Every major operating system - including Android, Windows, macOS, and Linux - has been silently leaking user activity through decades-old file-notification systems. Security researchers have discovered that these built-in mechanisms can be exploited to track keystrokes, monitor website visits, and steal credentials without requiring special privileges. The flaws allow unprivileged observers to spy on file events they should never have access to, creating a massive privacy blind spot.
A team at Graz University of Technology (TU Graz) revealed that attackers can exploit side-channel leaks in the subsystems designed to alert applications when files change. These include Linux inotify (introduced in 2005), Android FileObserver (2008), Microsoft ReadDirectoryChangesW (2000), and Apple FSEvents (2007). While the broadcasted events carry no actual file content, their timing and frequency create a rich side channel that exposes highly sensitive user behaviors.
We found decades-old bugs on these operating systems, all rooted in the file-notification subsystems that every modern OS ships to inform applications when files change.
- Sudheendra Raghav Neela, TU Graz
On Linux, researchers demonstrated that monitoring readable directories leaks events for files inside them, even without read permissions. By watching the /dev/input directory, attackers achieved an inter-keystroke timing attack with 93.1 to 100 percent accuracy locally, and 100 percent remotely over SSH. They also executed an authentication-prompt redress attack on KDE Plasma 6 under Wayland by monitoring /usr/bin/pkexec to spoof Polkit password dialogs.
Mobile devices inherit this exact behavior. Tests on a Google Pixel 9a and Samsung Galaxy A54 running Android 16 confirmed that watchers sitting on /sdcard can bypass scoped storage to view events outside an app's permitted view. Windows proved even more permissive, allowing unprivileged users to leak full file paths in unreadable directories, including other users' home folders. The researchers achieved a 97.8 percent F1 score when fingerprinting visits to the top 1,000 websites on Windows.
The full methodology is detailed in their upcoming ACM CCS 2026 paper, available at snee.la. Microsoft has stated that this side-channel leakage is considered "by design," meaning no immediate fix is planned from Redmond. Apple and Google have not issued public statements regarding the findings as of late September 2026.
How to Mitigate File-Notification Leaks
- Update Linux Kernels: The Linux kernel team released partial fixes in December 2025. Administrators must ensure their distributions are updated to kernels 5.10.248, 5.15.198, 6.1.160, 6.6.120, 6.12.64, or 6.18.3, which block access events for special files under
/dev. - Enforce Strict Directory Permissions: Since Windows and macOS lack immediate patches, defenders should rely on mandatory access controls and strict directory permissions to limit what unprivileged users can monitor.
- Isolate Applications: Utilize sandboxing to restrict how much of the file system an application can observe, particularly on Android devices where scoped storage can be bypassed.
The Ticking Time Bomb of Legacy APIs
The TU Graz findings expose a critical friction point in modern cybersecurity: features built for convenience in the early 2000s are fundamentally incompatible with today's zero-trust environments. When Microsoft ReadDirectoryChangesW and Linux inotify were designed, the threat model assumed that local users were inherently trusted. Today, a single compromised low-privilege app can weaponize these APIs to map out a user's entire digital life.
Microsoft's assertion that this behavior is "by design" is technically accurate but practically dangerous. It shifts the entire burden of defense onto enterprise security teams and app developers, who must now build complex sandboxing layers around OS-level flaws. This stance effectively normalizes side-channel leaks as an acceptable cost of doing business, leaving end-users vulnerable to sophisticated tracking.
Moving forward, operating system vendors will be forced to rethink how applications communicate. We are likely to see a shift toward granular, permission-based notification APIs that require explicit user consent, similar to how location and camera access are handled today. Until then, these decades-old subsystems remain an open window for anyone who knows how to listen.
