CVE-2025-68788: Linux Kernel Information Disclosure Flaw

CVE-2025-68788 Overview

A side-channel information disclosure vulnerability has been identified in the Linux kernel's fsnotify subsystem. The vulnerability allows users with read access to a parent directory (such as /dev) to subscribe to ACCESS/MODIFY events on child special files, even without direct read access to those files. This behavior differs from standard file access patterns and can potentially be exploited as a side-channel for information exfiltration.

Critical Impact

Attackers may leverage this side-channel to monitor file access patterns on special files like /dev/null, potentially enabling information exfiltration about system activity without requiring direct access to the target files.

Affected Products

• Linux Kernel (multiple versions)
• Systems with inotify/fanotify implementations
• Linux-based operating systems with fsnotify enabled

Discovery Timeline

• 2026-01-13 - CVE CVE-2025-68788 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2025-68788

Vulnerability Analysis

The fsnotify subsystem in the Linux kernel provides inotify and fanotify mechanisms that enable processes to monitor filesystem events. While these systems correctly prevent users without read access from directly subscribing to events on a file, they permit the same users to watch events on children when they have read access to the parent directory.

For regular files, this behavior aligns with expected security boundaries—users can still observe access and modification times via stat() calls. However, special files (such as device nodes in /dev) behave differently. When other users read or write to special files like /dev/null, the atime/mtime values typically do not change, except when explicitly modified via utimensat().

This inconsistency creates a side-channel where an attacker monitoring the parent directory can detect read/write operations on special files that would otherwise be invisible through normal stat() observations.

Root Cause

The root cause lies in the fsnotify implementation generating ACCESS/MODIFY events to parent directory watchers when read/write operations occur on special files. This behavior does not align with the actual visibility users would have through standard filesystem interfaces, where atime/mtime changes on special files are not observable during normal read/write operations.

Attack Vector

An attacker with read access to a parent directory containing special files (such as /dev) can set up an inotify or fanotify watch on that directory. When other users or processes interact with special files within that directory, the attacker receives ACCESS/MODIFY events that reveal activity patterns. This information could be used to:

• Infer timing of cryptographic operations
• Monitor inter-process communication patterns
• Track user activity through device file access
• Correlate events for timing-based attacks

The attack does not require elevated privileges beyond standard read access to directories containing special files.

Detection Methods for CVE-2025-68788

Indicators of Compromise

• Unusual inotify or fanotify watch registrations on directories containing special files
• Processes monitoring /dev or similar directories for extended periods
• Unexpected system calls related to inotify_add_watch() or fanotify_mark() targeting special file directories
• Anomalous read patterns from untrusted processes on system directories

Detection Strategies

• Monitor inotify_init() and fanotify_init() system calls for suspicious patterns
• Audit filesystem watch registrations on sensitive directories like /dev, /proc, and /sys
• Implement behavioral analysis to detect processes correlating filesystem events with other system activity
• Review process access patterns to identify potential reconnaissance activity

Monitoring Recommendations

• Enable auditd rules to log inotify and fanotify operations on critical directories
• Deploy endpoint detection solutions capable of identifying side-channel attack patterns
• Monitor for processes with unusual combinations of limited file permissions but extensive directory watch capabilities
• Implement SentinelOne's behavioral AI to detect anomalous filesystem monitoring patterns

How to Mitigate CVE-2025-68788

Immediate Actions Required

• Apply the latest Linux kernel patches that address this vulnerability
• Review and restrict access to sensitive parent directories where possible
• Audit existing inotify/fanotify watches on production systems
• Consider namespace isolation for sensitive processes to limit exposure

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability by aligning fsnotify event generation with actual stat() visibility. The patches prevent ACCESS/MODIFY events from being generated to parent watchers for read/write operations on special files, while still permitting these events when atime/mtime is explicitly modified via utimensat().

Relevant kernel commits are available:

• Linux Kernel Commit 635bc4d
• Linux Kernel Commit 6a7d7d9
• Linux Kernel Commit 7a93edb
• Linux Kernel Commit 82f7416
• Linux Kernel Commit e0643d4

Workarounds

• Restrict directory permissions on sensitive paths to limit who can establish watches
• Use SELinux or AppArmor policies to control inotify/fanotify capabilities
• Implement process isolation using containers or namespaces to limit watch scope
• Consider monitoring and alerting on suspicious inotify/fanotify activity as a compensating control

# Example: Audit inotify watch creation with auditd
auditctl -a always,exit -F arch=b64 -S inotify_add_watch -k fsnotify_watch
auditctl -a always,exit -F arch=b64 -S fanotify_mark -k fsnotify_watch