CVE-2023-35636: Microsoft Outlook Info Disclosure Flaw

CVE-2023-35636 Overview

CVE-2023-35636 is an information disclosure vulnerability affecting Microsoft Outlook and related Microsoft Office products. This vulnerability allows attackers to potentially extract sensitive information from affected systems through specially crafted content that exploits improper handling of data exposure mechanisms in Microsoft Outlook.

Critical Impact

Attackers can leverage this vulnerability to disclose sensitive information, potentially including NTLM credential hashes, which could be used for further attacks such as pass-the-hash or relay attacks against enterprise environments.

Affected Products

• Microsoft 365 Apps for Enterprise
• Microsoft Office 2016
• Microsoft Office 2019
• Microsoft Office Long Term Servicing Channel 2021

Discovery Timeline

• December 12, 2023 - CVE-2023-35636 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-35636

Vulnerability Analysis

This information disclosure vulnerability in Microsoft Outlook stems from improper handling of certain data exposure scenarios (CWE-200). When exploited, the vulnerability allows an attacker to extract sensitive information from the target system without requiring any special privileges on the affected system. The attack requires user interaction, meaning the victim must engage with malicious content for the exploit to succeed.

The vulnerability is particularly concerning for enterprise environments where Microsoft 365 Apps and Office suite products are widely deployed. Successful exploitation could lead to the disclosure of sensitive authentication information, which attackers can leverage for lateral movement or further compromise within the organization's network.

Root Cause

The root cause of CVE-2023-35636 lies in the improper handling of information exposure within Microsoft Outlook. The vulnerability is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor), indicating that the application fails to properly restrict access to sensitive data under certain conditions. This allows attackers to craft specific content that, when processed by Outlook, can leak confidential information to unauthorized parties.

Attack Vector

The attack vector for CVE-2023-35636 is network-based and requires user interaction. An attacker must convince a user to interact with specially crafted content, such as opening a malicious email or clicking on a link. The low attack complexity means that once user interaction is obtained, the vulnerability is relatively straightforward to exploit. The impact is focused on confidentiality, potentially exposing sensitive authentication credentials or other private data stored or processed by Outlook.

The exploitation typically involves:

1. Crafting malicious content designed to trigger the information disclosure
2. Delivering the payload to the target via email or other means
3. Waiting for user interaction with the malicious content
4. Capturing the disclosed information through attacker-controlled infrastructure

Detection Methods for CVE-2023-35636

Indicators of Compromise

• Unusual outbound network connections from Microsoft Outlook processes to external IP addresses
• Abnormal SMB traffic originating from workstations, particularly to external destinations
• Evidence of NTLM authentication attempts to unknown or suspicious domains
• Outlook process making unexpected network requests to non-Microsoft servers

Detection Strategies

• Monitor for outbound SMB connections from Outlook processes that deviate from normal baseline behavior
• Implement network traffic analysis to detect potential credential relay attempts
• Configure endpoint detection to alert on suspicious Outlook behaviors or unexpected child processes
• Enable detailed logging for authentication events to track potential credential harvesting attempts

Monitoring Recommendations

• Deploy network monitoring solutions to detect unusual outbound connections from Microsoft Office applications
• Implement SIEM rules to correlate Outlook-related network activity with authentication events
• Monitor for signs of credential harvesting or NTLM relay attacks within the environment
• Review email security gateway logs for suspicious attachments or links targeting this vulnerability

How to Mitigate CVE-2023-35636

Immediate Actions Required

• Apply the latest Microsoft security updates for all affected Office products immediately
• Ensure Microsoft 365 Apps are configured to receive automatic updates
• Review and restrict outbound SMB traffic where possible to limit credential exposure
• Educate users about the risks of interacting with suspicious emails or attachments

Patch Information

Microsoft has released security patches addressing CVE-2023-35636 as part of their regular update cycle. Organizations should consult the Microsoft Security Response Center Advisory for detailed patch information and apply updates to all affected products including Microsoft 365 Apps for Enterprise, Office 2016, Office 2019, and Office LTSC 2021.

Workarounds

• Block outbound SMB traffic (TCP port 445) at the network perimeter to prevent credential leakage to external attackers
• Implement network segmentation to limit the exposure of systems running vulnerable Office versions
• Consider disabling NTLM authentication where possible in favor of Kerberos
• Deploy email filtering solutions to block potential exploit delivery mechanisms

# Block outbound SMB traffic to external networks (Windows Firewall example)
netsh advfirewall firewall add rule name="Block Outbound SMB" dir=out action=block protocol=tcp remoteport=445 profile=any