CVE-2021-32537: Realtek HDA Driver DOS Vulnerability

CVE-2021-32537 Overview

CVE-2021-32537 is a Driver Vulnerability affecting the Realtek High Definition Audio (HDA) driver (RTKVHD64.sys). This vulnerability allows local attackers to send specially crafted strings to the kernel driver from user mode, resulting in improper handling of unexpected commands. The vulnerability stems from improper bounds checking (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer), which can cause the system to crash when exploited.

Critical Impact

Local attackers with low privileges can trigger a system crash (Blue Screen of Death), causing complete denial of service on affected Windows systems with vulnerable Realtek audio drivers installed.

Affected Products

• Realtek HDA Driver (RTKVHD64.sys)
• Windows systems with vulnerable Realtek High Definition Audio drivers

Discovery Timeline

• 2021-07-07 - CVE-2021-32537 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-32537

Vulnerability Analysis

This vulnerability exists within the Realtek HDA kernel driver (RTKVHD64.sys) and is classified as an out-of-bounds memory access issue. The driver fails to properly validate input received from user-mode applications before processing it, allowing attackers to trigger memory operations outside the intended boundaries.

When a local user sends specially crafted data to the driver through standard I/O control (IOCTL) mechanisms, the driver processes unexpected commands without adequate validation. This improper handling leads to memory corruption that causes a kernel panic, resulting in a system crash (BSOD - Blue Screen of Death).

The vulnerability requires local access to exploit, meaning an attacker must already have some level of access to the target system. However, only low-level privileges are needed to trigger the vulnerability, and no user interaction is required. While the vulnerability does not directly compromise confidentiality or integrity, it has a high impact on system availability due to its ability to crash the entire operating system.

Root Cause

The root cause is improper restriction of operations within the bounds of a memory buffer (CWE-119). The Realtek HDA kernel driver does not adequately validate the size or content of user-supplied data before performing memory operations. This allows attackers to provide input that causes the driver to access memory outside its allocated boundaries, leading to system instability and crashes.

Attack Vector

The attack vector is local, requiring the attacker to have existing access to the target system. The exploitation process involves:

1. An attacker with local access identifies a system running the vulnerable Realtek HDA driver
2. The attacker crafts a malicious payload containing unexpected command strings
3. The payload is sent to the kernel driver via device I/O control calls from user mode
4. The driver processes the malformed input without proper bounds checking
5. This triggers an out-of-bounds memory access in kernel space
6. The system experiences a kernel panic and crashes (BSOD)

The vulnerability is exploitable through standard Windows APIs for communicating with kernel drivers. Technical details regarding the specific exploitation mechanism are available in the Packet Storm Security Report.

Detection Methods for CVE-2021-32537

Indicators of Compromise

• System crashes (BSOD) with memory access violations referencing RTKVHD64.sys
• Unexpected IOCTL calls to the Realtek audio driver from non-standard processes
• Crash dump files indicating kernel-mode exceptions in the Realtek HDA driver module

Detection Strategies

• Monitor for abnormal patterns of device I/O control requests to audio drivers
• Implement endpoint detection rules to identify processes sending unusual payloads to kernel drivers
• Analyze Windows Event Logs and crash dumps for BSOD events involving RTKVHD64.sys
• Deploy SentinelOne Singularity Platform for real-time kernel driver activity monitoring and anomaly detection

Monitoring Recommendations

• Enable Windows Error Reporting to capture and analyze crash dump files
• Configure SentinelOne to alert on suspicious IOCTL patterns targeting audio subsystem drivers
• Implement driver inventory monitoring to track Realtek HDA driver versions across endpoints
• Review system stability reports for recurring crashes related to audio driver components

How to Mitigate CVE-2021-32537

Immediate Actions Required

• Inventory all systems with Realtek HDA drivers installed to identify affected endpoints
• Update Realtek High Definition Audio drivers to the latest version available from Realtek or your hardware manufacturer
• Implement least-privilege access controls to limit which users can interact with kernel drivers
• Deploy SentinelOne endpoint protection to detect and block exploitation attempts

Patch Information

Organizations should update to the latest Realtek HDA driver version that addresses this vulnerability. Check with your system or motherboard manufacturer for updated driver packages, as OEMs often distribute customized Realtek audio drivers. Consult the TW-CERT Security Advisory for additional guidance on remediation.

Workarounds

• Restrict local access to systems with vulnerable drivers to trusted users only
• Consider disabling or uninstalling Realtek audio drivers on critical servers where audio functionality is not required
• Implement application whitelisting to prevent unauthorized processes from communicating with kernel drivers
• Use network segmentation to limit the impact if a local attacker compromises a system with this vulnerability