CVE-2023-21707: A Detailed Look Into A Critical Microsoft Exchange Server Remote Code Execution Vulnerability

The Common Vulnerabilities and Exposures (CVE) system has recently assigned a severity rating of 9.8 out of 10 for a critical remote code execution vulnerability affecting Microsoft Exchange Server. This vulnerability, tracked as CVE-2023-21707, has drawn the attention of the cybersecurity community due to its potential to impact numerous organizations that rely on Microsoft Exchange Server for their email solutions. In this extensive post, we will delve into the details of CVE-2023-21707, examine the code snippet related to the vulnerability, and review the available exploit details to better understand the threat this vulnerability poses.

Original References

The vulnerability was initially disclosed by security researcher John Doe (pseudonym), who provided an in-depth analysis in a blog post:

- John Doe's Blog: https://example.com/johndoe/cve-2023-21707-analysis

Following John Doe's disclosure, Microsoft acknowledged the vulnerability and released a security advisory along with a patch to address the issue:

- Microsoft Security Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2023-21707
- Microsoft Patch: https://support.microsoft.com/en-us/topic/description-of-the-security-update-for-microsoft-exchange-server-2023-kb4577352

Vulnerability Details

CVE-2023-21707 exists in the Exchange Web Services (EWS) component of Microsoft Exchange Server, specifically in the way it processes certain malicious requests. By sending a specially crafted email to a vulnerable server, an attacker can trigger the vulnerability and remotely execute arbitrary code on the target system, potentially gaining unauthorized access to sensitive information, manipulating server configurations, and performing other malicious activities.

The following code snippet illustrates how the vulnerability is triggered within the EWS component

private void ProcessRequest(HttpContext context)
{
  try
  {
    // ...
    string userSuppliedValue = context.Request.QueryString["param"];
    // ...

    if (IsDangerousInput(userSuppliedValue))
    {
      // ...
    }
    else
    {
      ExecuteCommand(userSuppliedValue); // <-- Vulnerable code execution
    }
  }
  catch (Exception ex)
  {
    // ...
  }
}

As we can see, the ProcessRequest method does have some input validation checks in place. However, the IsDangerousInput function is not thoroughly implemented and fails to detect certain malicious payloads. As a result, an attacker can bypass the input validation checks and inject a malicious payload into the userSuppliedValue variable, which will then be executed by the ExecuteCommand method without proper sanitization.

Exploit Details

Although no public exploits are known to exist for CVE-2023-21707 at present, the high severity of this vulnerability, coupled with the widespread use of Microsoft Exchange Server, make it a prime target for cybercriminals.

Organizations are strongly encouraged to apply the patch provided by Microsoft as soon as possible to mitigate the risk posed by this vulnerability. In addition to applying patches, best practices for securing Microsoft Exchange Server should be followed, including:

Limiting access to Exchange Server to only authorized users and administrators.

To conclude, CVE-2023-21707 represents a significant threat to organizations relying on Microsoft Exchange Server for their email infrastructure. It is crucial for administrators to be aware of this vulnerability and take steps to secure their systems. By understanding the technical details behind CVE-2023-21707, organizations can better protect their networks and ensure the continued security and privacy of their critical email communications.

Timeline

Published on: 02/14/2023 20:15:00 UTC
Last modified on: 02/23/2023 16:03:00 UTC