CVE-2025-7091

Executive Summary

CVE-2025-7091 is a critical, unpatched stack-based buffer overflow vulnerability in the Belkin F9K1122 router (firmware version 1.00.33). The flaw resides in the formWlanMP function within the /goform/formWlanMP endpoint of the web server component. An unauthenticated, remote attacker can exploit this by sending a crafted HTTP POST request with oversized values in any of the 27 identified parameters (e.g., ateFunc, ateGain, ateTxCount, e2pTxPower1, etc.), leading to arbitrary code execution with root privileges on the device. Public exploit code is available, and the vendor has not responded to disclosure attempts. Given the lack of a patch and the criticality of the vulnerability, immediate mitigation through network segmentation and access controls is essential.

CVSS Breakdown & Threat Level

• CVSS Score: 10.0 (Critical)
• Attack Vector: Network (AV:N) – Exploitable remotely without physical access.
• Attack Complexity: Low (AC:L) – No special conditions required; a simple crafted request suffices.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – The vulnerable component and the impacted component are the same.
• Confidentiality Impact: High (C:H) – Full device compromise leads to data exfiltration.
• Integrity Impact: High (I:H) – Attacker can modify firmware, settings, or inject malicious code.
• Availability Impact: High (A:H) – Exploitation can cause denial of service or complete device takeover.

Threat Level: Critical. The vulnerability is trivial to exploit, has public proof-of-concept code, and affects a widely deployed consumer router. An attacker can gain full control of the device, pivot to internal networks, and conduct man-in-the-middle attacks.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow (CWE-121) in the formWlanMP function. This function is part of the router's web interface, specifically handling wireless configuration parameters. The endpoint /goform/formWlanMP accepts POST requests with parameters intended for manufacturing and testing (ATE – Automatic Test Equipment) and E2P (EEPROM) calibration data.

The vulnerable code likely uses a fixed-size stack buffer to copy user-supplied parameter values without proper bounds checking. By providing an excessively long string in any of the 27 parameters (e.g., ateFunc, ateGain, ateTxCount, ateChan, ateRate, ateMacID, e2pTxPower1 through e2pTxPower7, e2pTx2Power1 through e2pTx2Power7, ateTxFreqOffset, ateMode, ateBW, ateAntenna, e2pTxFreqOffset, e2pTxPwDeltaB, e2pTxPwDeltaG, e2pTxPwDeltaMix, e2pTxPwDeltaN, readE2P), an attacker can overwrite the return address on the stack, hijacking the program's execution flow.

The exploit is remote and unauthenticated, meaning any device on the network (or the internet if the router's web interface is exposed) can be targeted. The public exploit demonstrates overwriting the return address to jump to a ROP chain or shellcode, ultimately granting a root shell.

Vulnerable vs. Patched Code

Since no patch is available, a direct comparison is not possible. However, the vulnerable code pattern is typical of unsafe string copy functions like strcpy, sprintf, or memcpy without length validation.

Vulnerable Code (Hypothetical):

void formWlanMP(struct request *req) {
    char buffer[256];
    char *ateFunc = get_param(req, "ateFunc");
    strcpy(buffer, ateFunc); // No bounds check
    // ... process buffer
}

Patched Code (Hypothetical):

void formWlanMP(struct request *req) {
    char buffer[256];
    char *ateFunc = get_param(req, "ateFunc");
    if (strlen(ateFunc) >= sizeof(buffer)) {
        // Return error, do not copy
        return;
    }
    strncpy(buffer, ateFunc, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... process buffer
}

The patched version would use strncpy or snprintf with a size limit and validate the input length before copying.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Hardware: Belkin F9K1122 router with firmware 1.00.33.
2. Network: Connect the router to a isolated lab network. Assign it an IP (e.g., 192.168.2.1).
3. Tools: Python 3, pwntools, metasploit (optional), a debugger like gdb-multiarch (if you have a MIPS binary), or simply use the public exploit.

