CVE-2021-33656 in Linux
Summary
by MITRE • 07/18/2022
When setting font with malicous data by ioctl cmd PIO_FONT,kernel will write memory out of bounds.
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Analysis
by VulDB Data Team • 04/02/2025
This vulnerability exists in the Linux kernel's console font handling mechanism where the ioctl command PIO_FONT allows users to set custom fonts through kernel space operations. The flaw occurs when malicious data is passed to the kernel via this ioctl interface, specifically in the font data processing routines that handle font definitions and memory allocation. The vulnerability stems from insufficient bounds checking during the font data validation process, allowing an attacker to write beyond the allocated memory boundaries. This type of vulnerability falls under the CWE-787 category of out-of-bounds write conditions, which represents a critical security weakness in kernel memory management. The issue is particularly dangerous because it occurs within kernel space, where unauthorized memory access can lead to privilege escalation and system compromise.
The technical exploitation of CVE-2021-33656 involves crafting malicious font data that triggers an out-of-bounds memory write during the font setting operation. When the kernel processes the PIO_FONT ioctl command, it fails to properly validate the size and content of the font data buffer, allowing an attacker to overwrite adjacent memory locations. This can result in corruption of kernel data structures, potentially leading to denial of service conditions or more severe outcomes such as privilege escalation. The vulnerability is particularly concerning because it can be exploited by unprivileged users who have access to the console device or font manipulation interfaces, making it a potential vector for local privilege escalation attacks. The attack surface includes any system that allows font customization through kernel interfaces, particularly those with console or terminal management capabilities.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to encompass potential system compromise and data integrity violations. An attacker who successfully exploits this vulnerability could gain elevated privileges within the kernel space, potentially allowing them to execute arbitrary code with root-level permissions. This would enable complete system takeover and unauthorized access to sensitive data. The vulnerability also impacts system stability by introducing memory corruption that could cause unpredictable behavior, system crashes, or data loss. Organizations running Linux systems with console font customization capabilities are particularly at risk, especially in environments where untrusted users have access to system interfaces that might invoke the vulnerable ioctl commands.
Mitigation strategies for CVE-2021-33656 should focus on both immediate patching and operational security measures. The most effective solution is to apply the kernel patches released by the Linux kernel security team, which address the bounds checking deficiencies in the font handling code. Organizations should also implement restrictive access controls to console interfaces and font manipulation capabilities, limiting who can invoke the PIO_FONT ioctl commands. Monitoring for unusual font data patterns or system behavior that might indicate exploitation attempts should be implemented. Additionally, system administrators should consider disabling unnecessary font customization features when they are not required for system operation. The vulnerability demonstrates the importance of robust input validation in kernel space operations and aligns with ATT&CK techniques related to privilege escalation and kernel exploitation. Regular security audits of kernel interfaces and input validation mechanisms should be conducted to identify similar vulnerabilities in other system components.