CVE-2024-27123: QcalAgent XSS Vulnerability

CVE-2024-27123 Overview

CVE-2024-27123 is a cross-site scripting (XSS) vulnerability affecting QNAP QcalAgent. Local attackers can leverage the flaw to bypass security mechanisms or read application data. QNAP addressed the issue in QcalAgent version 1.1.9 and later. The weakness is categorized under CWE-79, Improper Neutralization of Input During Web Page Generation.

Critical Impact
Successful exploitation allows a local attacker to execute injected script content in the context of QcalAgent, exposing application data and enabling security control bypass.

Affected Products

QNAP QcalAgent versions prior to 1.1.9
QNAP NAS devices with QcalAgent installed
Environments relying on QcalAgent for calendar synchronization

Discovery Timeline

2026-09-18 - CVE-2024-27123 published to NVD
2026-09-18 - Last updated in NVD database

Technical Details for CVE-2024-27123

Vulnerability Analysis

The vulnerability is a stored or reflected cross-site scripting flaw in QcalAgent, a QNAP calendar synchronization utility. QcalAgent fails to properly neutralize user-supplied input before rendering it in the application interface. A local attacker who can supply crafted input to the application triggers execution of arbitrary script code within the QcalAgent context.

Once script execution succeeds, the attacker can read application data accessible to QcalAgent and bypass client-side security mechanisms. The attack requires user interaction, limiting mass exploitation but still enabling targeted abuse on shared or multi-user NAS deployments. The EPSS probability is 0.129%, indicating low observed exploitation activity to date.

Root Cause

The root cause is improper output encoding, tracked under CWE-79. QcalAgent renders attacker-controlled input directly into a web view or user interface component without sanitizing HTML, JavaScript, or event handler content. The application trusts input that should be treated as untrusted data.

Attack Vector

The attack vector is local. An attacker must have local access to interact with QcalAgent and requires user interaction to trigger the payload. Exploitation typically involves crafting a malicious payload embedded in calendar entries, event fields, or other input handled by QcalAgent. When a victim views the affected content, the browser or web view executes the attacker-controlled script.

No verified public exploit code is available. Refer to the QNAP Security Advisory QSA-24-54 for vendor technical details.

// No verified proof-of-concept code is publicly available for CVE-2024-27123.
// Consult the QNAP security advisory for technical details.

Detection Methods for CVE-2024-27123

Indicators of Compromise

Unexpected <script> tags, javascript: URIs, or HTML event handlers such as onerror and onload inside QcalAgent data fields
Anomalous outbound network requests originating from processes hosting QcalAgent web views
Unauthorized modifications to calendar entries or synchronization metadata

Detection Strategies

Inspect QcalAgent input fields and stored calendar data for HTML or JavaScript payloads
Review QNAP NAS logs for unusual QcalAgent activity, including unexpected local user sessions
Compare installed QcalAgent versions against the fixed release 1.1.9 across the fleet

Monitoring Recommendations

Enable verbose logging on QNAP NAS devices running QcalAgent and forward logs to a central analytics platform
Alert on new or modified files under QcalAgent data directories following user login events
Monitor for outbound connections from QcalAgent processes to previously unseen hosts

How to Mitigate CVE-2024-27123

Immediate Actions Required

Upgrade QcalAgent to version 1.1.9 or later on all affected QNAP NAS devices
Inventory QNAP appliances to identify installations exposed to local multi-user access
Restrict local access to trusted administrative users until patching completes

Patch Information

QNAP addressed CVE-2024-27123 in QcalAgent version 1.1.9 and later. Apply the update through the QNAP App Center or download it from the vendor. See the QNAP Security Advisory QSA-24-54 for release details and download links.

Workarounds

Disable or uninstall QcalAgent on NAS devices where calendar synchronization is not required
Limit local account access to the NAS to reduce the attack surface for local XSS exploitation
Enforce the principle of least privilege for user accounts that interact with QcalAgent

bash

# Verify installed QcalAgent version on a QNAP NAS via SSH
qpkg_cli --list | grep -i QcalAgent

# Remove QcalAgent if not required
qpkg_cli --remove QcalAgent