CVE-2023-36763 Overview
CVE-2023-36763 is an information disclosure vulnerability affecting Microsoft Outlook and related Microsoft 365 productivity applications. The flaw allows a remote, unauthenticated attacker to access confidential information from a targeted Outlook installation over the network without user interaction. Microsoft categorizes the issue under [CWE-200] Information Exposure, indicating that sensitive data is exposed to actors who should not have access.
The vulnerability affects Microsoft Outlook 2016, Microsoft Office 2019, Microsoft Office LTSC 2021, and Microsoft 365 Apps for Enterprise across both x86 and x64 builds. The EPSS score of 1.28% places this issue in the 79th percentile for exploitation likelihood.
Critical Impact
Remote, unauthenticated attackers can disclose confidential information from Outlook clients across multiple supported Microsoft Office versions without user interaction.
Affected Products
- Microsoft Outlook 2016 (x86, x64)
- Microsoft Office 2019 and Microsoft Office LTSC 2021 (x86, x64)
- Microsoft 365 Apps for Enterprise (x86, x64)
Discovery Timeline
- 2023-09-12 - CVE-2023-36763 published to NVD as part of Microsoft's September 2023 Patch Tuesday release
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-36763
Vulnerability Analysis
The vulnerability resides in Microsoft Outlook's processing of crafted content delivered over the network. An attacker who successfully exploits the issue can read sensitive information from the targeted Outlook session that should otherwise remain protected. According to Microsoft's advisory, exploitation requires no privileges and no user interaction, which expands the population of attackable clients to any reachable Outlook installation.
The weakness is classified under [CWE-200] Exposure of Sensitive Information to an Unauthorized Actor. Microsoft has not published code-level technical details, and no public proof-of-concept code is available at this time.
Root Cause
The root cause is an information exposure weakness in Outlook's handling of certain message or attachment content. Outlook releases data that the application should restrict, allowing an attacker to harvest confidential content from the client. Microsoft's advisory tracks the defect to the affected Office binaries shared across Outlook 2016, Office 2019, Office LTSC 2021, and Microsoft 365 Apps.
Attack Vector
The attack vector is network-based. An attacker delivers a specially crafted email or content payload to an Outlook user. Successful exploitation discloses confidential information from the client to the attacker. Because the attack complexity is low and no authentication or user interaction is required, the vulnerability can be triggered as soon as Outlook processes the malicious content.
No verified public exploitation code is available. Refer to the Microsoft Security Update Guide for CVE-2023-36763 for vendor-confirmed technical context.
Detection Methods for CVE-2023-36763
Indicators of Compromise
- Inbound emails containing crafted attachments or message bodies that target Outlook parsers, especially from untrusted external senders.
- Outlook processes establishing unexpected outbound network connections shortly after rendering a new message.
- Anomalous reads of Outlook data stores (.ost, .pst) or temporary attachment paths under %LOCALAPPDATA%\Microsoft\Outlook correlated with email delivery events.
Detection Strategies
- Inspect email gateways for messages exploiting Outlook content parsers and quarantine attachments with suspicious MIME or OLE structures.
- Correlate Outlook telemetry with endpoint EDR data to identify the OUTLOOK.EXE process performing unusual file reads or network egress.
- Hunt for Office client versions that predate the September 2023 cumulative security updates across the fleet.
Monitoring Recommendations
- Enable command-line, process, and network telemetry collection on workstations running Microsoft Office.
- Alert on Outlook spawning unexpected child processes or transmitting data to non-Microsoft domains immediately after message receipt.
- Track patch compliance for Microsoft 365 Apps, Office 2019, Office LTSC 2021, and Outlook 2016 builds against Microsoft's September 2023 baseline.
How to Mitigate CVE-2023-36763
Immediate Actions Required
- Apply the September 2023 Microsoft security updates to all affected Outlook, Office, and Microsoft 365 Apps installations.
- Inventory endpoints to confirm Office build numbers meet or exceed the patched versions listed in the Microsoft advisory.
- Increase mail filtering rigor for external senders until patch deployment is confirmed across the environment.
Patch Information
Microsoft has released security updates that remediate CVE-2023-36763 across all affected products. Patch metadata, KB identifiers, and download links are published in the Microsoft Security Update Guide for CVE-2023-36763. Administrators should deploy updates through Microsoft Update, WSUS, Microsoft Intune, or Configuration Manager according to their standard patch workflow.
Workarounds
- Read incoming mail in plain text mode to reduce parsing of rich content while patches are deployed.
- Block or sandbox emails containing complex attachments at the secure email gateway.
- Restrict Outlook outbound network access through host firewall rules to internal mail infrastructure only, limiting potential data exfiltration paths.
# Example: query installed Office build to verify patch level
reg query "HKLM\SOFTWARE\Microsoft\Office\ClickToRun\Configuration" /v VersionToReport
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
