (C pseudocode):

void setParentalRules(char *urlKeyword, char *domain, char *time) {
    char buffer[256];
    // Use snprintf with size limit
    snprintf(buffer, sizeof(buffer), "url=%s&domain=%s&time=%s", urlKeyword, domain, time);
    // Process buffer...
}

The patched version uses snprintf to limit the number of bytes written to the buffer, preventing overflow. Additionally, input validation should be added to reject overly long parameters.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Environment:

• TOTOLINK A950RG router with firmware V4.1.2cu.5204_B20210112.
• Attacker machine (Kali Linux) on the same network.
• Tools: curl, python3, gdb (for debugging if firmware is emulated).

Exploitation Steps:

1. Identify the target: Use Shodan or local network scanning to find the router.
2. Craft the payload: Since no public PoC exists, a basic DoS test can be performed:

   curl -X POST http://<router_ip>/cgi-bin/luci/;stok=/locale \
   -d "urlKeyword=$(python3 -c 'print("A"*500)')&domain=test&time=00:00"
   

3. Observe crash: The router may reboot or become unresponsive.
4. For RCE: Reverse engineer the firmware (e.g., using binwalk and Ghidra) to find the exact offset to the return address and develop a shellcode payload.

Note: No public exploit code is available. The above is a conceptual demonstration.

Blue Team (Defensive)

Detection & Response:

• Monitor for unusually long HTTP POST requests to /cgi-bin/luci/ or similar paths.
• Use network intrusion detection systems (NIDS) to alert on patterns like repeated 'A' characters or shellcode signatures.
• Isolate affected devices from critical networks until a patch is applied.

Honeypot Deployment:

• Set up a honeypot mimicking the TOTOLINK A950RG to capture exploitation attempts.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP POST requests to /cgi-bin/luci/ with parameters exceeding 256 bytes.
  • Repeated patterns of characters (e.g., long strings of 'A's) in the urlKeyword field.
  • Outbound connections from the router to unknown IP addresses (post-exploitation).
• Host IOCs:
  • Unexpected processes running on the router (e.g., reverse shells).
  • Modified firmware or configuration files.
  • Logs showing crashes or reboots without clear cause.

Detection Engineering

YARA Rule

rule CVE_2025_67189_TOTOLINK_Overflow {
    meta:
        description = "Detects buffer overflow attempts against TOTOLINK A950RG setParentalRules"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-67189"
    strings:
        $s1 = "urlKeyword=" ascii wide nocase
        $s2 = "setParentalRules" ascii wide nocase
        $overflow = { 41 41 41 41 41 41 41 41 } // 8 or more 'A's
    condition:
        $s1 and $s2 and #overflow > 10
}

Sigma Rule

title: TOTOLINK A950RG Buffer Overflow Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests with overly long urlKeyword parameter targeting TOTOLINK A950RG
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/cgi-bin/luci/'
        cs-uri-query|contains: 'urlKeyword='
        sc-bytes|>=: 500
    condition: selection
falsepositives:
    - Legitimate long URLs (unlikely)
level: high

Suricata Rule

alert http $EXTERNAL_NET any -> $HOME_NET $HTTP_PORTS (
    msg:"CVE-2025-67189 TOTOLINK A950RG Buffer Overflow Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/luci/"; http_uri;
    content:"urlKeyword="; http_client_body;
    pcre:"/urlKeyword=[A-Za-z0-9%]{500,}/Ri";
    sid:1000001;
    rev:1;
    reference:cve,2025-67189;
    priority:1;
)

Remediation & Mitigation

Immediate Actions:

1. Disable remote management: If not required, disable WAN access to the router's web interface.
2. Restrict access: Use firewall rules to allow only trusted IPs to access the management interface.
3. Monitor logs: Enable logging and review for suspicious POST requests.

Long-term Fix:

• Vendor patch: Await an official firmware update from TOTOLINK. Contact their support to expedite.
• Workaround: If patching is not possible, consider replacing the device with a more secure alternative.
• Network segmentation: Place the router in a DMZ and limit its exposure to internal networks.

Valters IT Signature
This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.