CVE-2021-47495 in Linux
Summary
by MITRE • 05/22/2024
In the Linux kernel, the following vulnerability has been resolved:
usbnet: sanity check for maxpacket
maxpacket of 0 makes no sense and oopses as we need to divide by it. Give up.
V2: fixed typo in log and stylistic issues
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Analysis
by VulDB Data Team • 09/29/2025
The vulnerability identified as CVE-2021-47495 resides within the Linux kernel's USB networking driver subsystem, specifically affecting the usbnet driver component that handles USB network adapters. This issue represents a fundamental input validation failure that occurs when processing USB endpoint descriptors during device initialization and configuration phases. The flaw manifests when the driver encounters a USB endpoint with a maximum packet size value of zero, which should never occur in valid USB device descriptors but can happen due to malformed hardware or malicious device behavior. The usbnet driver performs critical operations that require division by the maxpacket value, making this zero value particularly dangerous as it leads to division by zero errors that cause kernel oopses and system instability.
The technical implementation of this vulnerability stems from insufficient parameter validation within the usbnet driver's endpoint configuration handling logic. When a USB network device presents an endpoint descriptor with maxpacket set to zero, the driver fails to perform proper sanity checks before proceeding with operations that depend on this value. This particular flaw falls under CWE-369, known as Divide by Zero, where the code attempts to divide by a value that could potentially be zero. The kernel's usbnet driver does not validate that the maxpacket field contains a valid non-zero value before using it in arithmetic operations, creating an exploitable condition that can result in kernel panics and system crashes. The vulnerability demonstrates poor defensive programming practices where the driver assumes all USB descriptors conform to expected standards without proper validation.
Operationally, this vulnerability can result in significant system stability issues and potential denial of service conditions for systems running Linux kernels with affected usbnet drivers. The impact extends beyond simple system crashes to potentially affect network connectivity for devices relying on USB networking interfaces, including USB-to-Ethernet adapters, mobile broadband devices, and various USB network peripherals. Attackers could potentially exploit this vulnerability by connecting malicious USB devices that present malformed endpoint descriptors, leading to unauthorized system crashes or in more sophisticated scenarios, potentially enabling privilege escalation or persistent access to compromised systems. The vulnerability affects all Linux kernel versions that include the usbnet driver implementation and could be particularly concerning in embedded systems or devices where USB networking is prevalent.
Mitigation strategies for CVE-2021-47495 focus on implementing proper input validation and defensive programming practices within the kernel USB subsystem. The primary fix involves adding explicit checks to validate that maxpacket values are greater than zero before any arithmetic operations are performed, which aligns with ATT&CK technique T1068 by preventing exploitation of kernel-level vulnerabilities. System administrators should ensure their Linux systems are updated with patches that include the corrected usbnet driver implementation, which typically involves adding a simple conditional check to reject endpoint descriptors with zero maxpacket values. Additionally, organizations should implement USB device access controls and monitoring to detect anomalous USB device behavior, as this vulnerability could be exploited through malicious USB device insertion attacks. The fix also emphasizes proper error handling and logging mechanisms to ensure that invalid USB descriptors are properly rejected rather than causing kernel oopses, which supports the broader security principle of fail-safe design in kernel subsystems.