Nextcloud Calendar <6.0.3 Hash-based Participant Token Leak
CVE-2025-66511 Published on December 5, 2025

Nextcloud Calendar app used predictable proposal participant tokens
Nextcloud Calendar is a calendar app for Nextcloud. Prior to 6.0.3, the Calendar app generates participant tokens for meeting proposals using a hash function, allowing an attacker to compute valid participant tokens, which allowed them to request details and submit dates in meeting proposals. The tokens are not purely random generated. This vulnerability is fixed in 6.0.3.

NVD

Vulnerability Analysis

CVE-2025-66511 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is consided to have a high level of attack complexity. The potential impact of an exploit of this vulnerability is considered to have a small impact on confidentiality and integrity, and no impact on availability.

Attack Vector:
NETWORK
Attack Complexity:
HIGH
Privileges Required:
NONE
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
LOW
Integrity Impact:
LOW
Availability Impact:
NONE

Weakness Type

Use of Insufficiently Random Values

The software uses insufficiently random numbers or values in a security context that depends on unpredictable numbers. When software generates predictable values in a context requiring unpredictability, it may be possible for an attacker to guess the next value that will be generated, and use this guess to impersonate another user or access sensitive information.


Affected Versions

nextcloud security-advisories Version >= 6.0.0-rc.1, < 6.0.3 is affected by CVE-2025-66511

Exploit Probability

EPSS
0.03%
Percentile
6.66%

EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.