CVE-2021-33739
Microsoft Desktop Window Manager (DWM) Core Library Privilege Escalation Vulnerability
Description
CVE-2021-33739 is a HIGH vulnerability affecting Microsoft Windows, carrying a CVSS 3.1 score of 8.4. Microsoft Desktop Window Manager (DWM) Core Library contains an unspecified vulnerability that allows for privilege escalation. The NVD describes this issue as: Microsoft DWM Core Library Elevation of Privilege Vulnerability. Affected products include Microsoft Windows 10 1909, Microsoft Windows 10 2004, Microsoft Windows 10 20h2, Microsoft Windows 10 21h1, Microsoft Windows Server 2004, and 1 more. This CVE is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of 2021-11-17. With an EPSS score of 0.19488 (95.30th percentile), this vulnerability demonstrates significant real-world exploitation activity and should be prioritized for immediate remediation.
KEV Information
CVSS Score
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| microsoft | windows 10 1909 | < 10.0.18363.1621 |
| microsoft | windows 10 2004 | < 10.0.19041.1052 |
| microsoft | windows 10 20h2 | < 10.0.19042.1052 |
| microsoft | windows 10 21h1 | < 10.0.19043.1052 |
| microsoft | windows server 2004 | < 10.0.19041.1052 |
| microsoft | windows server 20h2 | < 10.0.19042.1052 |
Multiple CVSS Assessments
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
References
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33739(Patch, Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33739(US Government Resource)
Weakness Type
Unspecified Weakness Type
CVE-2021-33739 does not have a specific CWE classification assigned in the NVD database. The vulnerability affects Microsoft Windows and microsoft desktop window manager (dwm) core library contains an unspecified vulnerability that allows for privilege escalation. While the exact weakness type is not formally categorized, the vulnerability's characteristics — local attack vector, low complexity, and high severity — indicate a significant security flaw that enables privilege escalation, allowing attackers to gain unauthorized elevated access to the system.
The absence of a specific CWE classification does not diminish the severity of this vulnerability. Organizations should treat it with the same urgency as any actively exploited vulnerability listed in CISA's KEV catalog.
Impact Analysis
CVE-2021-33739 carries a CVSS 3.1 score of 8.4 (HIGH) with Unchanged Scope.
Confidentiality (HIGH): Successful exploitation grants the attacker extensive access to sensitive data processed by Microsoft Windows, including system files, credentials, and potentially data from connected systems.
Integrity (HIGH): Attackers can modify critical system files, install backdoors, alter configurations, or deploy malware on affected systems running Microsoft Windows.
Availability (HIGH): Complete disruption of the affected service or system is possible, including denial of service, system crashes, or rendering Microsoft Windows inoperable.
Scope Unchanged: The vulnerability's scope is Unchanged (U), meaning exploitation is contained within the vulnerable component. The impact, while significant, is limited to the Microsoft Windows environment itself.
With an EPSS score of 0.19488 (95.30th percentile), this vulnerability ranks among the most likely to be exploited in real-world attacks, underscoring the urgency of remediation.
Exploit Maturity
CVE-2021-33739 has confirmed active exploitation in the wild and is listed in CISA's Known Exploited Vulnerabilities catalog.
Exploit status: This vulnerability has been actively exploited, as confirmed by its inclusion in the KEV catalog. The EPSS score of 0.19488 (95.30th percentile) places it among the most exploited vulnerabilities tracked.
Ransomware association: As of the latest KEV data, no direct ransomware association has been confirmed for CVE-2021-33739. However, the confirmed exploitation in the wild means threat actors are actively using this vulnerability in attacks.
Attack surface: The local attack vector requires the attacker to have some form of access to the target system. While this limits the remote attack surface, it is commonly exploited in post-compromise scenarios where attackers have gained initial access through other means (phishing, other vulnerabilities) and then use CVE-2021-33739 for privilege escalation.
KEV deadline: CISA required federal agencies to remediate this vulnerability by 2021-11-17. All organizations should treat this deadline as a strong recommendation for their own remediation timelines.
Remediation
- Apply vendor patches immediately. Apply updates per vendor instructions. Consult the vendor advisory at portal.msrc.microsoft.com for specific patch guidance.
- Verify affected product versions in your environment. Identify all instances of Microsoft Windows in your infrastructure. Affected products include Microsoft Windows 10 1909, Microsoft Windows 10 2004, Microsoft Windows 10 20h2, Microsoft Windows 10 21h1, Microsoft Windows Server 2004, and 1 more. Use asset inventory and vulnerability scanning tools to ensure no instances are missed.
- Implement interim mitigations if patching is delayed. If immediate patching is not feasible, apply network-level controls such as restricting access to the affected component, enabling enhanced logging, and monitoring for indicators of compromise. Limit local user access and enforce principle of least privilege to reduce exploitation risk.
- Scan for signs of prior exploitation. Given the confirmed active exploitation of this vulnerability, review system logs and security monitoring data for evidence of compromise. Look for unexpected privilege escalation events, unauthorized administrative actions, or suspicious process creation.
- Update detection signatures and monitoring rules. Ensure intrusion detection and prevention systems, endpoint detection tools, and SIEM rules are updated to detect exploitation attempts targeting CVE-2021-33739.
- Conduct a post-remediation review. After patching, verify the fix is effective and document the remediation actions taken. Update your vulnerability management records and assess whether any additional hardening measures are warranted.
Technical Details
CVE-2021-33739 is a HIGH-severity vulnerability in Microsoft Windows that requires local access to exploit. The attack complexity is low, meaning no specialized conditions or preparation are required beyond the attack prerequisites. No prior authentication or privileges are needed to initiate the attack. No user interaction is required, allowing the attack to be fully automated.
Technical mechanism: Microsoft DWM Core Library Elevation of Privilege Vulnerability The exact technical mechanism is not fully specified in public advisories, but the vulnerability allows for elevation of privilege through exploitation of a flaw in the component.
CVSS 3.1 vector analysis: The vector (CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) reflects an Attack Vector of LOCAL, Attack Complexity of LOW, Privileges Required of NONE, User Interaction of NONE, Scope UNCHANGED, and impact ratings of HIGH/HIGH/HIGH for Confidentiality/Integrity/Availability respectively. The Unchanged scope means impact is contained within the vulnerable component itself.
Frequently Asked Questions
Is CVE-2021-33739 being actively exploited?
Yes. CVE-2021-33739 is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild. The EPSS score of 0.19488 (95.30th percentile) further confirms its high exploitation likelihood. Organizations should treat this as an urgent security matter requiring immediate attention.
What products are affected by CVE-2021-33739?
The affected products include Microsoft Windows 10 1909, Microsoft Windows 10 2004, Microsoft Windows 10 20h2, Microsoft Windows 10 21h1, Microsoft Windows Server 2004, and 1 more. Organizations running any of these versions should verify their exposure and prioritize remediation. Check vendor advisories for the complete and most current list of affected versions.
How do I fix CVE-2021-33739?
Apply updates per vendor instructions. Ensure all affected instances of Microsoft Windows are identified using vulnerability scanning and asset management tools. If immediate patching is not possible, implement network-level mitigations and enhanced monitoring. After patching, verify the fix and scan for indicators of prior compromise.
How severe is CVE-2021-33739?
CVE-2021-33739 is rated HIGH with a CVSS 3.1 score of 8.4. Its EPSS score of 0.19488 places it in the 95.30th percentile for exploitation likelihood. The vulnerability has confirmed active exploitation in the wild and was required to be remediated by federal agencies by 2021-11-17 per CISA's KEV directive.
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