Uptime Kuma Server-side Template Injection (SSTI) in Notification Templates Allows Arbitrary File Read
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
- Open Uptime Kuma → Notifications → Add or Edit an existing Webhook notification.
- Set notification type to Webhook and set Request Body to Custom Body.
- Paste the following JSON into the custom request body:
{
"Title": {% render '/etc/passwd' %}
}
- Click test.
- 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.
| Software | From | Fixed in |
|---|---|---|
uptime-kuma
|
2.0.0-dev.0 | 2.0.0-dev.0.x |
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