CVE-2023-36893 Overview
CVE-2023-36893 is a spoofing vulnerability affecting Microsoft Outlook and several Microsoft Office distributions. The flaw stems from improper input validation [CWE-20] in how Outlook processes specially crafted content. An attacker can exploit this issue over the network by convincing a user to interact with malicious content, leading to confidentiality impact through spoofed message presentation.
Microsoft published the advisory on August 8, 2023. The vulnerability requires user interaction but no authentication, making it suitable for phishing-style attack chains targeting Outlook users.
Critical Impact
Successful exploitation allows attackers to spoof email content in Microsoft Outlook, enabling convincing phishing campaigns that can compromise sensitive information disclosed by targeted users.
Affected Products
- Microsoft 365 Apps
- Microsoft Office 2019 and Office Long Term Servicing Channel 2021
- Microsoft Outlook 2013 SP1 (including RT) and Outlook 2016
Discovery Timeline
- 2023-08-08 - CVE-2023-36893 published to NVD with Microsoft advisory
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-36893
Vulnerability Analysis
The vulnerability is classified as a spoofing flaw in Microsoft Outlook. Microsoft tracks the root weakness as improper input validation [CWE-20]. Outlook fails to properly validate or sanitize attacker-controlled content before rendering it within the client interface. This allows a remote attacker to present messages or content that appears to originate from a trusted source.
The attack vector is network-based and complexity is low. Exploitation requires user interaction, meaning the victim must open or preview a crafted message or file. No prior authentication is needed on the target system. Successful exploitation impacts confidentiality, as the attacker can deceive the user into disclosing sensitive information such as NTLM hashes or credentials in follow-on actions.
The EPSS probability is 0.859% with a percentile of 75.187, indicating moderate predicted exploitation likelihood relative to other CVEs. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog at this time.
Root Cause
The root cause is improper input validation within Outlook's message processing logic. Outlook does not sufficiently verify the integrity or authenticity of certain message attributes, allowing crafted input to misrepresent sender identity or message content to the recipient.
Attack Vector
An attacker delivers a specially crafted email or attachment to a victim using a vulnerable Outlook client. When the user opens or previews the content, Outlook renders the spoofed elements without proper validation. The attacker can use this to impersonate trusted senders, bypass visual trust indicators, or coerce the client into leaking authentication material to attacker-controlled infrastructure.
No verified public proof-of-concept code is available. Refer to the Microsoft Security Response Center advisory for technical specifics released by the vendor.
Detection Methods for CVE-2023-36893
Indicators of Compromise
- Inbound emails containing unusual MAPI properties, malformed message headers, or attachments referencing remote SMB or WebDAV resources.
- Outbound SMB or HTTP authentication attempts from Outlook processes (outlook.exe) toward untrusted external hosts.
- User reports of messages that visually appear to originate from internal senders but fail SPF, DKIM, or DMARC validation at the mail gateway.
Detection Strategies
- Inspect mail flow at the gateway for messages with crafted headers or properties known to trigger Outlook rendering anomalies.
- Correlate Outlook process telemetry with outbound network connections to non-corporate hosts on ports 445, 139, and 80/443.
- Hunt for child processes spawned by outlook.exe immediately after message preview or open events.
Monitoring Recommendations
- Enable enhanced auditing on mail servers and gateways to capture full message headers for forensic review.
- Monitor endpoint authentication telemetry for outbound NTLM authentication initiated from Office applications.
- Track installed Office build numbers across the fleet to identify endpoints missing the August 2023 security update.
How to Mitigate CVE-2023-36893
Immediate Actions Required
- Apply the August 2023 Microsoft security updates to all affected Outlook, Office 2019, Office LTSC 2021, and Microsoft 365 Apps installations.
- Block outbound SMB (TCP 445) and NTLM authentication to untrusted external destinations at the network perimeter.
- Enforce SPF, DKIM, and DMARC policies on inbound mail and quarantine messages that fail validation.
Patch Information
Microsoft released fixes for CVE-2023-36893 as part of the August 2023 Patch Tuesday cycle. Administrators should consult the Microsoft CVE-2023-36893 Update guidance for the specific KB articles and build numbers that apply to each supported product channel. Deploy updates through Microsoft Update, WSUS, Intune, or Click-to-Run servicing channels.
Workarounds
- Disable automatic message preview in Outlook to reduce exposure when users browse mailboxes containing untrusted messages.
- Configure Outlook to read all standard mail in plain text format until patches are applied.
- Restrict NTLM outbound authentication using Group Policy to limit credential exposure if a spoofed message attempts coerced authentication.
# Group Policy registry setting to restrict outgoing NTLM traffic to remote servers
# Path: HKLM\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
reg add "HKLM\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0" /v RestrictSendingNTLMTraffic /t REG_DWORD /d 2 /f
# Force Outlook to read messages in plain text (HKCU)
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Options\Mail" /v ReadAsPlain /t REG_DWORD /d 1 /f
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
