The cybersecurity world is once again raising alarms after the disclosure of a new critical vulnerability in Microsoft Office: CVE-2025-29791. This bug allows local attackers to execute malicious code by exploiting a *type confusion* error. In this post, we'll break down how the vulnerability works, show you code snippets illustrating the risk, share exploit techniques, and provide trusted references—all in straightforward, accessible language.

What is CVE-2025-29791?

CVE-2025-29791 is a local code execution flaw found in certain versions of Microsoft Office, caused by Microsoft Office's improper handling of different resource types—commonly referred to as a "type confusion" vulnerability. An attacker who successfully exploits this bug can run arbitrary code with the same privileges as the local user. This means sensitive files, system configurations, and personal data could all be at risk.

How Does the Type Confusion Bug Work?

Type confusion occurs when a program (like Microsoft Office) accesses data using an incorrect or mismatched type. In some cases, specially crafted Office files (.docx, .xlsx, .pptx) can be made to confuse internal resource management, leading to corruption or outright control over memory—a foothold for running malicious code.

Imagine this simple (and dangerous) code scenario inside a document parser

// Conceptual C++ example of type confusion
class Resource {
public:
    virtual void parse() = ;
};

class TextResource : public Resource {
public:
    void parse() override {
        // Parse text...
    }
};

class ImageResource : public Resource {
public:
    void parse() override {
        // Parse image...
    }
};

void processResource(Resource* res) {
    // Incorrectly assume 'res' is TextResource and cast
    TextResource* textRes = static_cast<TextResource*>(res);
    textRes->parse(); // If 'res' is really an ImageResource, this leads to confusion!
}

If an attacker can trick Office into treating an image as if it were a text block (or any other incompatible type), they might bypass protection checks and inject their own routines into the execution flow.

Exploiting CVE-2025-29791: Attacker’s Perspective

In practice, an attacker could deliver a *malformed Office document*—say, by email or cloud storage. When the unwitting user opens this file, Office processes a resource using the wrong type, jumping to malicious code embedded in the file.

Attackers often use *malformed properties streams* or *unusual embeddings*. For example

<!-- Malicious OOXML snippet embedded in document -->
<o:DocumentProperties>
  <o:ResourceType>Image</o:ResourceType>
  <o:MaliciousPayload>{shellcode_here}</o:MaliciousPayload>
</o:DocumentProperties>

This malformed resource tricks Office into executing code sections with insufficient validation.

Evasion of antivirus: Well-crafted files may evade detection until exploited.

- Potential lateral movement: If used in a corporate environment, an attacker could pivot and attack internal resources.

Microsoft’s Official Advisory

- Microsoft Security Advisory (June 2025)
- Patch release details: Microsoft Update Catalog

Proof-of-Concept (PoC) — For Research Use Only

Researchers have published PoC files demonstrating exploitation. For educational purposes, here’s a *simplified* version.

> _Warning: Running untrusted Office files is dangerous!_

# This does not exploit the real Office bug but shows conceptual attack in Python
# For actual PoC, see Mitre or Github links below

class MaliciousResource:
    def parse(self):
        print("Launching payload: your system is compromised!")

# Simulated type confusion: pass MaliciousResource where TextResource is expected
def open_office_file(resource):
    if isinstance(resource, MaliciousResource):
        resource.parse()

# Simulate opening a malicious file
malicious = MaliciousResource()
open_office_file(malicious)

Further Reading & References

- CVE Details: CVE-2025-29791 at Mitre
- Microsoft Office Security Updates
- NIST National Vulnerability Database entry (check for official metrics and descriptions)
- OWASP: Type Confusion

Final Thoughts

*CVE-2025-29791 is a textbook example of how seemingly small programming mistakes, like type confusion, can open the door to serious attacks. The best protection is rapid patching and safe computing habits. If you're an admin or end user, double-check that your Microsoft Office is up to date, and always treat unexpected files with suspicion.*

Stay safe! 🚨

*Exclusive coverage by [YourSecuritySource.com]. Use for awareness, not unauthorized research.*

Timeline

Published on: 04/08/2025 18:16:05 UTC
Last modified on: 06/04/2025 17:52:42 UTC