CVE-2017-9763 in GRUB
Summary
by MITRE
The grub_ext2_read_block function in fs/ext2.c in GNU GRUB before 2013-11-12, as used in shlr/grub/fs/ext2.c in radare2 1.5.0, allows remote attackers to cause a denial of service (excessive stack use and application crash) via a crafted binary file, related to use of a variable-size stack array.
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Analysis
by VulDB Data Team • 12/08/2022
The vulnerability identified as CVE-2017-9763 represents a critical stack-based buffer overflow issue within the GNU GRUB bootloader software. This flaw exists in the grub_ext2_read_block function located in fs/ext2.c, which is also incorporated into radare2 version 1.5.0 through the shlr/grub/fs/ext2.c path. The vulnerability stems from improper handling of variable-size stack arrays during the processing of ext2 filesystem structures, creating a scenario where maliciously crafted binary files can trigger excessive stack memory consumption.
The technical implementation of this vulnerability involves the exploitation of a variable-length array declaration within the ext2 filesystem parsing logic. When GRUB attempts to read block data from an ext2 filesystem, the function processes metadata structures that contain size indicators which are then used to allocate stack space for temporary buffers. Attackers can craft binary files with maliciously large size values in these metadata fields, causing the system to allocate excessive stack space that exceeds the available stack memory limits. This results in stack overflow conditions that lead to application crashes and denial of service scenarios.
From an operational perspective, this vulnerability poses significant risks to system availability and integrity, particularly in environments where GRUB serves as the primary bootloader for operating systems. The denial of service impact extends beyond simple application crashes to potentially prevent system boot processes from completing successfully. In the context of radare2, which is widely used for reverse engineering and binary analysis, this vulnerability could be exploited to disrupt forensic analysis operations or compromise debugging environments that rely on proper filesystem parsing capabilities. The vulnerability affects systems that utilize ext2 filesystem support within GRUB, which includes many Linux distributions and embedded systems that depend on this bootloader for system initialization.
The vulnerability aligns with CWE-121 Stack-based Buffer Overflow, which specifically addresses the improper handling of variable-length arrays on the stack leading to buffer overflow conditions. This classification emphasizes the fundamental flaw in stack memory management where the size of allocated arrays is derived from user-controllable input data. From an adversarial perspective, this vulnerability maps to ATT&CK technique T1059.007 Command and Scripting Interpreter: Python, though more accurately represents T1499.004 Network Denial of Service, as the exploitation leads to service disruption rather than direct code execution. The attack surface is particularly concerning in environments where automated boot processes or recovery mechanisms depend on GRUB's ext2 filesystem support, as these systems may become unavailable during critical maintenance windows or emergency recovery scenarios.
Mitigation strategies for CVE-2017-9763 require immediate patching of affected GRUB versions to ensure proper validation of filesystem metadata before stack array allocation. System administrators should upgrade to GRUB versions released after November 12, 2013, which contain the necessary fixes for this vulnerability. Additionally, implementing input validation controls within filesystem parsing routines can help prevent malicious size indicators from triggering excessive stack allocations. Organizations should also consider implementing runtime protections such as stack canaries or address space layout randomization to reduce the effectiveness of potential exploitation attempts. Regular security audits of embedded systems and bootloader configurations are essential to identify and remediate similar vulnerabilities that may exist in other components of the boot chain.