Graz researchers show that Windows, Linux, Android and macOS let an unprivileged watcher reconstruct keystroke timing, browsing and WhatsApp activity from file events alone. On a single-user laptop that is a curiosity. On a virtual desktop host or a phone full of third-party apps it is a real exposure, and the vendors' responses are weakest exactly there.
Every modern operating system has a way for a program to ask when a file changes. Backup tools need it, as do sync clients, IDEs and antivirus scanners. A team at Graz University of Technology, led by doctoral student Sudheendra Raghav Neela, has shown that this plumbing, which dates back to Windows 2000 and Linux's 2005 kernel, tells an eavesdropper far more than intended. A program with no special privileges can watch file events it has no right to read and work out when you type, which websites you open and when you send a photo on WhatsApp.
The headlines call these decades-old flaws in every major OS. That is accurate, but not very useful. Nobody reads your files through this route and nothing runs remotely. The attacker needs code already on the machine. For most single-user laptops, the vendors' shrug is justified. It is wrong where "someone else's code on the same machine" is normal: shared Windows hosts and Android phones. In both, the vendor has either left the mitigation switched off or not shipped one at all.
The mechanism is simple. Linux's inotify, Android's FileObserver, Windows' ReadDirectoryChangesW and macOS's FSEvents check whether you can see a directory, not whether you can read what's in it. The events they report, including file names, access times and modifications, are metadata that reveals behaviour. Firefox writes to cache folders named after the sites you visit. A keyboard device file ticks each time a key is pressed. WhatsApp drops a named file into its media folder for every picture sent or received.
The numbers in the paper, due at the ACM CCS conference in The Hague in November, are strong. On Linux, watching the parent directory of the input devices recovered inter-keystroke timing with 93% to 100% accuracy across seven users, and 100% on SSH sessions. Keystroke timing is a known route to guessing passwords. On Windows, watching the root of the system drive reveals the full path of every file changed on the machine, including inside other users' home folders, and identified which of the top 1,000 websites a Firefox user was on with 97.8% accuracy. The team also built a UI-redress attack that uses the timing to steal credentials. SecurityWeek's summary sets out the results by platform.
The vendors' responses differ. Linux kernel maintainers were cooperative. A partial fix, tracked as CVE-2025-68788, landed in December 2025 across the long-term kernel branches and blocks access and modify events on device files. Microsoft told the researchers the Windows behaviour was "by design" and an "undocumented feature", The Register reported. It then added a registry policy, EnforceDirectoryChangeNotificationPermissionCheck, that closes the hole. The policy is disabled by default. Google and Apple have not shipped fixes. macOS leaks least because FSEvents only reports on files that are already readable to everyone. The research team's site documents the disclosure, which ran from August to October 2025.
The Microsoft decision has the widest commercial reach. Compatibility explains why the check is off. Plenty of legitimate software watches whole volumes, and a default change could break backup agents and indexers across millions of machines. That is a fair engineering concern on a laptop with one user. It fails on a multi-session host, such as Azure Virtual Desktop, Remote Desktop Services or a Citrix farm, where dozens of employees share one Windows instance by design. There, the "local attacker" Microsoft treats as an edge case is simply a colleague, a contractor or malware one careless user clicked on. Watching the system drive root on such a host would show what every other session is opening, in real time, with no alert raised.
Android is the more serious case in principle. Its security model depends on one app being unable to observe another. Scoped storage and the FUSE layer exist to enforce that, and FileObserver goes around both. The researchers showed that an app with no permissions could reconstruct a timeline of WhatsApp media sent and received, with file names and types. On a phone, the "local attacker" is any app in the store. Google has seen this kind of problem before. It locked down app access to process information under /proc after academic work in the mid-2010s showed apps could infer each other's activity from it. The same fix, restricting the API to directories the caller can actually read, is available here.
None of this has been seen in the wild, and an attacker who can run code locally has better options than inferring keystroke timing. On a personal laptop that is largely true. A process running as you can read your files directly, so the side channel adds little. The attack matters because it works across users and sandboxes, where direct reads are blocked. That is why it matters on shared hosts and phones and hardly at all elsewhere. The proof-of-concept code is public on GitHub, and techniques this cheap do not stay academic for long.
The technical debt here is conceptual rather than old code. Operating system permissions were built to protect file contents. The notification systems were added later for convenience and inherited only a directory-level check. Twenty years on, the metadata is often more revealing than the content, and the permission model has not caught up. The Graz group, led by Daniel Gruss, co-discovered Meltdown and Spectre. The pattern is familiar: a performance or convenience feature leaks, vendors argue about whether the leak is a bug, and fixes arrive once the costs cannot be avoided.
For IT teams the practical ranking is clear. Anyone running multi-session Windows or shared Linux build servers should turn on the Microsoft policy, and confirm current long-term kernels, this quarter after testing their backup and indexing tools. Single-user laptops can wait for vendor defaults. Mobile teams should assume any app on an unmanaged Android device can see when files land in another app's media folder.
We expect Google to move before Microsoft does. The Android leak breaks a sandbox promise that Google sells to users and regulators, while Microsoft can point to a policy switch that already exists. Look for an Android fix in a monthly security bulletin by the time the paper is presented in mid-November. Windows will stay off by default into 2027, and administrators of shared hosts will have to switch it on themselves.