CVE-2021-46790: Tuxera NTFS-3G Buffer Overflow Flaw

CVE-2021-46790 Overview

CVE-2021-46790 is a heap-based buffer overflow vulnerability affecting the ntfsck utility in NTFS-3G through version 2021.8.22. The vulnerability involves improper memory handling at buffer+512*3-2, which can lead to memory corruption when processing malformed NTFS file systems. While the upstream maintainer has deprecated ntfsck, the utility continues to be shipped by several major Linux distributions, making this vulnerability relevant for enterprise Linux environments.

Critical Impact

Local attackers with low privileges can exploit this heap-based buffer overflow to potentially achieve arbitrary code execution, leading to complete system compromise with high confidentiality, integrity, and availability impact.

Affected Products

• Tuxera NTFS-3G through version 2021.8.22
• Debian Linux 10.0 and 11.0
• Fedora 35 and 36

Discovery Timeline

• 2022-05-02 - CVE-2021-46790 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-46790

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), a category of memory corruption flaws where software writes data past the boundaries of allocated memory. In the context of ntfsck, the heap-based buffer overflow occurs during file system checking operations when processing NTFS metadata structures.

The vulnerable code path involves buffer arithmetic using the expression buffer+512*3-2, suggesting the issue relates to NTFS sector or cluster boundary calculations. When ntfsck processes a specially crafted NTFS file system image, it fails to properly validate input boundaries before writing to the heap-allocated buffer, allowing an attacker to corrupt adjacent memory regions.

Successful exploitation requires local access to the system and the ability to provide a malicious NTFS file system image to the ntfsck utility. This could occur when mounting untrusted USB drives or processing NTFS images from external sources.

Root Cause

The root cause is insufficient bounds checking in the ntfsck utility when handling NTFS file system structures. The buffer calculation buffer+512*3-2 indicates the code operates on sector-sized blocks (512 bytes) but fails to verify that write operations remain within allocated buffer boundaries. This allows specially crafted NTFS metadata to trigger writes beyond the heap buffer's allocated size.

Attack Vector

The attack requires local access to the target system. An attacker must provide a malicious NTFS file system image that triggers the vulnerable code path in ntfsck. This could be achieved through:

1. Physical access to insert a malicious USB drive containing a crafted NTFS partition
2. Tricking a user into running ntfsck on a downloaded disk image
3. Exploiting automated file system checking routines that process external storage

The vulnerability manifests in the NTFS file system checking routines where sector boundary calculations are performed. When ntfsck processes NTFS metadata structures, the buffer+512*3-2 arithmetic can result in heap memory corruption if the input file system contains malformed structures that cause the write offset to exceed buffer boundaries.

For detailed technical information about this vulnerability, refer to the GitHub Issue on NTFS-3G and the Openwall OSS Security Update.

Detection Methods for CVE-2021-46790

Indicators of Compromise

• Unexpected crashes or segmentation faults in the ntfsck process
• Core dumps generated during NTFS file system checking operations
• Suspicious NTFS disk images or external storage devices with unusual metadata structures
• Memory access violations logged by system monitoring tools during ntfsck execution

Detection Strategies

• Monitor for abnormal ntfsck process behavior including crashes, unexpected termination, or high memory usage
• Implement file integrity monitoring on NTFS-3G binaries to detect unauthorized modifications
• Use memory corruption detection tools such as AddressSanitizer (ASan) when testing NTFS file systems from untrusted sources
• Deploy endpoint detection and response (EDR) solutions to identify exploitation attempts targeting NTFS-3G utilities

Monitoring Recommendations

• Configure system logging to capture ntfsck invocations and their parameters
• Enable kernel auditing for file system operations involving external storage media
• Monitor for privilege escalation attempts following NTFS file system mounting operations
• Implement alerts for repeated ntfsck crashes which may indicate exploitation attempts

How to Mitigate CVE-2021-46790

Immediate Actions Required

• Update NTFS-3G to the latest available version in your distribution's package repository
• Avoid using the deprecated ntfsck utility on untrusted NTFS file systems
• Apply distribution-specific security patches from Debian and Fedora advisories
• Restrict execution of ntfsck to trusted administrators only

Patch Information

Security updates addressing CVE-2021-46790 have been released by multiple Linux distributions. Debian users should refer to Debian Security Advisory DSA-5160 for patching instructions. Fedora users can obtain updated packages through the standard package management system as announced in the Fedora Package Announcements.

Note that upstream considers ntfsck deprecated. Organizations should evaluate whether they require this utility and consider removing it entirely if not needed.

Workarounds

• Remove or disable the ntfsck binary if NTFS file system checking functionality is not required
• Use alternative methods for NTFS file system validation that do not rely on the vulnerable utility
• Implement strict access controls to prevent unprivileged users from invoking ntfsck
• Process only trusted NTFS file systems and avoid mounting or checking external media from unknown sources

# Remove or restrict ntfsck access
sudo chmod 700 /usr/bin/ntfsck
# Or remove the utility entirely if not needed
sudo rm /usr/bin/ntfsck

# Update NTFS-3G on Debian/Ubuntu
sudo apt update && sudo apt upgrade ntfs-3g

# Update NTFS-3G on Fedora
sudo dnf update ntfs-3g