CVE-2023-21704 - Breaking Down the Microsoft ODBC Driver for SQL Server RCE Vulnerability

In early 2023, Microsoft published a critical security vulnerability known as CVE-2023-21704. This flaw targets the Microsoft ODBC Driver for SQL Server and can let attackers run malicious code remotely on affected systems. If you use Microsoft SQL Server or develop apps that connect to SQL with ODBC, this is a must-read.

Let's dig into what CVE-2023-21704 is, why it’s dangerous, and how attackers could exploit it. We’ll also look at sample code snippets and show you how to protect yourself.

What is CVE-2023-21704?

CVE-2023-21704 is a Remote Code Execution (RCE) vulnerability in the Microsoft ODBC Driver for SQL Server. Specifically, if an attacker gets you to connect to a rogue (malicious) SQL server, they can exploit a bug in the ODBC driver and run code on your machine — with your user’s rights.

| Item              | Value                                        |
| ----------------- | ------------------------------------------- |
| CVSS Score        | 8.1 (High)                                  |
| Affected Versions | ODBC Driver 17 and 18 for SQL Server        |
| Impact            | Remote Code Execution                       |
| Exploited         | In the Wild: No (as of writing)             |

- Microsoft Advisory: MSRC CVE-2023-21704
- NIST Entry: NVD - CVE-2023-21704

How Does The Vulnerability Work?

The bug exists in how the ODBC Driver handles authentication when connecting to a SQL Server. If you connect to a fake (malicious) SQL Server, and the attacker controls what's sent, the server can trick the driver into running arbitrary code — for example, by sending a payload during login.

Attacker sets up a fake SQL Server somewhere on the Internet.

2. Victim app or user connects to attacker’s server (maybe a supply chain compromise or typo in connection string).

Suppose you have a C# app connecting to SQL Server by ODBC

using System;
using System.Data.Odbc;

class OdbcTest {
    static void Main() {
        // WARNING: Don't actually connect to unknown servers!
        string connString = "Driver={ODBC Driver 17 for SQL Server};Server=attacker.example.com,1433;UID=sa;PWD=Password123;";
        using (OdbcConnection conn = new OdbcConnection(connString)) {
            conn.Open();
            Console.WriteLine("Connected.\nRunning a test query...");
            using (OdbcCommand cmd = new OdbcCommand("SELECT 1", conn)) {
                Console.WriteLine(cmd.ExecuteScalar());
            }
        }
    }
}

An attacker, controlling attacker.example.com, could exploit CVE-2023-21704 at the driver level — before your app even runs its first query.

What Could Happen If Exploited?

- Malware install: The attacker could run a payload, installing ransomware or a remote access tool.

Move deeper: Use control of the client machine to pivot farther into a corporate network.

The scary part: This attack works with just a *connection* to the wrong server — you don't have to accept any files or click “OK” on a dialog box.

Exploitation Details

While there’s no public “exploit in the wild” (as of June 2024), the public advisories and reverse engineering of the patch let us outline a typical technique:

- The vulnerability involves how ODBC handles response packets in the pre-login handshake. If the attacker crafts a response with unusual/unexpected data (like an oversized buffer or invalid structure), the driver’s memory handling fails.

This leads to a classic buffer overflow or use-after-free, giving code execution to the attacker.

Security researchers @tenable and @Rapid7 have analyzed the patch and confirmed it mitigates this issue by tightening input validation.

Download

- ODBC Driver 17 for SQL Server
- ODBC Driver 18 for SQL Server

2. Never Connect to Untrusted Servers

Double check your connection strings! Only connect to servers you control. Don't hardcode server names you don’t recognize.

3. Monitor Network Traffic

Watch for outbound connections to unknown or suspicious SQL Server ports (default is 1433/TCP). Use firewalls or endpoint protection to block unexpected destinations.

References and Further Reading

- Microsoft Security Response Center: CVE-2023-21704
- NIST NVD: CVE-2023-21704
- Tenable Blog: What You Need to Know About CVE-2023-21704
- Rapid7 Patch Tuesday Analysis, Jan 2023
- Microsoft Docs: ODBC Driver Updates

Summary Table

| What should you do?                   | Action                                                      |
| ------------------------------------- | ----------------------------------------------------------- |
| Are you running an affected driver?   | Check with: odbcad32.exe > Drivers tab                    |
| Need to fix?                          | Download and install the latest driver from Microsoft link   |
| Unsure about connections in code?     | Search your code for Unicode: "Server=" or "DSN="       |
| Monitor for attacks?                  | Set up alerts for unexpected SQL Server network activity     |

Final Thoughts

CVE-2023-21704 is a classic example of why client-side security matters. Even a simple connection to a malicious SQL Server can spell disaster. Patch your ODBC drivers ASAP, be careful which servers you talk to, and stay up to date on security news.

*Share this with your IT and development teams. A single patch could save your network!*

Timeline

Published on: 02/14/2023 20:15:00 UTC
Last modified on: 02/23/2023 15:01:00 UTC