A sophisticated browser-based surveillance FROST technique, short for Fingerprinting Remotely using OPFS-based SSD Timing, that enables malicious websites to silently profile user activity by exploiting solid-state drive timing patterns entirely from within JavaScript.
FROST, developed by researchers at Graz University of Technology, targets the Origin Private File System (OPFS) API, a relatively new browser feature that allows web applications to create and store files on a user’s local disk without triggering any permission dialog.
While OPFS was designed for legitimate productivity applications like in-browser IDEs and video editors, the researchers discovered it could be weaponized to generate measurable SSD contention, turning the drive’s own access latency into a covert surveillance channel.
Sites Track Users Through SSD
The attack begins the moment a victim visits a malicious webpage. The attacker’s JavaScript silently creates an oversized OPFS file that occupies up to 60% of the available disk space on Chrome and Safari, and continuously performs random 4 KB reads against it.
Because the file exceeds the operating system’s page cache capacity, every read forces a genuine disk access.
When the victim opens other websites or applications simultaneously, their SSD I/O competes with the attacker’s reads, producing detectable latency spikes that leak behavioral fingerprints, Hannes Weissteiner said.
What distinguishes FROST from prior SSD side-channel research is its complete elimination of the need for native code execution. Earlier attacks relied on privileged kernel interfaces like Linux’s io_uring API, restricting their real-world applicability.
FROST operates entirely within the browser sandbox with no special privileges, no user clicks, and no suspicious permission prompts.
To achieve high-resolution timing measurements, the attack configures Cross-Origin Opener Policy (COOP) and Cross-Origin Embedder Policy (COEP) HTTP headers on the attacker’s server, unlocking nanosecond-level timers in the browser and bypassing default timer-fuzzing protections.
Critically, because contention occurs at the hardware storage level, FROST works cross-browser; the malicious site running in Chrome can spy on activity happening in Safari or any other application accessing the same SSD.
The researchers validated FROST across two categories of attacks on macOS Sonoma (Apple M2, 8 GB RAM) and Linux (AMD Ryzen 7 5800X3D, 32 GB RAM):
- Covert Channel Capacity: FROST achieved a true channel throughput of 661.63 bits/s on Linux and 891.77 bits/s on macOS using OPFS, enabling data exfiltration from a native application to a malicious website without any user interaction.
- Website Fingerprinting: A Convolutional Neural Network (CNN) trained on OPFS timing traces identified which of the top 50 websites a macOS user visited, achieving F1 scores of 88.95% (closed-world) and 86.95% (open-world).
- Application Fingerprinting: FROST identified which native macOS application from a set of 10, including Safari, Keynote, Maps, and Music, a user had launched with a 95.83% F1 score, demonstrating surveillance capability beyond just web browsing.
The Hannesweissteiner disclosed their findings to Google Chromium, Mozilla Firefox, and Apple Safari security teams.
The responses were largely dismissive: Google stated it does not consider fingerprinting a security vulnerability; Apple called the attack currently out of scope while hinting at potential future mitigations; Mozilla acknowledged the findings but has not yet implemented fixes.
Mitigations
- Limit OPFS file sizes to amounts that fit within system RAM (e.g., 1 GB), preventing page cache bypass.
- Restrict high-resolution timers when OPFS is actively in use
- Implement cross-origin OPFS tracking, alerting users when large amounts of storage are rapidly consumed across multiple origins
- Require explicit user permission before any site accesses OPFS, though researchers warn this would disrupt legitimate browser applications
Until browser vendors treat storage-based fingerprinting as a genuine privacy threat, everyday users browsing on macOS and Linux remain silently exposed to this class of attack every time they visit an untrusted website.
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