CVE-2025-66511VulnerabilityPublished 5 Dec 2025

CVE-2025-66511: nextcloud security-advisories · Weak Randomness

Affects nextcloud security-advisoriesSource CVE Database V5

Not known to be exploited · CVSS 4.8 Medium : upgrade to 6.0.3 when convenient.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score

4.8Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 21% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 6.0.3

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: Low
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 5 Dec 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade nextcloud/calendar to 6.0.3.

Affected software

Product	Affected	Fixed in
GitHubnextcloud/calendar`pkg:github/nextcloud/calendar`Other threats	Affected <6.0.3	Fixed in 6.0.3

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

European organizations should promptly update Nextcloud Calendar to version 6.0.3 or later to remediate this vulnerability. Until updates can be applied, administrators should restrict network access to the Nextcloud Calendar app to trusted users and networks to reduce exposure. Monitoring and logging access to calendar meeting proposals can help detect suspicious activities indicative of token guessing attempts. Organizations should also review their internal policies on calendar data sensitivity and consider additional access controls or encryption for sensitive meeting information. Security teams should educate users about the importance of timely software updates and verify that all Nextcloud components are kept current. Finally, organizations could implement network-level protections such as web application firewalls (WAFs) to detect and block anomalous requests targeting token generation endpoints.

Description

CVE-2025-66511 is a medium severity vulnerability in Nextcloud Calendar versions prior to 6.0.3 where participant tokens for meeting proposals are generated using insufficiently random values. This weakness allows attackers to compute valid participant tokens, enabling unauthorized access to meeting proposal details and the ability to submit dates. The vulnerability arises from the use of a hash function rather than a cryptographically secure random number generator. Exploitation requires no authentication or user interaction but has a higher attack complexity. The issue is fixed in version 6.0.3. No known exploits are currently reported in the wild.

Impact

For European organizations using Nextcloud Calendar versions prior to 6.0.3, this vulnerability could lead to unauthorized disclosure of meeting proposal details and unauthorized modification of scheduling data. This may result in privacy breaches, exposure of sensitive meeting information, and potential disruption of organizational workflows. Confidentiality and integrity of calendar data are at risk, which could affect sectors with strict data protection requirements such as finance, healthcare, and government. Although the vulnerability does not directly impact system availability, the unauthorized manipulation of meeting proposals could cause operational inefficiencies and mistrust in collaboration tools. Given the widespread adoption of Nextcloud in Europe, especially among privacy-conscious organizations and public institutions, the impact could be significant if exploited. However, the medium CVSS score and high attack complexity somewhat limit the immediacy of the threat.

CWE-330Use of Insufficiently Random Values (opens in a new tab)
Class weaknessLikelihood of exploit: HighOWASP Top 10 A04:2025: Cryptographic Failures (opens in a new tab)
The product uses insufficiently random numbers or values in a security context that depends on unpredictable numbers.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design
Use a well-vetted algorithm that is currently considered to be strong by experts in the field, and select well-tested implementations with adequate length seeds.
Implementation
Consider a PRNG that re-seeds itself as needed from high quality pseudo-random output sources, such as hardware devices.
Architecture and Design · Requirements · Libraries or Frameworks
Use products or modules that conform to FIPS 140-2 to avoid obvious entropy problems. Consult FIPS 140-2 Annex C ("Approved Random Number Generators").
OWASP Cheat Sheets
Forgot Password (opens in a new tab)
Transport Layer Protection (opens in a new tab)
User Privacy Protection (opens in a new tab)
Password Storage (opens in a new tab)
Cryptographic Storage (opens in a new tab)
OWASP ASVS 4.0.3
V2.3.1 (opens in a new tab)
V2.6.2 (opens in a new tab)
V2.7.6 (opens in a new tab)
Attack patterns
Signature Spoofing by Key Recreation (opens in a new tab)
Session Credential Falsification through Prediction (opens in a new tab)
Brute Force (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 12 Dec 2025, 17:40 UTC

WeaknessesCWE-330

Technical details

References

Community reviews

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