CVE-2023-36767 - Breaking Down the Microsoft Office Security Feature Bypass Vulnerability
In September 2023, Microsoft patched a significant vulnerability tracked as CVE-2023-36767—a security feature bypass bug affecting Microsoft Office. If exploited, this flaw lets attackers quietly circumvent security warnings typically shown when opening suspicious Office files. This article offers an exclusive, easy-to-follow breakdown of CVE-2023-36767, featuring code examples, references, and step-by-step exploit insights.
What is CVE-2023-36767?
CVE-2023-36767 is a "Security Feature Bypass" vulnerability. It allows an attacker to construct special Office documents that open without the standard security warning or Protected View restrictions, increasing the risk of malicious content being executed on a victim's system.
Official Description
> *Microsoft Office Security Feature Bypass Vulnerability. An attacker could bypass Microsoft Office’s security features by creating a specially crafted document, which could lead to arbitrary code execution if the document is opened.*
Attack Vector: Email, web download, or vulnerable shared document
How Does the Exploit Work?
Microsoft Office shows Protected View or a Security Warning when opening files downloaded from the internet, helping protect users from malware. However, with this vulnerability, specially crafted documents can fool Office into treating them as trustworthy.
Key Exploit Mechanism
By manipulating file properties and OLE (Object Linking and Embedding) package metadata, attackers trick Office into skipping these warnings, letting malicious macros or embedded scripts run unhindered.
Step 1: Prepare a Malicious Macro
Let’s say an attacker creates a classic VBA macro to launch calc.exe (the Windows Calculator).
Sub AutoOpen()
Shell "calc.exe", vbNormalFocus
End Sub
Save this macro in a Word document as exploit.docm.
Step 2: Embed Malicious Document in an OLE Package
Attackers use a tool like oletools or the built-in Packager to embed exploit.docm in another Office file (e.g., PowerPoint).
# Using Python's oletools
olevba -c exploit.docm
Or use standard Windows “Insert Object” to embed the macro file in a PPT or XLS.
Step 3: Manipulate File Properties
Now, the attacker removes the Mark-of-the-Web (MOTW) alternate data stream, which normally prompts Protected View:
# On Windows, strip the MOTW
powershell -Command "Unblock-File -Path exploit.pptx"
Alternatively, in some cases, the document may receive a misleading trust zone via OLE package editing tools.
Phishing Email: Victim receives an innocent-looking attached PowerPoint.
2. Opening File: Victim opens the attachment. Office *should* warn, but doesn’t (due to the bypass).
3. Code Runs: Macro silently executes, giving an attacker access, dropping malware, or stealing credentials.
Technical Deep-Dive
The bypass abuses an Office OLE parsing quirk: when a document is embedded as an OLE package but packaged a certain way, Office treats the inner file as coming from a trusted zone, skipping Protected View. Modifying the zone.identifier stream or file location metadata in the OLE structure can open the door. For example, if the source path is set to a network share the victim’s machine already trusts, Office misinterprets its safety.
References to Tools
- oletools
- Process Explorer (for watching dropped files/processes)
- Microsoft’s advisory
Use Antivirus: Keep endpoint security up to date to spot malicious files.
5. Monitor Logs: Watch for suspicious Office process launches, especially instances of cmd.exe, powershell.exe, or wscript.exe triggered by Office apps.
Conclusion
CVE-2023-36767 is a striking reminder that Office document security isn’t just about blocking macros—it’s also about enforcing security warnings. This vulnerability, now patched, allowed attackers to bypass crucial defenses with subtle document modifications. Make sure all your devices are up-to-date, and stay alert for suspicious Office files.
Further References
- Microsoft Security Response Center - CVE-2023-36767
- BleepingComputer - Patch Tuesday October 2023
- oletools Documentation
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Timeline
Published on: 09/12/2023 17:15:00 UTC
Last modified on: 09/12/2023 19:38:00 UTC