CVE-2008-3531 in FreeBSD
Summary
by MITRE
Stack-based buffer overflow in sys/kern/vfs_mount.c in the kernel in FreeBSD 7.0 and 7.1, when vfs.usermount is enabled, allows local users to gain privileges via a crafted (1) mount or (2) nmount system call, related to copying of "user defined data" in "certain error conditions."
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Analysis
by VulDB Data Team • 12/01/2024
This vulnerability exists in the FreeBSD kernel's virtual file system mount implementation where a stack-based buffer overflow occurs in the sys/kern/vfs_mount.c file. The flaw specifically manifests when the vfs.usermount parameter is enabled, creating a dangerous condition that allows local attackers to escalate privileges. The vulnerability stems from improper handling of user-defined data during error conditions in mount and nmount system calls, where the kernel fails to properly validate the length of data being copied to a stack buffer. This represents a classic stack buffer overflow vulnerability that can be exploited to overwrite adjacent memory locations and potentially execute arbitrary code with kernel privileges.
The technical implementation of this vulnerability involves the kernel's handling of user-supplied data during file system mounting operations. When vfs.usermount is enabled, the system allows non-root users to mount file systems, but the kernel's processing of user-defined mount options creates an opportunity for attackers to supply data that exceeds the allocated stack buffer size. The overflow occurs specifically during error handling scenarios where the system attempts to copy user-provided data into a fixed-size stack buffer without proper bounds checking. This flaw is categorized under CWE-121 Stack-based Buffer Overflow, which is a well-known vulnerability type that has been extensively documented in security literature and represents a fundamental weakness in memory management practices.
The operational impact of this vulnerability is significant as it provides local privilege escalation capabilities to attackers who can already execute code on the target system. An attacker with normal user privileges can exploit this vulnerability to gain kernel-level privileges, effectively compromising the entire system. The attack vector requires the attacker to have local access and the ability to execute mount or nmount system calls with crafted parameters, but this is achievable since the vulnerability is in the kernel's handling of user mount operations. The exploitation process typically involves crafting malicious mount options that trigger the buffer overflow condition during error handling, potentially allowing for code execution with the highest system privileges.
Mitigation strategies for this vulnerability should focus on both immediate system hardening and long-term architectural improvements. The most effective immediate solution is to disable the vfs.usermount parameter through sysctl configuration, which prevents the vulnerable code path from being executed. System administrators should also ensure that FreeBSD systems are updated to patched versions that address the buffer overflow condition in the kernel's mount handling code. Additional defensive measures include implementing proper input validation and bounds checking in kernel code, using stack canaries for buffer overflow detection, and employing kernel address space layout randomization to make exploitation more difficult. Organizations should also consider implementing monitoring and logging of mount system calls to detect potential exploitation attempts. This vulnerability aligns with ATT&CK technique T1068 for privilege escalation and demonstrates the critical importance of kernel-level security in operating system design. The flaw underscores the necessity of rigorous code review processes and defensive programming practices when implementing system call handlers that process user-supplied data.