Exploit Steps:

1. Identify Target: Scan for the router's web interface on port 80.
2. Craft Payload: Use the public exploit code (if available) or write a custom one. The exploit sends a POST request to /goform/formWlanMP with an oversized ateFunc parameter containing shellcode and a return address.
3. Execute: Run the exploit against the target IP.
4. Post-Exploitation: Obtain a root shell, extract credentials, modify settings, or install a backdoor.

Example Exploit Snippet (Python):

import requests

target = "http://192.168.2.1/goform/formWlanMP"
payload = b"A" * 256 + b"\x00\x00\x00\x00"  # Placeholder for ROP chain
data = {"ateFunc": payload}
requests.post(target, data=data)

Blue Team (Defensive)

Detection:

• Monitor for HTTP POST requests to /goform/formWlanMP with abnormally long parameter values (e.g., > 256 bytes).
• Use network intrusion detection systems (NIDS) like Suricata to alert on oversized POST parameters.
• Check logs for crashes or restarts of the web server process.

Mitigation:

• Immediate: Block external access to the router's web interface. Use a firewall to restrict access to only trusted internal IPs.
• Network Segmentation: Place the router in a separate VLAN with no direct internet exposure.
• Firmware Update: Monitor Belkin's website for a patch. If none is available, consider replacing the device.
• WAF: Deploy a web application firewall (WAF) in front of the router (if possible) to filter oversized requests.

Indicators of Compromise (IOCs)

• Network:
  • HTTP POST requests to /goform/formWlanMP with parameter values exceeding 256 bytes.
  • Unusual outbound connections from the router to unknown IPs (e.g., on ports 4444, 31337).
  • Repeated HTTP 500 errors or connection resets from the router's web interface.
• Host (on the router):
  • Unexpected processes running (e.g., /bin/sh spawned by the web server).
  • Modified firmware or configuration files.
  • Presence of a reverse shell or backdoor binary.
• Logs:
  • Web server crash logs indicating a buffer overflow (e.g., "Segmentation fault" in /var/log/messages).

Detection Engineering

YARA Rule

rule CVE_2025_7091_Belkin_BOF {
    meta:
        description = "Detects exploit attempts for CVE-2025-7091 in HTTP POST requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-7091"
    strings:
        $s1 = "/goform/formWlanMP" ascii wide nocase
        $s2 = "ateFunc=" ascii wide nocase
        $s3 = "ateGain=" ascii wide nocase
        $s4 = "e2pTxPower1=" ascii wide nocase
        $long_param = /[a-zA-Z0-9_]{256,}/  // Parameter value longer than 255 bytes
    condition:
        $s1 and ($s2 or $s3 or $s4) and $long_param
}

Sigma Rule

title: CVE-2025-7091 Belkin Router Buffer Overflow Attempt
id: 8a2b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP POST requests to /goform/formWlanMP with oversized parameters
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: accesslog
detection:
    selection:
        cs-method: POST
        cs-uri-stem: '/goform/formWlanMP'
    condition: selection and (len(cs-uri-query) > 1024 or any(cs-uri-query, len(.) > 256))
falsepositives:
    - Legitimate large configuration updates (unlikely)
level: critical

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-7091 Belkin Router Buffer Overflow Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/goform/formWlanMP"; http_uri;
    pcre:"/[a-zA-Z0-9_%]{256,}/Ri";
    classtype:attempted-admin;
    sid:1000001;
    rev:1;
    reference:cve,2025-7091;
)

Remediation & Mitigation

1. Immediate Actions:

   • Disable remote management and UPnP on the router.
   • Restrict access to the web interface to only trusted IPs via firewall rules.
   • Monitor for any suspicious activity on the network.

2. Long-Term Actions:

   • Contact Belkin support and demand a firmware patch. If unresponsive, consider replacing the device with a vendor that provides timely security updates.
   • Implement network segmentation to isolate IoT devices from critical systems.
   • Deploy a WAF or reverse proxy to filter malicious requests.

3. Detection:

   • Deploy the provided YARA, Sigma, and Suricata rules to detect exploitation attempts.
   • Enable logging on the router (if possible) and centralize logs for analysis.

4. User Awareness:

   • Educate users not to expose router management interfaces to the internet.
   • Encourage regular firmware updates and use of strong admin passwords.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert