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Introduction

In early 2024, Microsoft quietly patched a critical vulnerability—CVE-2024-21420—in the Windows Defender Application Control (WDAC) OLE DB Provider for SQL Server. This bug lets attackers run arbitrary code remotely, a classic "Remote Code Execution" (RCE) risk. If you run apps that use OLE DB to talk to SQL Server, this vulnerability is a big deal.

This post breaks down the technical aspects, includes code snippets, shows how the exploit works, and gives you exclusive, plain-language details. We'll also talk about mitigating the risk. All information is synthesized from Microsoft's advisories and security research, with easy-to-follow examples.

What Is CVE-2024-21420?

* Vulnerability Type: Remote Code Execution (RCE)
* Impact: If exploited, attackers can execute code in the context of the application that uses the OLE DB provider to connect to SQL Server. This could mean total compromise of the host, depending on permissions.
* Component: Microsoft WDAC OLE DB Provider for SQL Server (MSOLEDBSQL)
* Severity: Critical (CVSS base score: typically above 8.)
* Affected Software: All unpatched versions of Windows using WDAC-enabled OLE DB for SQL Server.
* Attack Vector: Remote, needs user to connect to a malicious SQL Server or attacker-controlled host.

Official Microsoft advisory:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-21420

Why Does This Matter?

OLE DB is a standard way Windows software talks to databases like SQL Server. Database client code is often trusted and runs with high privileges. If an attacker can get a target to connect to a malicious SQL Server (or man-in-the-middle a connection), they can trigger this bug and run code on your machine.

This vulnerability is in the client—the code you run when connecting to SQL. You don’t need to be running a SQL Server—just connecting to *any* SQL Server from a vulnerable client is enough.

Technical Details

Microsoft's WDAC OLE DB provider failed to properly validate certain data from a SQL Server. If a client connects to a malicious server and receives a specially crafted response, the client might process malicious data, leading to memory corruption and code execution.

Attacker controls a SQL Server—either on their own machine or as a man-in-the-middle.

2. Victim’s client application connects to the malicious SQL Server using OLE DB (e.g., from an Office macro or custom .NET app).

Here's a classic way to connect to SQL Server using MSOLEDBSQL in C#

using System.Data.OleDb;

string connString = "Provider=MSOLEDBSQL;Server=malicious.example.com;Database=TestDB;User Id=testuser;Password=testpass;";
using (OleDbConnection conn = new OleDbConnection(connString))
{
    conn.Open(); // Vulnerable step if server is malicious
    // Further queries...
}

If malicious.example.com is attacker-controlled, just opening the connection could result in code execution. The exploit doesn't require authorization—all that's needed is connection.

How Does the Exploit Work?

According to analysts and PoC discussions, the exploit payload is delivered through the server’s response during the pre-login or login negotiation phase, exploiting flaws in how WDAC OLE DB provider parses the packets.

Assuming the attacker listens for incoming SQL Server connections and delivers a crafted TDS payload

# Pseudocode outline for malicious server using Python
import socket

HOST = '...'
PORT = 1433  # SQL Server default port

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen(1)
    print('Waiting for connection...')
    conn, addr = s.accept()
    with conn:
        print('Connected by', addr)
        # Send crafted TDS packet triggering vulnerability
        malicious_payload = b'\x12\x34\x56...' # (shortened for brevity)
        conn.sendall(malicious_payload)

When a vulnerable client connects, the malicious payload triggers memory corruption, and the attacker's code can run.

Proof-of-Concepts:
- PoC details are not officially public, but check recent discussions on packetstorm or GitHub repositories for OLE DB CVEs.
- More technical exploration: Microsoft TDS documentation.

Apply updates: Microsoft released a fix in February 2024.

- Security update guide for CVE-2024-21420
- Ensure all endpoints using MSOLEDBSQL are up-to-date, especially developer/test machines.

Look for unusual MSOLEDBSQL connection attempts to unknown servers in logs.

- Use Microsoft Defender for Endpoint to monitor and alert on suspicious database connection patterns.

Conclusion

CVE-2024-21420 is one of the most critical OLE DB vulnerabilities discovered in recent years. It can be exploited silently, just by getting your client to connect to a malicious SQL Server. Always keep your systems updated, and restrict your client machines from connecting to unknown database servers.

If you're a developer or an admin, confirm that your systems are patched—not just backend SQL Servers, but every client application that connects to SQL using OLE DB!

For more in-depth details, check the official Microsoft advisory at:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-21420

References

- Microsoft Security Response Center (MSRC): CVE-2024-21420
- MSOLEDBSQL documentation
- TDS Protocol Specification


Note:
This post is for educational purposes only. Use this knowledge to defend, not attack.

Timeline

Published on: 02/13/2024 18:16:00 UTC
Last modified on: 02/13/2024 18:22:43 UTC