In August 2023, Microsoft disclosed a serious vulnerability, CVE-2023-38180, impacting both .NET applications and the Visual Studio IDE. Denial of Service (DoS) issues like this can cause applications and tools to crash or become unusable, impacting productivity and potentially opening the door to further attacks. This long-read post breaks down what this vulnerability means, how it works, how it can be exploited, and what developers can do to stay safe.

What is CVE-2023-38180?

CVE-2023-38180 is a Denial of Service (DoS) vulnerability that affects the .NET ecosystem, specifically certain .NET libraries and the Visual Studio development environment. If triggered, it allows attackers to cause the application—or even Visual Studio itself—to crash, hang, or go into an infinite loop by passing it a specially crafted input.

Microsoft officially classified it as Important and urged developers to patch their systems. While it does not allow remote code execution or data theft directly, DoS vulnerabilities can be just as damaging, especially on exposed services or in CI/CD environments.

Impact: Application crash, infinite loop, or resource exhaustion

Microsoft’s Advisory:  
> https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-38180

Where Exactly is the Problem?

The vulnerability is rooted in the way .NET’s core libraries handle input—most often, network or file-based input. In technical releases, Microsoft pointed to issues in the regular expression engine and certain collection classes. By sending a carefully crafted payload, an attacker can trigger expensive operations—like catastrophic backtracking in regex, or endless loops in data parsing routines.

A common DoS attack in .NET involves exploiting regular expressions. Consider the following pattern

using System.Text.RegularExpressions;

string pattern = @"(a+)+$";
Regex regex = new Regex(pattern);

string input = new string('a', 100) + "!";
bool isMatch = regex.IsMatch(input);

With an input string of many 'a's followed by a non-matching character, the regex engine gets stuck trying all possible matches, spending excessive CPU and memory, resulting in application unresponsiveness. CVE-2023-38180 targets such inefficient algorithms that are exposed without mitigation.

How Might This Be Exploited?

Attackers can exploit this by sending malicious files, HTTP payloads, or even by tricking developers into opening a file or project inside Visual Studio. Let’s see a plausible scenario for web applications and developers using Visual Studio.

1. Remote Web Application Exploit

Suppose you have a .NET web API endpoint expecting a JSON payload. Somewhere in your API logic, you use a regex to validate incoming data, or parse strings from user input.

An attacker could POST a crafted input like the long string above, causing the API process to hang or crash, denying service to legitimate users.

2. Visual Studio DoS

Similarly, Visual Studio could hang or become unusable if a solution or file with maliciously crafted content is opened. This is rarer, but possible—especially in collaborative environments or if a malicious NuGet package is imported into a project.

Code Snippet: Vulnerable Validation

// Bad: Using an unbounded regex to validate email
Regex unsafeEmailRegex = new Regex(@"([a-zA-Z-9_.+-]+)@(([a-zA-Z-9-]+\.)+[a-zA-Z-9]{2,4})");

string input = "...."; // crafted input triggers catastrophic backtracking
if (unsafeEmailRegex.IsMatch(input))
{
  // Do something
}

Fix: Add Regex Timeout

// Good: Using RegexOptions and Timeout
Regex safeEmailRegex = new Regex(
    @"([a-zA-Z-9_.+-]+)@(([a-zA-Z-9-]+\.)+[a-zA-Z-9]{2,4})",
    RegexOptions.None, 
    TimeSpan.FromSeconds(2) // Limit regex execution
);

if (safeEmailRegex.IsMatch(input))
{
  // Do something
}

Here's a minimal C# PoC that demonstrates how easy it is to stall a .NET application via regex

using System;
using System.Text.RegularExpressions;

public class DoSExample
{
    public static void Main()
    {
        string pattern = @"(+)+$";
        Regex regex = new Regex(pattern);

        // Long string of zeros followed by an 'x'
        string input = new string('', 50000) + "x";

        Console.WriteLine("Testing regex...");
        bool result = regex.IsMatch(input);

        Console.WriteLine("Done!");
    }
}

Running this will freeze or significantly delay the process, a classic DoS effect.

Microsoft released security patches as part of their August 2023 Patch Tuesday updates

- .NET 6 & .NET 7 SDK/runtime updates

Visual Studio 2022 patches

You can see the full update details here:  
https://learn.microsoft.com/en-us/security-updates/securitybulletins/2023/ms23-aug

If you’re using any of the above-mentioned tools or runtimes, update immediately.

How to Protect Your Apps & Tools

1. Patch Early, Patch Often  
Apply the latest .NET and Visual Studio updates. Always keep your libraries up-to-date to benefit from security and performance fixes.

2. Sanitize All Inputs  
Never trust external inputs—this includes user data, files, or network payloads.

3. Use Regex Timeouts  
Always use the Regex constructor that accepts a timeout value to prevent catastrophic backtracking.

4. Code Review and Audit  
Review code for risky patterns—especially regular expressions and file/network parsers.

5. Monitor and Alert  
Set up monitoring and alerts for unusual application crashes or high CPU usage—early detection can help you react before attackers succeed.

References and Further Reading

- Microsoft CVE-2023-38180 Advisory
- Microsoft Security Update Guide
- Common .NET DoS Patterns (OWASP)
- .NET Release Notes
- Secure Regex .NET Documentation

Conclusion

CVE-2023-38180 is a real-world example of how poorly handled input—especially via complex regular expressions or unchecked data parsing—can cripple applications and development tools. Denial of Service is often overlooked, but its impacts can be very real in terms of downtime and lost productivity. Stay vigilant, keep software updated, and always code defensively.

Timeline

Published on: 08/08/2023 19:15:00 UTC
Last modified on: 08/08/2023 20:39:00 UTC