VulnerabilityPublished 5 Dec 2025

CVE-2025-66558: nextcloud security-advisories · Insecure Direct Object Reference

Affects nextcloud security-advisoriesSource CVE Database V5

Not known to be exploited · CVSS 3.1 Low : upgrade to 1.4.2 when convenient.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 3.1 base score

3.1Low

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 20% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.4.2

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

Attack Vector: Network
Attack Complexity: High
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
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: 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: 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: None: There is no loss of confidentiality within 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 26 Oct 2026.

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 or the CVSS exploit maturity 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/twofactor_webauthn to 1.4.2.

Affected software

Product	Affected	Fixed in
GitHubnextcloud/twofactor_webauthn`pkg:github/nextcloud/twofactor_webauthn`Other threats	Affected <1.4.2	Fixed in 1.4.2

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 immediately verify their Nextcloud Twofactor WebAuthn versions and upgrade to 1.4.2 or 2.4.1 or later to remediate this vulnerability. Beyond patching, organizations should enforce strong random generation and protection of 2FA device keys to prevent guessing attacks. Monitoring authentication logs for unusual 2FA device re-registrations can help detect exploitation attempts. Implementing network segmentation and limiting access to Nextcloud administrative interfaces reduces exposure. Additionally, educating users about unexpected 2FA prompts and establishing rapid incident response procedures for authentication anomalies will mitigate operational impacts. Organizations should also consider multi-layered authentication strategies and continuous security assessments of their 2FA implementations. Finally, maintaining up-to-date backups and recovery plans ensures resilience against potential disruptions caused by such vulnerabilities.

Description

CVE-2025-66558 is a low-severity authorization bypass vulnerability in Nextcloud's Twofactor WebAuthn provider affecting versions prior to 1.4.2 and 2.4.1. The flaw arises from a missing ownership check that allows an attacker to take control of a victim's 2FA WebAuthn device by guessing a long random key string (80-128 characters). Although the attacker cannot authenticate as the victim, they can force the victim to re-register a new 2FA device upon next login, potentially disrupting user experience and 2FA integrity. The vulnerability requires remote network access, low privileges, and high attack complexity, with no user interaction needed. No known exploits are reported in the wild, and patches are available in versions 1.4.

Impact

For European organizations, the impact of this vulnerability is primarily operational and related to the integrity of two-factor authentication processes. While the attacker cannot directly access or impersonate the victim, the ability to take over a 2FA device registration can cause denial of service to legitimate users by forcing them to re-register their 2FA devices. This disruption can lead to temporary loss of access, increased helpdesk workload, and potential user frustration. In environments with strict security policies or regulatory requirements (such as GDPR), any weakening or disruption of authentication mechanisms can raise compliance concerns. Organizations relying heavily on Nextcloud for collaboration and sensitive data sharing may face increased risk if attackers exploit this flaw to undermine trust in authentication controls. However, since confidentiality and availability are not directly compromised, and no known exploits exist, the overall risk remains low but non-negligible. The vulnerability could be leveraged as part of a broader attack chain if combined with other weaknesses.

CWE-639Authorization Bypass Through User-Controlled Key (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #24OWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.
Architecture and Design
For each and every data access, ensure that the user has sufficient privilege to access the record that is being requested.
Architecture and Design · Implementation
Make sure that the key that is used in the lookup of a specific user's record is not controllable externally by the user or that any tampering can be detected.
Architecture and Design
Use encryption in order to make it more difficult to guess other legitimate values of the key or associate a digital signature with the key so that the server can verify that there has been no tampering.
OWASP Cheat Sheets
Access Control (opens in a new tab)
Authorization Testing Automation (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Insecure Direct Object Reference Prevention (opens in a new tab)
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V4.1.2 (opens in a new tab)
V4.2.1 (opens in a new tab)
V8.3.8 (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, 19:15 UTC

WeaknessesCWE-639

Technical details

References

Community reviews

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