Summary

In some Notification types (e.g., Webhook, Telegram), the send() function allows user-controlled renderTemplate input. This leads to a Server-side Template Injection (SSTI) vulnerability that can be exploited to read arbitrary files from the server.

Details

The root cause is how Uptime Kuma renders user-controlled templates via renderTemplate(). The function instantiates a Liquid template engine and parses the template argument without sanitization:

async renderTemplate(template, msg, monitorJSON, heartbeatJSON) {
    const engine = new Liquid();
    const parsedTpl = engine.parse(template);

    // ...
}

In some Notification flows, the send() implementation passes user-editable fields directly into renderTemplate():

// webhook.js
if (notification.webhookContentType === "form-data") {
    const formData = new FormData();
    formData.append("data", JSON.stringify(data));
    config.headers = formData.getHeaders();
    data = formData;
} else if (notification.webhookContentType === "custom") {
    data = await this.renderTemplate(notification.webhookCustomBody, msg, monitorJSON, heartbeatJSON); //<- this line cause SSTI
}

Because notification can be edited by users and is rendered by the Liquid engine without proper sandboxing or a whitelist of allowed operations, an attacker can supply a crafted template that causes the server to read arbitrary files. In particular, Liquid’s template tags (e.g. {% render ... %}) can be abused to include server-side files if the engine is not restricted, resulting in Server-side Template Injection (SSTI) that leaks sensitive file contents.

PoC

1. Open Uptime Kuma → Notifications → Add or Edit an existing Webhook notification.
2. Set notification type to Webhook and set Request Body to Custom Body.
3. Paste the following JSON into the custom request body:

{
  "Title": {% render '/etc/passwd' %}
}

4. Click test.
5. Your webhook will receive the file content

Impact

This is a post-authentication Server-side Template Injection (SSTI) vulnerability that allows an authenticated user to perform arbitrary file read on the server.