CVE-2023-21715 - Unmasking the Microsoft Publisher Security Features Bypass Vulnerability

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Introduction

Security flaws in popular software can leave millions of users exposed to cyberattacks. In early 2023, Microsoft reported a notable vulnerability in Microsoft Publisher, a component of the Office suite that's often used by businesses and home users to design publications. This flaw, tracked as CVE-2023-21715, allows attackers to bypass security features and potentially run malicious code with minimal user interaction.

In this long read, I’ll break down exactly what went wrong, provide code snippets and technical details, and show you how attackers might exploit this vulnerability. I’ll also link you to the official references for further reading. Let’s dive right in.

Understanding CVE-2023-21715

CVE-2023-21715 concerns a vulnerability in Microsoft Publisher (a graphical design application shipped with Office), in which certain macro security policies can be bypassed. Microsoft Publisher, like other Office apps, supports macros—scripts that can automate tasks. Because macros are a common source of malware, Office apps have multiple security features to block macros in downloaded or untrusted files.

The Problem

In this vulnerability, an attacker can craft a malicious Publisher file (.pub), trick a user into opening it, and then circumvent security features—specifically, macro policies that are designed to prevent scripts from executing without user approval.

Microsoft's official advisory: Microsoft Security Response Center – CVE-2023-21715

> “An attacker who successfully exploited this vulnerability could bypass Office macro policies used to block untrusted or malicious files.”

How Does the Exploit Work?

The heart of the problem is that Microsoft Publisher does not handle macro block policies the same as other Office apps. When a file is downloaded from the internet, Windows adds a ‘Mark-of-the-Web’ (MOTW) attribute. Office apps use this to warn users or block macros. However, Publisher failed to properly enforce these restrictions.

The attacker gets a victim to download this file (via phishing, for example).

3. Victim opens file in Publisher. Due to the flawed policy handling, macros may run without prompt—even though its origin is the internet.

Example Proof-of-Concept (PoC) Snippet

> Note: This is a hypothetical PoC for educational purposes only.

Suppose you have a malicious Visual Basic for Applications (VBA) macro you want to include

Sub AutoOpen()
    MsgBox "This is a test—malicious code would execute here!"
    Shell "cmd.exe /c calc.exe" ' For demonstration, launches Calculator
End Sub

Normal Office would block this if MOTW is present.

Exploitation Details

Here’s an overview of how an attacker might prepare the exploit.

Due to the vulnerability, macros execute without prompt.

Because Publisher is not as commonly scrutinized as Word or Excel, many organizations overlook it in their macro policies, increasing exposure.

Real-World Risk

While Publisher isn't exploited as frequently as Word or Excel, it's included in many business Office installations. For targets who use Publisher (such as marketing, design, or admin staff), this vulnerability is a powerful tool for attackers to initially compromise a system and gain a foothold in a corporate network.

> It’s a classic example of attackers seeking the "weakest link"—Publisher's macro enforcement is weaker than its Office siblings.

Mitigation and Patch

Microsoft released a patch for this vulnerability on February 14, 2023 as part of Patch Tuesday. All users are strongly advised to update Office to the latest version.

References

- Microsoft Advisory: CVE-2023-21715
- CERT/CC Vulnerability Note VU#451632

Conclusion

CVE-2023-21715 is an important reminder that security controls must be consistent across all software components in a suite as widely used as Microsoft Office. Attackers are always looking for under-the-radar entry points, and Publisher’s inconsistent macro handling provided exactly that.

Patch your systems, educate your users, and keep a watchful eye on less commonly used apps—because that's sometimes where attackers quietly slip through.

For more detailed info

- Microsoft Original Advisory
- Mitre CVE Entry

Timeline

Published on: 02/14/2023 20:15:00 UTC
Last modified on: 02/23/2023 15:44:00 UTC