New FROST Attack Lets Websites Track What Sites and Apps You Open via SSD Timing
Researchers say a malicious site can infer what other sites and apps a user opens by measuring SSD timing from inside the browser.
Intelligence analysis by GPT-5.4 Mini
FROST is a browser-based side-channel attack that uses JavaScript and OPFS storage timing to detect activity on the same disk. The researchers report high identification accuracy on macOS and say the attack needs no native code, extension, or permission prompt.
A website can act like it is listening to a busy hallway. If other apps or sites make the hard drive busy, tiny delays show up. FROST uses those delays like footprints to guess what is open on the computer.
Analysis
How FROST works
The attack abuses the browser’s Origin Private File System (OPFS), a storage feature available in major desktop browsers. A malicious page creates a file larger than RAM, forcing reads to hit the SSD instead of staying in cache. It then loops over random 4 KB reads and measures timing changes with JavaScript. When a user opens a site or launches an app that also touches the same drive, the disk becomes busier and the read timings shift in ways the page can detect.
To make those measurements precise enough, the attacker can enable cross-origin isolation on its own page, which restores better timer resolution. A neural network trained on those timing traces then classifies the activity. The researchers from Graz University of Technology say the attack works in the browser sandbox, so it does not need native code or a browser extension.
What the paper claims
The paper reports strong results on macOS: a closed-world test against the top 50 websites reached an F1 score of 88.95%, and an open-world test with 300 unseen sites still scored 86.95%. For ten pre-installed macOS apps, the classifier reached 95.83%. The team also built a covert channel over the same signal, with throughput of 661.63 bit/s on Linux and 719.27 bit/s on macOS.
Limits and defenses
The attack only sees activity on the same disk as the OPFS file, so separate drives can reduce exposure. On Linux, profile-sync-daemon can incidentally blunt the zero-click version by keeping browser profiles in RAM. The article says browser makers were notified before publication, but no CVE exists and there is no public evidence of in-the-wild use. Proposed mitigations include capping OPFS size, throttling timers while OPFS is in use, or adding a permission prompt, but each would trade away some speed or usability.
Key points
- FROST is a browser-based side-channel attack that uses SSD timing to infer what sites and apps a user opens.
- It runs in JavaScript, needs no extension or native code, and abuses Origin Private File System storage.
- The researchers report strong macOS results, including high site and app classification accuracy.
- The paper also demonstrates a covert channel over the same timing signal.
- Browser makers were notified, but there is no CVE and no public evidence of real-world abuse.
The disclosure may push browser makers to rethink how much disk access OPFS should allow and whether stronger limits or prompts are needed. If mitigations land, normal web apps could keep useful offline storage while reducing the chance of silent tracking.
If browser vendors keep treating this as out of scope, websites may continue gaining near-native visibility into local activity through timing tricks. The broader risk is that OPFS is only one example of a pattern where web features intended for convenience also create new side channels.
