CVE-2025-66550

Executive Summary

CVE-2025-66550 is a medium-severity vulnerability in the Nextcloud Calendar application that allows a malicious user to craft a calendar event with a specially crafted attachment. When a victim views this event, the attachment triggers an automatic download of a file from the same Nextcloud server without requiring user confirmation. This bypasses the standard user consent mechanism for file downloads, potentially leading to unauthorized data exfiltration.

The vulnerability affects Nextcloud Calendar versions prior to 4.7.17 and 5.2.4. The CVSS score is 5.7 (Medium), with a low attack complexity and low privileges required. The EPSS score is 0.0%, indicating a low probability of active exploitation in the wild at the time of analysis. No public exploit code or Nuclei templates are currently available.

Supply Chain & Package Security

Nextcloud is a self-hosted, open-source file sync and share platform written in PHP and JavaScript. The Calendar app is a standard component distributed through the Nextcloud App Store. This vulnerability does not directly affect the core Nextcloud server but impacts the Calendar app module. Organizations using Nextcloud should verify their Calendar app version and apply the patch as part of their regular update cycle.

CVSS Breakdown & Threat Level

• CVSS Score: 5.7 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:N/A:N
• Attack Vector (AV): Network – The attack is carried out over the network.
• Attack Complexity (AC): Low – No special conditions are required for exploitation.
• Privileges Required (PR): Low – The attacker needs a valid Nextcloud user account.
• User Interaction (UI): Required – The victim must view the malicious calendar event.
• Scope (S): Unchanged – The vulnerability does not affect other components.
• Confidentiality (C): High – Successful exploitation can lead to unauthorized file access.
• Integrity (I): None – No data modification is possible.
• Availability (A): None – No denial of service is possible.

Threat Level: Medium. The vulnerability requires user interaction and a valid account, but it can lead to significant data leakage if exploited in a targeted attack.

Deep Dive & Mechanics

The vulnerability resides in the Nextcloud Calendar app's handling of event attachments. When a user creates a calendar event, they can attach files from the Nextcloud server. The attachment mechanism uses a download link that, when clicked, triggers a file download. In vulnerable versions, the download occurs automatically without the user confirming the action.

The attack flow is as follows:

1. An authenticated malicious user creates a calendar event.
2. The attacker adds an attachment that links to a sensitive file on the same Nextcloud server (e.g., a shared document or a file in a public folder).
3. The attacker shares this calendar event with a victim user.
4. When the victim views the event (e.g., in the Calendar app or via a shared link), the attachment is automatically downloaded without any user interaction or confirmation.
5. The victim's browser downloads the file, potentially exposing sensitive data.

The root cause is a missing or insufficient user confirmation mechanism for attachment downloads. The Calendar app should require explicit user consent before initiating a download, but in vulnerable versions, it bypasses this check.

Vulnerable vs. Patched Code

The vulnerability is fixed by adding a confirmation dialog or requiring explicit user action before downloading attachments. The exact code change is not publicly available, but the patch likely involves modifying the JavaScript or PHP code that handles attachment download requests.

Vulnerable Code (Conceptual):

// Vulnerable: Automatically triggers download
function handleAttachment(attachment) {
    var link = document.createElement('a');
    link.href = attachment.downloadUrl;
    link.download = attachment.filename;
    document.body.appendChild(link);
    link.click();
    document.body.removeChild(link);
}

Patched Code (Conceptual):

// Patched: Requires user confirmation
function handleAttachment(attachment) {
    if (confirm('Do you want to download this file?')) {
        var link = document.createElement('a');
        link.href = attachment.downloadUrl;
        link.download = attachment.filename;
        document.body.appendChild(link);
        link.click();
        document.body.removeChild(link);
    }
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• A Nextcloud instance running a vulnerable version (Calendar < 4.7.17 or < 5.2.4).
• A valid attacker account on the Nextcloud instance.
• A victim account that will view the malicious event.

Exploitation Steps:

1. Log in to Nextcloud as the attacker.
2. Navigate to the Calendar app.
3. Create a new event.
4. Add an attachment by selecting a file from the Nextcloud server (e.g., a sensitive document).
5. Share the event with the victim user.
6. When the victim views the event, the file will be automatically downloaded.

Note: No public exploit code is available. The above steps describe the manual exploitation process.

Blue Team

Detection:

• Monitor Nextcloud access logs for unusual download patterns, especially for files that are not typically accessed.
• Look for calendar events with attachments that are shared across users.
• Use the detection rules provided in the Detection Engineering section.

Prevention:

• Apply the patch immediately by upgrading to Calendar 4.7.17 or 5.2.4.
• Implement additional access controls on sensitive files.
• Educate users about the risks of viewing calendar events from untrusted sources.

Indicators of Compromise (IOCs)

• File Downloads: Unusual file downloads triggered by calendar events.
• Access Logs: Entries showing file downloads from the Calendar app without corresponding user interaction.
• Network Traffic: HTTP requests to /apps/calendar/... followed by file download requests.
• User Behavior: Multiple users reporting unexpected file downloads after viewing calendar events.

Detection Engineering

YARA Rule

rule CVE_2025_66550_Nextcloud_Calendar {
    meta:
        description = "Detects potential exploitation of CVE-2025-66550 in Nextcloud Calendar"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-66550"
    strings:
        $s1 = "calendar" nocase
        $s2 = "attachment" nocase
        $s3 = "download" nocase
        $s4 = "auto" nocase
    condition:
        all of them
}

Sigma Rule

title: Nextcloud Calendar Automatic File Download
id: 8f4a3b2c-1d5e-4f6a-8b7c-9d0e1f2a3b4c
status: experimental
description: Detects automatic file downloads triggered by Nextcloud Calendar events
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-66550
logsource:
    product: nextcloud
    service: access
detection:
    selection:
        uri|contains: '/apps/calendar/'
        method: 'GET'
        status: 200
        response_body|contains: 'download'
    condition: selection
falsepositives:
    - Legitimate file downloads from calendar events
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-66550 Nextcloud Calendar Automatic File Download";
    flow:to_server,established;
    content:"/apps/calendar/"; http_uri;
    content:"download"; http_uri;
    classtype:attempted-user;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Nextcloud Calendar to version 4.7.17 or 5.2.4.
2. If immediate upgrade is not possible, disable the Calendar app temporarily.
3. Review and restrict file sharing permissions to limit exposure.

Long-term Measures:

• Implement a security awareness program to educate users about social engineering attacks.
• Regularly update all Nextcloud components.
• Monitor security advisories from Nextcloud and apply patches promptly.

Workarounds:

• Disable the Calendar app for untrusted users.
• Use a web application firewall (WAF) to block suspicious download requests.

Valters IT