isolated-vm sandbox escape (GHSA-864f-rcv7-6rh4): a TOCTOU type-confusion in ExternalCopy's transferList marshaling breaks the V8 Isolate guest/host boundary — the sandbox underneath a wide range of AI-agent and low-code automation platforms

isolated-vm gives each untrusted-JavaScript sandbox its own V8 Isolate — a separate heap with no shared object graph with the host, the same primitive Chrome uses to separate tabs. Endor Labs (credited discoverer, no CVE assigned as of this run) found that the isolation primitive itself held; what broke is the C++ marshaling code that copies values across the boundary: "we did not break the V8 Isolate. We broke the code that carries data into it" (Endor Labs, 2026-08-20).

ExternalCopy's transferList option — a postMessage-style zero-copy ArrayBuffer transfer — walks the transfer-list array twice in src/external_copy/serializer.cc: the first walk validates each element with IsArrayBuffer() and registers it; the second performs an unchecked handle.As<ArrayBuffer>() reinterpret-cast with no re-validation. Because transfer_list is a real JavaScript array and both walks read it via a genuine property access that invokes JS accessors, a guest can define transferList[0] as a getter that returns a real ArrayBuffer on the first read — satisfying walk 1's check — and an attacker-chosen value on the second read; the unchecked cast then treats that value as an ArrayBuffer, yielding a controlled-address read/write primitive: "As <ArrayBuffer>() is not a checked conversion. It is a bare reinterpret-cast that tells V8, 'trust me, this is an ArrayBuffer.' The code assumes it is safe because walk 1 already checked, but that assumption only holds if the two walks see the same values" (Endor Labs, 2026-08-20). Endor Labs escalated this from a single ivm.Reference — the minimum capability any embedder must hand a sandbox for it to do anything at all — to full host control-flow hijacking, demonstrating a complete guest-to-host escape.

Fixed in 7.0.1 (mainline) and 6.2.0 (6.x backport), both released 2026-08-08, by wrapping ExternalCopy::Copy in a v8::Isolate::DisallowJavascriptExecutionScope that prevents any guest JS — getters, proxies, interceptors — from running during the copy, closing the time-of-check-to-time-of-use window outright: "the maintainer responded quickly and shipped a fix in versions 7.0.1 and 6.2.0. The patch wraps ExternalCopy::Copy in a v8::Isolate::DisallowJavascriptExecutionScope, which prevents any user JavaScript (getters, proxies, interceptors) from running during the copy" (Endor Labs, 2026-08-20).

isolated-vm (1M+ weekly downloads) is the sandbox of record for a wide range of AI-agent and automation platforms that execute model- or user-generated code: Endor Labs names n8n (which recommends isolated-vm for its Code-node task runners), Activepieces, Mastra AI (its "code mode" tool-orchestration execution), Budibase (which migrated off the deprecated vm2), Sim.ai, Directus and Rocket.Chat as production consumers whose guest/host boundary is this exact library. The absence of a CVE identifier does not soften the exposure: an anonymous, single-call escape from the isolation primitive that a widely-deployed class of self-hosted automation platforms advertises as its safety boundary is the class of flaw that demands action ahead of a routine patch cycle regardless of a formal severity score.

Triage: any self-hosted platform advertising sandboxed code execution via isolated-vm should confirm its patched version directly — a version check against the platform's own release notes rather than against isolated-vm's version alone, since embedders bundle it at different cadences. There is no telemetry-side discriminator for this flaw once patched; the mitigation is entirely upgrade-based, since the vulnerable code path executes inside the sandbox implementation itself rather than producing an externally observable behavioral signature before the escape completes.