CVE-2023-29343 Overview
CVE-2023-29343 is an elevation of privilege vulnerability in Microsoft SysInternals Sysmon for Windows. Sysmon is a widely deployed system monitoring service used by defenders to log process creation, network connections, and file system activity. A local, authenticated attacker can abuse a link-following weakness ([CWE-59]) to escalate privileges on the host. Successful exploitation grants high impact to confidentiality, integrity, and availability. The vulnerability has an EPSS percentile of 95.255, indicating elevated likelihood of exploitation relative to other CVEs. Microsoft addressed the issue through an updated Sysmon release published via the Microsoft Security Response Center.
Critical Impact
A local attacker with low privileges can leverage improper link resolution in Sysmon to gain elevated privileges on affected Windows systems.
Affected Products
- Microsoft SysInternals Sysmon for Windows (versions prior to the May 2023 fix)
- Windows endpoints running vulnerable Sysmon installations
- Windows servers with Sysmon deployed for security telemetry
Discovery Timeline
- 2023-05-09 - CVE-2023-29343 published to the National Vulnerability Database
- 2023-05-09 - Microsoft publishes security update guidance for CVE-2023-29343
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-29343
Vulnerability Analysis
The vulnerability is classified under [CWE-59] (Improper Link Resolution Before File Access, also known as link following). Sysmon runs as a privileged Windows service and writes telemetry, configuration, and archive data to file system locations. When the service performs file operations without correctly validating whether a target path resolves through a symbolic link, junction, or hard link, a low-privileged local user can redirect those operations to attacker-chosen locations. The result is that a SYSTEM-level service performs file writes or modifications on behalf of an unprivileged user.
Root Cause
The root cause is improper link resolution before file access in Sysmon's privileged file handling routines. The service trusts file paths under user-influenced directories without checking for reparse points. An attacker who can create or replace directory entries used by Sysmon can plant a junction or symlink that redirects privileged I/O. This pattern is a recurring source of local privilege escalation in Windows services that operate on user-writable paths.
Attack Vector
The attack vector is local and requires low privileges with no user interaction. An attacker first obtains code execution as a standard user. They then prepare a directory structure containing a crafted reparse point that points to a SYSTEM-owned resource. When Sysmon performs a file operation that follows the link, the privileged service writes to or modifies the target the attacker selected. This can be chained into arbitrary file write primitives that yield SYSTEM-level code execution. No public proof-of-concept exploit is listed in the enriched data, and the vulnerability is not present in the CISA Known Exploited Vulnerabilities catalog. Refer to the Microsoft Security Update for CVE-2023-29343 for vendor technical details.
Detection Methods for CVE-2023-29343
Indicators of Compromise
- Creation of junctions, symbolic links, or hard links in directories accessed by the Sysmon service by non-administrative users.
- Unexpected file writes by the Sysmon process (Sysmon.exe or Sysmon64.exe) to paths outside its normal working directories.
- Sysmon service crashes, restarts, or configuration changes that do not correspond to administrative activity.
Detection Strategies
- Monitor for use of mklink, CreateSymbolicLink, or NTFS reparse point APIs originating from non-administrative user contexts.
- Audit file system activity in directories used by Sysmon for archives, configuration, and temporary state.
- Correlate local logon events with subsequent privilege changes or process creations under SYSTEM in close temporal proximity.
Monitoring Recommendations
- Enable Windows Object Access auditing on Sysmon-related directories and review for anomalous handle requests.
- Track installed Sysmon versions across the fleet and flag hosts running builds prior to the May 2023 patch.
- Alert on unexpected modifications to files owned by SYSTEM that are performed shortly after a standard user creates a reparse point.
How to Mitigate CVE-2023-29343
Immediate Actions Required
- Upgrade Sysmon to the version released alongside the May 2023 Microsoft Security Update for CVE-2023-29343 on all Windows endpoints and servers.
- Inventory hosts running Sysmon and prioritize patching systems where standard users have interactive logon rights.
- Restrict local interactive access on servers where Sysmon is deployed to reduce the population of accounts able to stage the attack.
Patch Information
Microsoft published the fix through the Microsoft Security Response Center. Administrators should download the updated Sysmon binary from the official SysInternals distribution channel and replace the deployed version. See the Microsoft Security Update for CVE-2023-29343 for version details and download links. After upgrading, validate the running service version with sysmon.exe -? | findstr /i version or by checking the file properties of the deployed binary.
Workarounds
- Where patching is delayed, remove or disable Sysmon on hosts that do not require it until the updated version can be deployed.
- Restrict the ability of standard users to create symbolic links by reviewing the SeCreateSymbolicLinkPrivilege assignment in local security policy.
- Apply strict NTFS permissions on directories used by Sysmon so that only administrators and SYSTEM can create or modify entries within them.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
