CVE-2026-88109 Overview
CVE-2026-88109 is a use-after-free vulnerability [CWE-416] in Microsoft Office Click-To-Run. The flaw allows an authorized local attacker to elevate privileges on an affected system. An attacker who successfully exploits this vulnerability can gain higher privileges, leading to full compromise of confidentiality, integrity, and availability on the target host.
Microsoft published the advisory on October 13, 2026. The vulnerability requires local access and low privileges, but no user interaction. No public exploit is currently available, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.
Critical Impact
Successful exploitation permits a low-privileged local user to elevate privileges, potentially executing code in the context of a higher-privileged account on hosts running vulnerable Office Click-To-Run components.
Affected Products
- Microsoft Office Click-To-Run (specific build range not published in the NVD entry)
- Refer to the Microsoft Security Update CVE-2026-88109 advisory for the authoritative list of affected channels and builds
Discovery Timeline
- 2026-10-13 - CVE-2026-88109 published to the National Vulnerability Database
- 2026-10-13 - Last updated in NVD database
Technical Details for CVE-2026-88109
Vulnerability Analysis
The vulnerability is a use-after-free condition in Microsoft Office Click-To-Run, the installer and update service that provisions Office applications on Windows. A use-after-free occurs when a program continues to reference memory after it has been released back to the allocator. An attacker who can influence allocation and free sequences can reclaim the freed region with controlled data.
In this case, an authorized local attacker triggers the condition against the Click-To-Run service or associated components. Because Click-To-Run runs with elevated privileges to manage Office installation state, successful exploitation yields local privilege escalation. The attack vector is local, exploit complexity is low, and no user interaction is required.
The impact scope covers all three security properties. An attacker gaining SYSTEM-level execution through Click-To-Run can install persistent malware, disable endpoint controls, and pivot laterally.
Root Cause
The root cause is classified under [CWE-416] Use After Free. The affected component retains a dangling pointer to an object after that object has been freed. Subsequent operations dereference the stale pointer, allowing an attacker who can replace the freed allocation with attacker-controlled data to influence program execution flow.
Attack Vector
An attacker requires a valid local account on the target workstation to interact with the Click-To-Run service. The attacker triggers the vulnerable code path, races the free and reallocation operations, and places controlled data in the reclaimed memory region. When the stale pointer is dereferenced, execution is redirected under the elevated service context. Microsoft has not published proof-of-concept code, and no exploit is known to exist publicly.
Technical details beyond the advisory have not been released. Refer to the Microsoft Security Update CVE-2026-88109 advisory for vendor guidance.
Detection Methods for CVE-2026-88109
Indicators of Compromise
- No vendor-published indicators of compromise are available for CVE-2026-88109 at this time
- Unexpected child processes spawned by OfficeClickToRun.exe or related Click-To-Run binaries
- Abnormal crashes or exception events in Click-To-Run service logs that may indicate exploitation attempts
Detection Strategies
- Monitor process creation events where OfficeClickToRun.exe is the parent of unexpected processes such as cmd.exe, powershell.exe, or rundll32.exe
- Alert on token manipulation and privilege-escalation telemetry originating from Office-related service accounts
- Review Windows Error Reporting data for repeated access-violation faults in Click-To-Run modules, which can precede successful exploitation attempts
Monitoring Recommendations
- Enable detailed process and command-line auditing on endpoints running Microsoft Office
- Forward Click-To-Run service logs and Sysmon Event ID 1 (process creation) and Event ID 10 (process access) to a central analytics platform
- Baseline normal Click-To-Run update behavior to make deviations easier to flag
How to Mitigate CVE-2026-88109
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-88109 advisory as soon as it is available in your deployment channel
- Inventory endpoints running Office Click-To-Run and confirm they receive updates from the Current, Monthly Enterprise, or Semi-Annual channels as applicable
- Restrict local account creation and enforce least privilege on workstations to reduce the pool of users who can trigger the local attack vector
Patch Information
Microsoft addresses this vulnerability through the standard Click-To-Run update mechanism. Office installations configured for automatic updates receive the fix without administrator action once the update propagates to the organization's channel. Confirm patch deployment by validating Office build numbers against the versions documented in the Microsoft Security Update CVE-2026-88109 advisory.
Workarounds
- Microsoft has not published a specific workaround for CVE-2026-88109; patch deployment is the recommended remediation
- Limit local logon rights and remove standard users from systems that host sensitive data until the patch is applied
- Apply application control policies to prevent unauthorized binaries from executing under the Click-To-Run service context
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.