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A new security flaw, CVE-2025-29792, has been reported in Microsoft Office. This vulnerability, involving a Use-After-Free issue, could let a local, authorized attacker elevate their privileges on the affected system. In this post, we'll break down what this vulnerability means, how it works, and even walk through a simple exploit example.

What is a Use-After-Free?

Use-After-Free (UAF) bugs happen when an application keeps using a piece of memory after it's been freed. This can let attackers execute their own code by manipulating the memory before the application uses it again.

Impact: Privilege Escalation (local)

- Pre-requisite: Attacker must have local access and be authorized (not guest/anonymous)

Status: Patch pending

When an attacker successfully exploits this, they can gain higher privileges—possibly SYSTEM rights—by triggering Office to use a freed memory chunk in a malicious way.

How Does Exploitation Work?

Through a crafted document or add-in, an attacker could cause Office to free memory, then trick the program into using that same memory for malicious code. A simple scenario:

Pseudo Code Example

// Simplified example of UAF
void *obj = allocate_object();
free(obj);           // Object memory is freed
malicious_fill(obj); // Attacker fills freed memory
use(obj);            // Office uses the object again (now attacker controlled)

Attackers can use this to point the program's control flow towards their own payload.

Step 1: Craft a Malicious Office Add-in

Attackers may deliver a custom Office add-in that performs quick alloc/free operations to replace the memory content.

Step 2: Force Use After Free

Using Office automation, they cause Office to reference the freed object, which they now control.

Step 3: Escalate Privileges

By overwriting key memory pointers, the attacker gets code to run as a higher-privilege process.

Example Snippet in Python (using pywin32 for demo)

import win32com.client

office_app = win32com.client.Dispatch("Excel.Application")
workbook = office_app.Workbooks.Add()

# Simulating UAF by closing and re-opening objects
chart = workbook.Charts.Add()
del chart  # free chart object

# Attacker code triggers reallocation
# This part is symbolic, actual exploitation would require low-level memory access

# If Office references 'chart' again, it could execute attacker code in privileged context

Original References

- _Microsoft Security Advisory portal_ (pending, placeholder link)
- Microsoft Office Security Best Practices

Closing Thoughts

CVE-2025-29792 is an important reminder that even "simple" local bugs can lead to serious privilege escalation. If you're a sysadmin or security enthusiast, stay tuned for official patches, and always exercise defense-in-depth.

Have you encountered Office oddities or suspect exploitation? Share your story in the comments.

Timeline

Published on: 04/08/2025 18:16:05 UTC
Last modified on: 05/06/2025 17:03:09 UTC