CVE-2017-0263
Microsoft Win32k Privilege Escalation Vulnerability
Description
CVE-2017-0263 is a HIGH severity privilege escalation vulnerability in the Microsoft Win32k kernel-mode driver, carrying a CVSS 3.1 score of 7.8. Classified under CWE-416 (Use After Free), the vulnerability occurs because the Windows kernel-mode driver fails to properly handle objects in memory. A local attacker with low-level privileges can exploit this flaw to execute arbitrary code in kernel mode, effectively gaining complete control over the affected system. Affected products include Windows 7, Windows 8.1, Windows 10 (versions 1507 through 1703), and Windows Server 2008 through 2016. This CVE was actively exploited in targeted attacks attributed to the APT28 (Fancy Bear) threat group and was added to CISA's KEV catalog on February 10, 2022. The EPSS score is 17.77% (95.03rd percentile).
KEV Information
CVSS Score
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| microsoft | windows 10 1507 | - |
| microsoft | windows 10 1511 | - |
| microsoft | windows 10 1607 | - |
| microsoft | windows 10 1703 | - |
| microsoft | windows 7 | - |
| microsoft | windows 8.1 | - |
| microsoft | windows rt 8.1 | - |
| microsoft | windows server 2008 | -; r2 |
| microsoft | windows server 2012 | -; r2 |
| microsoft | windows server 2016 | - |
Multiple CVSS Assessments
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
References
- http://www.securityfocus.com/bid/98258(Broken Link, Third Party Advisory, VDB Entry)
- http://www.securitytracker.com/id/1038449(Broken Link, Third Party Advisory, VDB Entry)
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2017-0263(Patch, Vendor Advisory)
- https://www.exploit-db.com/exploits/44478/(Exploit, Third Party Advisory, VDB Entry)
- https://xiaodaozhi.com/exploit/117.html(Exploit, Third Party Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2017-0263(US Government Resource)
Weakness Type
CWE-416: Use After Free
CVE-2017-0263 is classified under CWE-416 — Use After Free. This weakness occurs when software references memory after it has been freed, which can cause a program to crash, use unexpected values, or execute arbitrary code. In the kernel context, use-after-free vulnerabilities are particularly dangerous because they allow attackers to manipulate kernel memory layouts.
In the Win32k driver, the vulnerability arises from improper handling of window manager objects in memory. When certain objects are freed but subsequently referenced by kernel-mode code, an attacker can manipulate the freed memory region to gain control over the execution flow. Since Win32k operates at the kernel level, successful exploitation grants the attacker the highest privilege level on the Windows operating system, bypassing all user-mode security controls.
Impact Analysis
CVE-2017-0263 carries a CVSS 3.1 base score of 7.8 (HIGH) with full impact across all three security dimensions when exploited locally.
Confidentiality (High): Kernel-mode code execution provides unrestricted access to all data on the system, including protected memory regions of other processes, stored credentials, encryption keys, and data belonging to all users on the machine.
Integrity (High): With kernel-level privileges, an attacker can modify any file, process, or system configuration. This includes disabling security software, installing rootkits that operate below the operating system's visibility, and manipulating audit logs.
Availability (High): Kernel-mode access allows an attacker to cause blue-screen crashes, corrupt critical system structures, or render the system completely inoperable. The attacker can also deploy ransomware with system-level privileges, making recovery more difficult.
Exploitation context: The local attack vector (AV:L) and low privilege requirement (PR:L) mean an attacker needs initial code execution on the target, typically achieved through a companion vulnerability such as the document-based exploit CVE-2017-0262. The combination of a remote access vulnerability with this privilege escalation forms a powerful attack chain.
The EPSS score of 17.77% (95.03rd percentile) indicates significant exploitation activity, consistent with its use in targeted APT campaigns.
Exploit Maturity
CVE-2017-0263 has a high exploit maturity with confirmed use in targeted nation-state attacks.
APT exploitation: CVE-2017-0263 was exploited in targeted attacks attributed to the APT28 (Fancy Bear) threat group, a well-known Russian state-sponsored cyber espionage unit. The vulnerability was used as a privilege escalation component in a multi-stage attack chain.
Attack chain: In observed campaigns, CVE-2017-0263 was combined with CVE-2017-0262 (a Microsoft Office remote code execution vulnerability via crafted EPS files). The Office vulnerability provided initial code execution, and CVE-2017-0263 escalated privileges to kernel level, granting complete system control.
Public exploits available: Exploit code is available on Exploit-DB (44478) and other sources, demonstrating the use-after-free exploitation technique in Win32k.
