CVE-2025-66550VulnerabilityPublished 5 Dec 2025

CVE-2025-66550: nextcloud security-advisories · Improper Handling of Unexpected Data Type

Affects nextcloud security-advisoriesSource CVE Database V5

Not known to be exploited · CVSS 5.7 Medium : upgrade to 5.2.4 or 4.7.17 when convenient.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score

5.7Medium

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

EPSS: 0.3% chance of exploitation in 30 days – higher than 27% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 5.2.4, 4.7.17

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: Required
Scope: Unchanged
Confidentiality: None
Integrity: High
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: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
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: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to 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 5.2.4 or 4.7.17.

Affected software

Product	Affected	Fixed in
GitHubnextcloud/calendar`pkg:github/nextcloud/calendar`Other threats	Affected <4.7.17 <5.2.4	Fixed in 4.7.17 5.2.4

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 prioritize upgrading Nextcloud Calendar to versions 4.7.17 or 5.2.4 or later to eliminate this vulnerability. Additionally, implement strict access controls to limit calendar event creation privileges to trusted users only. Employ monitoring and alerting for unusual calendar event creation patterns or unexpected file downloads within Nextcloud logs. Educate users about the risks of interacting with calendar attachments from untrusted sources and encourage verification before opening attachments. Consider deploying network-level controls to restrict unauthorized file downloads from internal servers. Regularly audit Nextcloud instances for outdated components and apply security patches promptly. For environments with high security requirements, consider isolating Nextcloud services and enforcing multi-factor authentication to reduce the risk of account compromise that could enable exploitation.

Description

Nextcloud Calendar is a calendar app for Nextcloud. Prior to 4.7.17 and 5.2.4, when a malicious user creates a calendar event with a crafted attachment that links to a download link of a file on the same Nextcloud server, the file would be downloaded without the user confirming the action. This vulnerability is fixed in 4.7.17 and 5.2.4.

Impact

For European organizations, this vulnerability could lead to unauthorized data modification or leakage within Nextcloud environments, particularly affecting collaborative workflows that rely on calendar events. Since Nextcloud is widely used in Europe for secure file sharing and collaboration, exploitation could undermine data integrity and trust in internal communications. The automatic download of files without user consent could also expose users to further risks if the downloaded files are malicious or sensitive. Organizations in sectors with strict data protection regulations, such as finance, healthcare, and government, may face compliance risks if this vulnerability is exploited. Although the attack requires some privileges and user interaction, insider threats or compromised accounts could leverage this flaw to propagate malicious files or disrupt operations. The absence of known exploits reduces immediate risk but does not eliminate the potential for targeted attacks, especially in environments with outdated Nextcloud Calendar versions.

CWE-241Improper Handling of Unexpected Data Type (opens in a new tab)
Base weakness
The product does not handle or incorrectly handles when a particular element is not the expected type, e.g. it expects a digit (0-9) but is provided with a letter (A-Z).
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
Implementation · Input Validation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174).
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
OWASP ASVS 4.0.3
V1.5.1 (opens in a new tab)
Attack patterns
Passing Local Filenames to Functions That Expect a URL (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 12 Dec 2025, 17:39 UTC

WeaknessesCWE-241

Technical details

References

Community reviews

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