CVE-2024-49996: Linux Kernel Buffer Overflow Vulnerability

CVE-2024-49996 Overview

A buffer overflow vulnerability has been discovered in the Linux kernel's CIFS (Common Internet File System) subsystem, specifically in the handling of NFS reparse points. The vulnerability exists in the cifs_strndup_from_utf16() function, which incorrectly calculates buffer sizes when parsing reparse point data structures. This leads to memory access beyond the intended buffer boundaries, potentially allowing attackers with local access to compromise system confidentiality, integrity, and availability.

The flaw stems from improper length calculation when processing ReparseDataLength, where the code fails to subtract the InodeType size before accessing the DataBuffer. Additionally, insufficient bounds checking before accessing InodeType and calling reparse_mkdev() for major/minor device values creates multiple paths to invalid memory access.

Critical Impact

Local attackers can exploit this buffer overflow vulnerability to read or write memory outside intended buffer boundaries, potentially leading to privilege escalation, information disclosure, or system crashes.

Affected Products

• Linux Kernel (multiple versions)
• Systems using CIFS filesystem with NFS reparse point functionality
• Debian Linux (affected versions covered by LTS announcements)

Discovery Timeline

• October 21, 2024 - CVE-2024-49996 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2024-49996

Vulnerability Analysis

This buffer overflow vulnerability (CWE-120) occurs within the CIFS filesystem driver when processing NFS reparse points. The core issue lies in how the kernel calculates data buffer sizes from the ReparseDataLength field.

When parsing reparse point structures, ReparseDataLength represents the combined size of InodeType and DataBuffer. The vulnerable code path in cifs_strndup_from_utf16() uses the full ReparseDataLength value to access buf->DataBuffer, without first subtracting the size of InodeType. This arithmetic error causes the function to read memory beyond the actual end of the DataBuffer, resulting in an out-of-bounds memory access.

Furthermore, the code lacks proper validation before accessing the InodeType member and before calling reparse_mkdev() for major and minor device values. These additional missing checks create multiple vectors for triggering invalid memory access conditions.

Root Cause

The root cause is a classic buffer length miscalculation combined with insufficient bounds checking. The ReparseDataLength field contains the total size of both InodeType and DataBuffer components. However, when the code attempts to access only the DataBuffer portion, it fails to subtract sizeof(InodeType) from the length calculation. This results in the code using an oversized length parameter, causing memory access past the legitimate buffer boundary.

The fix requires three corrections:

1. Properly subtract InodeType size from ReparseDataLength before accessing DataBuffer
2. Verify ReparseDataLength is large enough before accessing InodeType
3. Validate reparse buffer size before calling reparse_mkdev() for major/minor values

Attack Vector

The vulnerability requires local access to the system where an attacker can trigger CIFS operations involving specially crafted NFS reparse points. An attacker with low privileges can potentially exploit this by:

1. Mounting a malicious CIFS share containing crafted reparse point data
2. Triggering filesystem operations that cause the kernel to parse the malformed reparse point structure
3. The incorrect length calculation causes out-of-bounds memory access in kernel space

Since this is a local attack vector requiring user interaction with crafted filesystem data, exploitation depends on the attacker's ability to influence CIFS mount operations or provide malicious network share content.

Detection Methods for CVE-2024-49996

Indicators of Compromise

• Unexpected kernel oops or panics related to CIFS or filesystem operations
• Memory corruption errors in kernel logs referencing cifs_strndup_from_utf16 or reparse point functions
• Suspicious CIFS mount activity involving unusual or untrusted network shares

Detection Strategies

• Monitor kernel logs for buffer overflow warnings or memory access violations in CIFS subsystem
• Deploy kernel address sanitizer (KASAN) in development/testing environments to detect out-of-bounds access
• Implement network monitoring for unusual SMB/CIFS traffic patterns to untrusted servers
• Use Linux audit subsystem to track CIFS mount operations

Monitoring Recommendations

• Enable kernel crash dump analysis to capture exploitation attempts
• Monitor for unusual CIFS mount operations, especially from non-standard sources
• Implement endpoint detection for kernel-level anomalies
• Review system logs for repeated CIFS-related errors that could indicate exploitation attempts

How to Mitigate CVE-2024-49996

Immediate Actions Required

• Update Linux kernel to patched versions as soon as possible
• Restrict CIFS mount operations to trusted network shares only
• Limit local user access to systems where CIFS functionality is enabled
• Consider disabling NFS reparse point support if not required for operations

Patch Information

Multiple kernel patches have been released across different stable branches to address this vulnerability. The patches correctly implement the length subtraction for InodeType and add proper bounds checking before accessing structure members.

Official patches are available from the Linux kernel stable repository:

• Kernel Git Commit c6db81c550ce
• Kernel Git Commit e2a8910af016
• Kernel Git Commit 803b3a39cb09
• Kernel Git Commit c173d47b69f0

Debian users should refer to the Debian LTS Security Announcements for distribution-specific updates.

Workarounds

• Restrict CIFS mount permissions using mount options and access controls
• Disable mounting of remote CIFS shares from untrusted sources
• Use network segmentation to limit exposure to potentially malicious CIFS servers
• Implement mandatory access control (SELinux/AppArmor) policies for CIFS operations

# Restrict CIFS mount capability to root only
chmod 750 /sbin/mount.cifs

# Example: Block CIFS traffic from untrusted networks using iptables
iptables -A INPUT -p tcp --dport 445 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 445 -j DROP