CVE-2026-63833 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
ntfs3: reject direct userspace writes to reserved $LX* xattrs
NTFS3 uses $LXUID, $LXGID, $LXMOD and $LXDEV as internal WSL permission metadata and reloads them into i_uid, i_gid and i_mode from ntfs_get_wsl_perm().
Because the empty-prefix xattr handler also lets file owners call setxattr() on these names directly, an unprivileged writer on a writable ntfs3 mount can plant root ownership and S_ISUID on their own file and gain euid 0 after inode reload.
Reject direct userspace writes to the reserved $LX* names. Internal ntfs3 metadata updates are unchanged because ntfs_save_wsl_perm() writes them via ntfs_set_ea() directly.
[[email protected]: added an additional check for non privileged users]
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Analysis
by VulDB Data Team • 07/20/2026
This vulnerability resides within the NTFS3 file system implementation in the Linux kernel, specifically affecting how reserved extended attributes are handled. The issue stems from improper access controls around internal WSL (Windows Subsystem for Linux) permission metadata identifiers that begin with the $LX prefix. These metadata fields include $LXUID for user ID, $LXGID for group ID, $LXMOD for file mode, and $LXDEV for device information. The kernel's ntfs_get_wsl_perm() function properly loads these values into inode attributes i_uid, i_gid, and i_mode during file operations, but the corresponding extended attribute handler allows direct user-space modification through setxattr() calls.
The technical flaw represents a privilege escalation vulnerability that violates fundamental security principles of access control and privilege separation. When unprivileged users can directly write to these reserved $LX* extended attributes through standard file system interfaces, they can manipulate inode metadata to gain elevated privileges. This occurs because the system reloads these values into the inode structure during file operations, allowing an attacker to set root ownership (i_uid = 0) and setuid bits (S_ISUID) on their own files, effectively creating a mechanism for privilege escalation to euid 0. The vulnerability is particularly dangerous because it leverages legitimate file system operations while exploiting the gap between internal kernel metadata handling and user-space access controls.
The operational impact of this vulnerability extends beyond simple privilege escalation as it provides attackers with persistent root-level access on systems running NTFS3 mounts. This represents a critical security flaw in the Linux kernel's file system subsystem that could be exploited across various deployment scenarios including desktop systems, servers, and cloud environments where NTFS3 support is enabled. The vulnerability affects any system with writable NTFS3 mounts and unprivileged user access, making it particularly concerning for multi-user environments and shared computing resources. According to CWE classification, this represents a weakness in privilege management (CWE-276) combined with improper access control (CWE-284), while the ATT&CK framework would categorize this under privilege escalation techniques using file system manipulation.
The mitigation strategy involves implementing direct restrictions on user-space write operations to the reserved $LX* extended attribute names, preventing unauthorized modification through standard file system interfaces while maintaining internal kernel functionality. The fix ensures that ntfs_save_wsl_perm() continues to function correctly by writing these metadata values directly via ntfs_set_ea() without user-space interference. This approach preserves the legitimate internal operations while closing the security gap that allowed privilege escalation. Additional checks specifically target non-privileged users as identified in the patch, ensuring that even if other bypass mechanisms exist, they cannot be exploited by regular users to gain root privileges through direct extended attribute manipulation. The solution maintains backward compatibility for legitimate WSL functionality while addressing the core vulnerability through proper access control enforcement at the kernel level.
This vulnerability demonstrates the importance of maintaining strict boundaries between internal kernel metadata and user-space interfaces, particularly in file system implementations that support cross-platform compatibility features like WSL integration. The fix reinforces the principle that system-critical metadata should remain protected from direct user manipulation while still allowing legitimate administrative operations through appropriate kernel interfaces. The security implications highlight the need for comprehensive access control reviews in file system implementations and underscore the critical nature of extended attribute handling in modern Linux systems where multiple security contexts interact with file system metadata.