# Researchers turn ordinary file-change notifications into a way to watch other users

> Graz researchers show file-change notifications on Linux, Windows, Android and macOS can reveal typing rhythm, browsing and app activity. macOS leaks the least.

HisnLabs Security & Research Team (HisnLabs) · Published 2026-09-26
Canonical: https://hisnlabs.com/en/news/file-notifications-leak-user-activity-graz

Every modern operating system lets an app ask to be told when a file is created, changed or deleted, a feature used by text editors, file managers, sync clients and antivirus products, and one that needs no special privileges beyond read access to the folder being watched. Researchers at Graz University of Technology, in Austria, have shown that this ordinary feature, on Linux, Android, Windows and macOS, can be used to monitor other people on the same system, [SecurityWeek reported](https://www.securityweek.com/windows-linux-android-file-notification-systems-leak-user-activity/) on 25 September.

## Background

File-change notification is a standard part of every mainstream operating system: it is how a cloud-sync client notices a new file without constantly re-scanning a disk, and how a text editor reloads a document someone else just edited. Because reading which files changed, and when, has never required administrator rights, it has not traditionally been treated as a channel an attacker could use to spy on somebody else's activity. The Graz team's research, published with a demonstration site and proof-of-concept code on GitHub, tests that assumption across four operating systems [[1]](https://www.securityweek.com/windows-linux-android-file-notification-systems-leak-user-activity/).

## Findings

None of the attacks read what is inside a file. What they collect is file names and the timing of events, which the researchers showed is enough to reconstruct a lot of what a user, an app or the system is doing. Most scenarios assume the attacker already runs code on the machine, under a separate account; on Android, the attacking app needs no permissions at all [[1]](https://www.securityweek.com/windows-linux-android-file-notification-systems-leak-user-activity/).

- On Linux, watching the folder around the device files that represent keyboard input reliably revealed when a key was pressed, across seven test typists, though not which key. Keystroke timing has been studied for more than two decades as a way to help infer what someone types; input that shows nothing on screen, such as a hidden sudo password, was not captured.
- Also on Linux, the set of system fonts Firefox loads for a page reliably identified which of the top 100 sites was being visited, and the team demonstrated a fake password prompt on KDE Plasma 6 (Wayland), drawn over the real one at the right moment.
- On Android phones, tested on a Google Pixel and Samsung Galaxy models, an app with no permissions could watch another app’s private storage folder. Against WhatsApp, that revealed when photos, videos and documents were sent, received or deleted, along with their file names.
- On Windows, monitoring the root of the system drive reported the full path of every changed file on the machine, including inside other users’ home folders. Because Firefox stores data in folders named after websites, another user’s browsing could be followed in real time; the same test against Edge worked, but less reliably, because Edge creates folders for fewer sites.

macOS leaked the least of the four systems, the researchers found, because only files that are readable by everyone can be monitored there. Even so, they could still track application launches, interactions with apps, and changes to settings [[1]](https://www.securityweek.com/windows-linux-android-file-notification-systems-leak-user-activity/).

> Whatever an app learns on your Mac, it has to send it somewhere to use it. Try FireAI free for 17 days and watch where your apps connect. [Download FireAI for Mac](https://hisnlabs.com/fireai/en/download)

## Implications for Mac users

The precondition matters more on a Mac than the leak itself: almost every scenario in this research starts with someone else's code already running on the machine, under a separate account, which is not the situation of a single person using their own Mac with no other local users. Where it applies most directly is shared Macs, such as a family or office computer with several accounts, and the researchers found macOS the most resistant of the four systems tested, since a monitoring app there is limited to files that are readable by everyone [[1]](https://www.securityweek.com/windows-linux-android-file-notification-systems-leak-user-activity/).

## Recommendations

1. Install software only from sources you trust; nearly every scenario in this research requires an attacker’s code to already be running on the machine.
2. Give each person who uses a shared Mac their own account rather than one shared login.
3. Keep macOS up to date so any future hardening of file-notification behaviour reaches your machine.
4. On Linux systems you manage, check whether the kernel update addressing CVE-2025-68788 has been applied.

## Relevance to FireAI

FireAI does not stop an app from watching file-change events on a Mac; that happens entirely on the device, before any network connection is made, and no firewall can see it. What FireAI controls is what apps send out over the network, which is where information gathered this way has to go before it is of any use to whoever collected it.

FireAI asks the first time any app tries to reach the internet and remembers the answer, shows every destination on the [world map](https://hisnlabs.com/fireai/en/docs/world-map), and lets a user block an app outright. A small utility installed last week that suddenly wants to talk to a server abroad is exactly the kind of connection it puts in front of the user, whatever the utility is doing on disk beforehand.

> A firewall can’t see what an app watches on disk, but it can see where that app sends data next. Try FireAI free for 17 days. [Download FireAI for Mac](https://hisnlabs.com/fireai/en/download)

## Limitations

> **Note:** The Linux kernel has been partly hardened so device files no longer generate access and modify events, which the researchers say addresses the most severe issue there (CVE-2025-68788); they list no fixes for Android or macOS. Microsoft told the researchers the Windows behaviour is by design, saying the technique "requires an attacker to already have the ability to run code locally on a device under a separate user account and does not provide access to file contents." Apple and Google had not responded to SecurityWeek by the time of its report. The researchers say they are not aware of anyone using this technique in the wild.

Try [FireAI, by HisnLabs](https://hisnlabs.com/fireai/en/download) free for 17 days.

## Sources

- [SecurityWeek (Eduard Kovacs), 25 September 2026: Windows, Linux, Android File Notification Systems Leak User Activity](https://www.securityweek.com/windows-linux-android-file-notification-systems-leak-user-activity/)