# Windows Win32k Zero-Day Exploited in Wild

Published: 2021-10-12 · Severity: high
Canonical: https://vorant.io/reports/add1921a-3433-5e2a-9660-6dd7bd42de87/windows-win32k-zero-day-exploited-in-wild

> Microsoft's October 2021 Patch Tuesday fixes CVE-2021-40449, a Win32k driver flaw already being exploited in the wild.

IPA (Japan's Information-technology Promotion Agency) issued an alert on October 13, 2021 regarding Microsoft's monthly security update, which addresses multiple vulnerabilities across Microsoft products. Among these, CVE-2021-40449, a Win32k elevation-of-privilege vulnerability, was confirmed by Microsoft to be actively exploited in the wild at the time of disclosure.

Exploitation of the vulnerabilities covered in this update could allow attackers to crash applications or gain control of affected systems, potentially leading to a range of damages. Due to the confirmed active exploitation of CVE-2021-40449, IPA urged users and administrators to apply Microsoft's patches urgently to prevent further damage, and referred users to Windows Update guidance for patch application.

## Mentioned in this report

- Vulnerabilities: CVE-2021-40449 (KEV)

## Detection guidance (public sample)

### Application Crash Referencing win32k.sys (Possible EoP Exploit Attempt)

ATT&CK: T1068

Detects Windows Error Reporting events where the faulting module is win32k.sys, which can indicate exploitation attempts against the Win32k kernel component such as CVE-2021-40449. Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.

```yaml
title: Application Crash Referencing win32k.sys
logsource:
  product: windows
  service: application
detection:
  selection:
    EventID: 1000
    Source: Application Error
    FaultingModule|endswith: win32k.sys
  condition: selection
falsepositives:
- Legitimate driver bugs or compatibility issues unrelated to exploitation
- Third-party software causing win32k faults through unrelated crashes
level: medium
tags:
- attack.t1068
id: 4ee47425-c307-5c0e-8c00-fdcfdc931ab9
status: experimental
author: Vorant
```

### Windows Kernel Bugcheck Referencing win32k.sys Driver

ATT&CK: T1068

Detects Windows kernel bugcheck (blue screen) events that reference win32k.sys as the failing module, which can occur during exploitation of Win32k elevation-of-privilege vulnerabilities. Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.

```yaml
title: Windows Kernel Bugcheck Referencing win32k.sys Driver
logsource:
  product: windows
  service: system
detection:
  selection:
    Provider_Name: Microsoft-Windows-WER-SystemErrorReporting
    EventID: 1001
  keyword_filter:
    Message|contains: win32k.sys
  condition: selection and keyword_filter
falsepositives:
- Non-malicious driver crashes or hardware-related kernel faults involving win32k.sys
level: medium
tags:
- attack.t1068
id: 581fade8-0cdd-5df1-b1d2-ddfddd9d69bd
status: experimental
author: Vorant
```

### Unexpected SYSTEM Integrity Process Spawned from Medium Integrity Parent

ATT&CK: T1068

Detects a process running at System integrity level being spawned from a parent process normally running at Medium integrity (e.g., user session shell or browser), consistent with successful privilege escalation via a Win32k exploit. Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.

```yaml
title: Unexpected SYSTEM Integrity Process Spawned from Medium Integrity Parent
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    IntegrityLevel: System
    ParentImage|endswith:
    - \\explorer.exe
    - \\iexplore.exe
    - \\chrome.exe
    - \\winword.exe
    - \\excel.exe
  filter:
    Image|endswith:
    - \\services.exe
    - \\svchost.exe
    - \\dllhost.exe
  condition: selection and not filter
falsepositives:
- Legitimate UAC elevation flows or scheduled tasks configured to run as SYSTEM from
  user-initiated actions
- Endpoint management agents that legitimately elevate from user shell context
level: medium
tags:
- attack.t1068
id: 5795cd40-4eb2-517b-a6ec-e8bd08b046e3
status: experimental
author: Vorant
```

Behavioural rules are generated from public reporting — validate in your environment before deploying.

Source reporting: https://www.ipa.go.jp/archive/security/security-alert/2021/20211013-ms.html

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