CVE-2025-26639: Windows 10 21h2 Privilege Escalation Flaw

CVE-2025-26639 Overview

CVE-2025-26639 is an integer overflow or wraparound vulnerability in the Windows USB Print Driver that enables an authorized attacker to elevate privileges locally. This vulnerability affects a wide range of Windows operating systems including Windows 10, Windows 11, and Windows Server editions, making it a significant concern for enterprise environments with USB printing infrastructure.

Critical Impact

Local privilege escalation through integer overflow in Windows USB Print Driver allows attackers with low-level access to gain elevated system privileges, potentially compromising system integrity and confidentiality.

Affected Products

• Microsoft Windows 10 21H2
• Microsoft Windows 10 22H2
• Microsoft Windows 11 22H2
• Microsoft Windows 11 23H2
• Microsoft Windows 11 24H2
• Microsoft Windows Server 2022
• Microsoft Windows Server 2022 23H2
• Microsoft Windows Server 2025

Discovery Timeline

• 2025-04-08 - CVE-2025-26639 published to NVD
• 2025-07-03 - Last updated in NVD database

Technical Details for CVE-2025-26639

Vulnerability Analysis

This vulnerability stems from improper handling of integer values within the Windows USB Print Driver component. When processing certain input data, the driver fails to properly validate integer boundaries, leading to an overflow condition. This overflow can result in a heap-based buffer overflow (CWE-122), which attackers can exploit to corrupt memory structures and ultimately gain elevated privileges on the affected system.

The local attack vector means an attacker must already have some level of access to the target system, but the low complexity and lack of user interaction requirements make this vulnerability relatively easy to exploit once initial access is achieved. Successful exploitation grants the attacker complete control over confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of CVE-2025-26639 is an integer overflow or wraparound condition in the Windows USB Print Driver. When arithmetic operations on integer values exceed the maximum representable value, the result wraps around to a small or negative number. In this case, the wrapped value is subsequently used for memory allocation or buffer size calculations, resulting in an undersized buffer being allocated. When data is written to this undersized buffer, a heap-based buffer overflow occurs, enabling memory corruption that can be leveraged for privilege escalation.

Attack Vector

The attack vector for CVE-2025-26639 is local, requiring an authenticated attacker to have prior access to the target system. The exploitation scenario involves:

1. An attacker with low-privilege access to a Windows system identifies the vulnerable USB Print Driver
2. The attacker crafts malicious input designed to trigger the integer overflow condition
3. When the driver processes this input, the integer overflow leads to improper memory allocation
4. The resulting heap-based buffer overflow allows the attacker to corrupt adjacent memory structures
5. By carefully controlling the overflow, the attacker can execute arbitrary code with elevated SYSTEM privileges

The vulnerability does not require user interaction, meaning an attacker can trigger it programmatically without social engineering.

Detection Methods for CVE-2025-26639

Indicators of Compromise

• Unusual process activity involving USB print driver components (usbprint.sys or related driver files)
• Unexpected privilege escalation events from low-privilege user accounts
• Anomalous memory access patterns or crashes in the Windows Print Spooler service
• System event logs showing driver errors or memory access violations related to USB printing

Detection Strategies

• Monitor Windows Event Logs for privilege escalation attempts and driver-related errors
• Implement endpoint detection rules to identify suspicious interactions with USB print driver components
• Deploy memory integrity monitoring to detect heap corruption attempts
• Use behavioral analysis to identify processes attempting to exploit local privilege escalation paths

Monitoring Recommendations

• Enable advanced auditing for process creation and privilege use events
• Configure SentinelOne Singularity platform to monitor driver load events and suspicious print spooler activity
• Implement real-time alerting for any attempts to load untrusted drivers or modify print driver configurations
• Regularly review security logs for patterns consistent with local privilege escalation attempts

How to Mitigate CVE-2025-26639

Immediate Actions Required

• Apply the latest Microsoft security updates from the April 2025 Patch Tuesday release immediately
• Restrict local access to systems where USB print functionality is not required
• Consider disabling USB print drivers on systems that do not require USB printing capabilities
• Implement the principle of least privilege to limit the impact of potential exploitation

Patch Information

Microsoft has released security updates to address CVE-2025-26639. Organizations should refer to the Microsoft Security Update Guide for CVE-2025-26639 for detailed patch information and download links specific to each affected product version. The patches address the integer overflow condition by implementing proper bounds checking before memory allocation operations.

Workarounds

• Disable USB print driver functionality on systems where it is not business-critical using Group Policy or device installation restrictions
• Implement application control policies to prevent unauthorized code execution in the context of print driver operations
• Restrict physical USB port access and implement USB device whitelisting where possible
• Consider using network-based printing solutions as an alternative to USB printing on sensitive systems

# Disable USB Print Driver via Registry (use with caution - impacts USB printing)
reg add "HKLM\SYSTEM\CurrentControlSet\Services\usbprint" /v Start /t REG_DWORD /d 4 /f

# Restart the system to apply changes
shutdown /r /t 0