KEV listing: CISA added this vulnerability to the KEV catalog on February 10, 2022, with a remediation deadline of August 10, 2022.
Ransomware association: CISA's KEV does not indicate a direct ransomware association, though kernel-level privilege escalation vulnerabilities are commonly used in ransomware attack chains.
EPSS context: The EPSS score of 17.77% (95.03rd percentile) reflects significant real-world exploitation, primarily in targeted rather than mass exploitation campaigns.
Remediation
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Apply Microsoft security update immediately. Install the patch from the Microsoft Security Response Center advisory for all affected Windows versions. This corrects the memory handling in the Win32k kernel-mode driver.
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Patch CVE-2017-0262 concurrently. Since this vulnerability was exploited in combination with CVE-2017-0262 (Office EPS vulnerability), ensure both vulnerabilities are addressed simultaneously to break the attack chain.
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Implement exploit mitigation technologies. Enable Windows Defender Exploit Guard, Credential Guard, and other kernel-level protections. Ensure kernel address space layout randomization (KASLR) is active to complicate exploitation.
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Restrict local access and enforce least privilege. Minimize the number of users with local login access to servers and workstations. Use standard user accounts for daily operations and require explicit privilege elevation for administrative tasks.
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Deploy endpoint detection and response (EDR). Monitor for suspicious Win32k exploitation patterns, including unusual kernel memory access patterns, privilege escalation attempts, and post-exploitation behaviors such as credential dumping.
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Audit for historical compromise. Given the APT28 attribution, organizations in government, defense, and political sectors should perform thorough forensic analysis to determine if this vulnerability was previously exploited against their systems.
Technical Details
CVE-2017-0263 is a use-after-free vulnerability in the Win32k kernel-mode driver, which handles the Windows window manager subsystem.
Attack vector and prerequisites: The attack is local (AV:L) with low complexity (AC:L), requires low privileges (PR:L), and no user interaction (UI:N). The attacker needs initial code execution on the target system, typically obtained through a companion remote vulnerability.
Use-after-free mechanism: The vulnerability exists in how the Win32k driver manages window manager objects. Under specific conditions, the driver frees a kernel memory object but retains a reference to it. An attacker can trigger this condition, then reclaim the freed memory with controlled data. When the driver subsequently accesses the stale reference, it uses the attacker-controlled data, enabling arbitrary code execution in kernel mode.
Exploitation technique: The attacker typically manipulates the Windows window manager to create the use-after-free condition, then uses heap spraying or pool manipulation techniques to place controlled data at the freed memory location. The controlled data redirects execution flow to attacker-supplied shellcode running at Ring 0 (kernel level).
Attack chain context: In documented APT28 campaigns, the attack began with a spear-phishing email containing a Microsoft Office document with a malicious EPS image (CVE-2017-0262). Upon opening the document, the EPS vulnerability executed initial shellcode, which then triggered CVE-2017-0263 to escalate to SYSTEM privileges.
Affected platforms: Windows 7, 8.1, 10 (1507-1703), Server 2008, 2008 R2, 2012, 2012 R2, and Server 2016.
Frequently Asked Questions
What is CVE-2017-0263?
CVE-2017-0263 is a high-severity use-after-free privilege escalation vulnerability in the Microsoft Win32k kernel-mode driver. It allows a local attacker with low privileges to execute arbitrary code with kernel-level permissions, gaining complete control over the affected Windows system. The CVSS score is 7.8.
How was CVE-2017-0263 exploited in the wild?
CVE-2017-0263 was exploited by the APT28 (Fancy Bear) threat group in targeted attacks. The attack chain combined CVE-2017-0262 (an Office EPS vulnerability) for initial access with CVE-2017-0263 for privilege escalation to kernel level. Spear-phishing emails with malicious Office documents were the delivery mechanism.
How do I fix CVE-2017-0263?
Apply the Microsoft security update from the MSRC advisory for CVE-2017-0263. Also patch CVE-2017-0262 to address the full attack chain. Enable exploit mitigation technologies like Windows Defender Exploit Guard and KASLR. Implement least-privilege access policies and deploy EDR solutions to detect kernel exploitation attempts.
How severe is CVE-2017-0263?
CVE-2017-0263 is rated HIGH with a CVSS score of 7.8. While the local attack vector limits direct exposure, the confirmed use in APT28 nation-state campaigns demonstrates its severity. The EPSS score of 17.77% (95.03rd percentile) reflects significant real-world exploitation. Kernel-level privilege escalation enables complete system takeover.
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