CVE-2021-33655 in Linux
Summary
by MITRE • 07/18/2022
When sending malicous data to kernel by ioctl cmd FBIOPUT_VSCREENINFO,kernel will write memory out of bounds.
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Analysis
by VulDB Data Team • 09/03/2025
This vulnerability resides in the Linux kernel's framebuffer subsystem where the FBIOPUT_VSCREENINFO ioctl command fails to properly validate input parameters before writing to kernel memory. The flaw occurs when user-space applications submit malicious data through this interface, causing the kernel to write beyond the allocated memory boundaries. This out-of-bounds write condition represents a critical security weakness that can be exploited to corrupt kernel memory structures and potentially escalate privileges.
The technical implementation involves the kernel's handling of video screen information structures where insufficient bounds checking allows arbitrary data to overwrite adjacent memory locations. When the ioctl command processes malformed input parameters, the kernel's memory management routines fail to validate the size or content of the submitted data structure, leading to memory corruption that can be leveraged for privilege escalation attacks. This vulnerability specifically affects the fbdev subsystem and demonstrates poor input validation practices that align with CWE-121, which addresses stack buffer overflow conditions.
The operational impact of this vulnerability extends beyond simple memory corruption as it provides attackers with potential pathways to gain elevated system privileges. An attacker with access to the framebuffer device could exploit this issue to execute arbitrary code in kernel space, effectively compromising the entire system. The vulnerability is particularly dangerous because it requires minimal privileges to trigger and can be exploited through legitimate system interfaces that are commonly accessible to unprivileged users.
Mitigation strategies include applying the latest kernel security patches that implement proper input validation for the FBIOPUT_VSCREENINFO command and restrict access to framebuffer devices through appropriate permissions. System administrators should also consider implementing runtime protections such as kernel address space layout randomization and stack canaries to reduce exploit reliability. Additionally, monitoring for suspicious ioctl operations on framebuffer devices can help detect potential exploitation attempts. This vulnerability aligns with ATT&CK technique T1068 which covers the use of local privilege escalation techniques through kernel exploits, making it a significant concern for system security posture.