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CVE-2026-4692 is a critical protection-mechanism failure in Mozilla Firefox and Thunderbird's Responsive Design Mode component. The flaw allows a compromised restricted content process to cause execution to cross the browser sandbox boundary into the privileged parent process. The corrected validation restricts the affected transaction to the parent process, requires a top-level browsing context, and rejects transactions whose source is a child process. Fixed releases include Firefox 149, Firefox ESR 115.34 and 140.9, and Thunderbird 149 and 140.9.
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1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This is a nine-file, standalone Firefox browser-exploitation proof of concept for CVE-2026-4692, not a framework module. Its primary component, bctx.html, is a malicious web page that uses compact prebuilt WebAssembly modules from wasm-bytes.js to establish arbitrary native memory read/write and native function-call capabilities in a compromised Firefox content process. The page discovers hard-coded Firefox/XUL process state, opens a popup, derives that popup's top-level BrowsingContext ID, and sends a forged PContent::CommitBrowsingContextTransaction directly through Firefox IPC. The transaction selects field index 23 and sets InRDMPane to true, exploiting the missing parent-side authorization validation in vulnerable builds. forge_bctx.py generates and self-verifies the static serialized message template (bctx.bin and bctx.json), leaving only the BrowsingContext ID for runtime patching. srv.py exposes the assets with a simple request-logging HTTP server on port 8781. mdrive2.py launches a local, custom Firefox build with Marionette on localhost:2828 and navigates to the page after a delay. irun orchestrates profile setup, browser launch, LLDB attachment to the parent process, log collection, and evidence extraction. profile.user.js configures Fission, multi-process behavior, popup allowances, and diagnostic output. The code is operational but narrowly build-dependent: offsets, function addresses, paths, architecture, and build revision are hard-coded. It contains no shell payload. Its demonstrated effect is a privileged browser-state forgery; the README describes a possible follow-on attack in which forged RDM state permits synthesized touch events to target privileged browser chrome UI.
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12 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Firefox sandbox escape in Responsive Design Mode.
A critical browser sandbox-escape vulnerability that allows code executing in a restricted child/content process to gain parent-process privileges, enabling arbitrary commands and access to local browser data and files.
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Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
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Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.