Overview
CVE-2025-66511 is a medium severity vulnerability found in Nextcloud Calendar, a popular calendar application for the Nextcloud platform. This vulnerability, affecting versions prior to 6.0.3, stems from the predictable generation of participant tokens used in meeting proposals. An attacker could potentially compute valid tokens, allowing them to gain unauthorized access to meeting details and submit dates on behalf of others.
Technical Details
The core issue lies in how the Calendar app generates participant tokens for meeting proposals. Instead of employing a cryptographically secure random number generator, the application utilizes a hash function that, while not explicitly stated, is likely vulnerable to reverse engineering or brute-force attacks due to a limited entropy source. This predictability allows a malicious actor to compute potential participant tokens. With a valid token, an attacker can then request meeting details and even propose alternative dates, potentially disrupting or manipulating scheduled events.
CVSS Analysis
The Common Vulnerability Scoring System (CVSS) assigned a score of 4.8 to CVE-2025-66511, indicating a medium severity vulnerability. The CVSS vector likely reflects the following characteristics:
- Attack Vector (AV): Network (N)
- Attack Complexity (AC): High (H) – Calculating the tokens requires some effort but is feasible.
- Privileges Required (PR): None (N)
- User Interaction (UI): None (N)
- Scope (S): Unchanged (U)
- Confidentiality Impact (C): Low (L) – Meeting details are exposed.
- Integrity Impact (I): Low (L) – Attacker can submit dates.
- Availability Impact (A): None (N)
While the impact on confidentiality and integrity is relatively low, the potential for disruption and unauthorized access justifies the “Medium” severity rating.
Possible Impact
Exploitation of CVE-2025-66511 could have the following impacts:
- Unauthorized Access to Meeting Details: Attackers could view sensitive information contained within meeting proposals, such as participant lists, topics discussed, and proposed dates.
- Meeting Disruption: Attackers could submit fraudulent or conflicting date proposals, causing confusion and potentially disrupting scheduled events.
- Impersonation: By submitting dates on behalf of others, attackers could potentially impersonate legitimate participants and manipulate meeting outcomes.
Mitigation and Patch Steps
The vulnerability is fixed in Nextcloud Calendar version 6.0.3. The recommended course of action is to immediately upgrade to version 6.0.3 or later. This patch replaces the vulnerable token generation mechanism with a more secure, cryptographically random approach.
- Upgrade Nextcloud Calendar: The primary mitigation is to update the Nextcloud Calendar app to version 6.0.3 or a later version. This can be done through the Nextcloud app store interface.
- Verify Successful Update: After upgrading, verify that the Calendar app version is indeed 6.0.3 or higher.
- Monitor for Suspicious Activity: Keep an eye on your Nextcloud logs for any unusual activity related to meeting proposals or calendar entries.
