CVE-2021-3713 in QEMU
Summary
by MITRE • 08/25/2021
An out-of-bounds write flaw was found in the UAS (USB Attached SCSI) device emulation of QEMU in versions prior to 6.2.0-rc0. The device uses the guest supplied stream number unchecked, which can lead to out-of-bounds access to the UASDevice->data3 and UASDevice->status3 fields. A malicious guest user could use this flaw to crash QEMU or potentially achieve code execution with the privileges of the QEMU process on the host.
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Analysis
by VulDB Data Team • 10/25/2024
The vulnerability identified as CVE-2021-3713 represents a critical out-of-bounds write flaw within the USB Attached SCSI (UAS) device emulation component of QEMU virtualization software. This issue affects QEMU versions prior to 6.2.0-rc0 and stems from inadequate input validation mechanisms within the UAS device implementation. The flaw specifically manifests when the UAS device emulation processes guest-supplied stream numbers without proper bounds checking, creating a scenario where malicious input can trigger memory corruption in the host system.
The technical nature of this vulnerability can be classified under CWE-787 Out-of-bounds Write, which occurs when a program writes data past the end of a buffer or array. In this case, the UASDevice->data3 and UASDevice->status3 fields serve as the vulnerable memory locations where the out-of-bounds write occurs. The root cause lies in the failure to validate the stream number provided by the guest operating system before using it as an index into fixed-size arrays or buffers. This unchecked guest input allows an attacker to manipulate memory locations that should remain protected, potentially leading to arbitrary code execution or system crashes.
The operational impact of this vulnerability extends beyond simple system instability, as it provides a potential path for privilege escalation within virtualized environments. When a malicious guest user exploits this flaw, they can cause QEMU to write data beyond the intended memory boundaries, which may result in a denial of service condition or more severe consequences including code execution with the same privileges as the QEMU process running on the host system. This represents a significant concern for cloud environments and server virtualization platforms where multiple tenants share the same physical infrastructure, as it could enable attackers to compromise host systems and potentially escalate their access to other virtual machines or underlying resources.
From an attack perspective, this vulnerability aligns with ATT&CK technique T1059 Command and Scripting Interpreter, as exploitation typically requires guest-level access to execute malicious commands that manipulate the UAS device emulation. The attack chain would involve a compromised guest operating system or malicious user gaining the ability to send crafted UAS commands to the virtualized USB device, triggering the out-of-bounds write condition. Organizations using QEMU-based virtualization solutions should prioritize patching to version 6.2.0-rc0 or later, as this release includes the necessary mitigations to prevent the unchecked stream number processing that enables this vulnerability.
The broader implications of this flaw highlight the importance of input validation in virtualization software components, particularly those handling device emulation. Virtualization platforms must implement robust bounds checking mechanisms to prevent guest users from manipulating host memory structures through device emulation interfaces. Security practitioners should consider this vulnerability as part of comprehensive virtualization security assessments, examining not only the specific UAS implementation but also other device emulations within QEMU that might suffer from similar unchecked input handling patterns. Additionally, organizations should implement monitoring solutions to detect anomalous UAS device activity that could indicate exploitation attempts, as the vulnerability's impact extends to both availability and confidentiality aspects of virtualized environments.