CVE-2021-3697 in grub2
Summary
by MITRE • 07/06/2022
A crafted JPEG image may lead the JPEG reader to underflow its data pointer, allowing user-controlled data to be written in heap. To a successful to be performed the attacker needs to perform some triage over the heap layout and craft an image with a malicious format and payload. This vulnerability can lead to data corruption and eventual code execution or secure boot circumvention. This flaw affects grub2 versions prior grub-2.12.
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Analysis
by VulDB Data Team • 05/19/2025
The vulnerability identified as CVE-2021-3697 represents a critical heap-based buffer underflow flaw within the grub2 bootloader implementation that processes JPEG image files. This weakness specifically manifests when the JPEG reader component encounters a malformed image file that triggers an underflow condition in its data pointer management. The underlying technical mechanism involves the JPEG parser failing to properly validate the boundaries of its internal data structures, causing a pointer to move beyond its allocated memory region. This mismanagement creates a scenario where user-controlled data can be written to heap memory locations that should remain protected from external influence. The vulnerability resides in the grub2 bootloader version prior to grub-2.12, making a significant portion of systems relying on this bootloader susceptible to exploitation.
The operational impact of this vulnerability extends beyond simple data corruption to potentially enable complete system compromise through code execution capabilities. When an attacker successfully manipulates the heap layout through careful crafting of JPEG payloads, they can achieve arbitrary code execution within the bootloader context, which represents a critical escalation of privileges. The attack requires sophisticated triage operations to understand the target heap layout before crafting the malicious JPEG file, indicating that this is not a trivial vulnerability to exploit but rather requires significant attacker skill and preparation. The vulnerability's potential to circumvent secure boot mechanisms makes it particularly dangerous in enterprise and government environments where system integrity is paramount. This flaw essentially allows an attacker to bypass the initial boot security measures that are designed to prevent unauthorized code execution before the operating system loads.
The technical nature of this vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and can be mapped to ATT&CK technique T1068, which covers exploit for privilege escalation. The vulnerability demonstrates the classic characteristics of heap manipulation attacks where attackers exploit memory management flaws to write controlled data to arbitrary memory locations. The requirement for heap layout triage and payload crafting indicates this vulnerability operates within the category of advanced persistent threat techniques that demand both deep system knowledge and precise exploitation skills. Organizations affected by this vulnerability must prioritize immediate remediation through grub2 version updates to grub-2.12 or later, as the underlying memory corruption mechanisms provide attackers with pathways to achieve system compromise. The broader implications suggest that similar vulnerabilities may exist in other bootloader implementations that process image files, highlighting the need for comprehensive security reviews of memory management practices in system-level software components.