CVE-2021-32559 in pywin32
Summary
by MITRE • 07/06/2021
An integer overflow exists in pywin32 prior to version b301 when adding an access control entry (ACE) to an access control list (ACL) that would cause the size to be greater than 65535 bytes. An attacker who successfully exploited this vulnerability could crash the vulnerable process.
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Analysis
by VulDB Data Team • 03/27/2025
The vulnerability identified as CVE-2021-32559 represents a critical integer overflow condition within the pywin32 library ecosystem, specifically affecting versions prior to b301. This flaw manifests during the process of adding access control entries to access control lists, creating a scenario where the calculated size of the ACL exceeds the maximum allowable limit of 65535 bytes. The pywin32 library serves as a Python extension for Windows programming, providing access to Windows API functions and system resources, making it a critical component in many Windows-based applications and automation scripts. The vulnerability stems from inadequate input validation and size calculation handling within the library's ACL management functions, particularly when processing user-supplied data that influences the size of access control entries.
The technical exploitation of this integer overflow occurs when an attacker crafts malicious input that, when processed by the vulnerable pywin32 functions, causes the internal size calculation to wrap around and exceed the 65535 byte threshold. This overflow condition results in unpredictable behavior within the memory management system of the affected process. The vulnerability maps directly to CWE-190, which specifically addresses integer overflow conditions that can lead to buffer overflows and memory corruption. When the integer overflow occurs during ACL processing, the system attempts to allocate memory based on the erroneous calculated size, potentially leading to heap corruption or stack overflow conditions that manifest as process crashes or more severe system instability. The vulnerability is particularly concerning because it operates at the kernel level of Windows security mechanisms, where access control lists are fundamental to system security and privilege management.
The operational impact of this vulnerability extends beyond simple denial of service, as it represents a potential entry point for more sophisticated attacks within Windows environments. When a vulnerable process crashes due to this integer overflow, it can disrupt legitimate system operations and potentially provide attackers with opportunities to escalate privileges or execute arbitrary code. The vulnerability affects any application or script that utilizes pywin32 to manage Windows access control lists, including system administration tools, network security applications, and enterprise automation platforms. From an adversary perspective, this vulnerability aligns with ATT&CK technique T1068, which involves local privilege escalation through exploitation of system vulnerabilities, and T1499, focusing on network denial of service attacks that can disrupt system availability. The impact is particularly severe in enterprise environments where pywin32 is commonly used for Windows automation, system monitoring, and security tool development, as these applications often run with elevated privileges.
Mitigation strategies for CVE-2021-32559 primarily focus on immediate version updates to pywin32 b301 or later, which contain the necessary patches to prevent the integer overflow condition. Organizations should conduct comprehensive vulnerability assessments to identify all systems utilizing vulnerable pywin32 versions and implement remediation protocols across their infrastructure. Additional protective measures include implementing strict input validation for any user data that might influence ACL operations, monitoring system logs for unusual process termination patterns, and applying network segmentation to limit potential attack vectors. The vulnerability highlights the importance of proper integer handling in security-critical code and underscores the need for comprehensive code review processes that specifically examine memory management and size calculation routines. System administrators should also consider implementing application whitelisting policies to restrict execution of untrusted Python scripts that might interact with Windows security mechanisms, thereby reducing the attack surface for exploitation of this class of vulnerability.