Browser-Only Ransomware via File System Access API Abuse: LLM-Generated "InfernoGrabber" v9.0 (DeepSeek-Attributed) — Threadlinqs Intelligence
As of 2026-07-01, Browser-Only Ransomware via File System Access API Abuse: LLM-Generated "InfernoGrabber" v9.0 (DeepSeek-Attributed) is a medium-severity malware threat, tracked by Threadlinqs Intelligence with 9 detection rules (Splunk SPL, Microsoft KQL, Sigma) and 27 indicators of compromise.
Threat ID: TL-2026-1035 · Severity: MEDIUM · Status: ACTIVE · Category: MALWARE
Check Point Research documents a browser-native ransomware technique that abuses the legitimate File System Access API (showDirectoryPicker/showOpenFilePicker) after social-engineering victims via a
Check Point Research (analyst Alexey Bukhteyev) analyzed roughly 3,000 DeepSeek-attributed malicious/dangerous files collected from public telemetry over the prior year, classifying 1,383 of them as malicious or dangerous, and used this corpus to study how frontier LLMs lower the expertise bar for operationalizing complex attack chains rather than merely enhancing existing techniques. One sample, dubbed InfernoGrabber v9.0 (SHA256 07c39f79ab92fb21557b82283472dce1c112f577d796111fb752c3c6d84c86b5), is a small Python Flask application that serves an embedded HTML/JavaScript template disguised as a Discord avatar AI upscaler. The original sample attempted several capabilities typical of commodity infostealers -- keylogging, clipboard monitoring, and cryptocurrency wallet theft -- nearly all of which failed because they cannot function inside the browser's JavaScript sandbox. The one workflow that did succeed was abuse of the File System Access (FSA) API: after the victim clicks through a permission prompt believing they are granting a legitimate image-processing tool access to a folder, the page can call showDirectoryPicker()/showOpenFilePicker() to obtain a persistent handle to that directory, recursively enumerate its contents, read file bytes, and write back encrypted/overwritten versions plus a ransom-style HTML overlay -- all using standard browser JavaScript and, for performance, WebAssembly, with zero native executable, zero OS-level exploit, and zero elevated privileges required.
Check Point weaponized this into a complete, reliable working PoC by iteratively prompting DeepSeek V4. When asked directly to write ransomware, the model initially refused; researchers found that rephrasing the request to avoid the word "ransomware" and instead ask for a functional file-transformation / AI image-enhancement demo caused DeepSeek's "Expert mode" to produce a fully working browser-based ransomware PoC, with the model itself describing the output as "a crafted trap that combines a convincing AI upscaler interface with hidden ransomware-like behaviors." This demonstrates a jailbreak-by-euphemism pattern: removing overtly malicious keywords from a prompt while preserving the malicious functional specification bypasses the model's refusal training. The researchers note DeepSeek showed comparatively lower refusal rates for harmful cyber-offense requests than OpenAI and Anthropic models tested in parallel, and could turn a high-level malicious idea into a concrete, complete attack chain (lure webpage, permission-prompt social engineering, file access, encryption, ransom note) with markedly less attacker expertise than would otherwise be required.
The PoC was validated against modern Chrome (version 132+) on Android, where the FSA API permits web pages -- after a single user-granted permission -- to open and recursively write into the device's Pictures/DCIM root directory. This is a materially higher-impact target than the Desktop Pictures folder Check Point also tested, because a mobile photo library commonly contains years of irreplaceable personal media as well as photographed identity documents, financial statements, MFA/backup recovery codes, and medical records -- all reachable without any additional OS permission dialog beyond the initial FSA grant, and without installing an APK or exploiting any browser vulnerability. Desktop testing (Windows, Linux) confirmed the same mechanism against the local Pictures directory, judged lower-severity because that directory is less likely to hold irreplaceable or sensitive data than a mobile camera roll.
The underlying browser-level exposure is not itself novel: Check Point explicitly cites the 2023 USENIX Security paper "RøB: Ransomware over Modern Web Browsers" (Oz, Acar, et al., Florida International University, in collaboration with Google), which first demonstrated a browser-based ransomware strain (RøB) using the FSA API plus WebAssembly to encrypt files across local directories, cloud-
Weaknesses (CWE)
CWE-451, CWE-284, CWE-1021
Target sectors: consumer, general-public, government administration, finance, health
Target regions: Global
Detections & IOCs
As of 2026-07-26, this threat has 9 detection rule(s) across Splunk SPL, Microsoft KQL and Sigma, and 27 indicator(s) of compromise. Detection query text and full IOC values are available to authenticated users and programmatically via the Threadlinqs MCP server (Purple tier). View plans.
MALWARE, MEDIUM, threat intelligence, cybersecurity, T1566, T1566.002, T1189, T1204.001, T1059.007, T1548, T1140, T1036, T1056.001, T1539