VulnerabilityPublished 5 Dec 2025Updated 8 Dec 2025

CVE-2025-66554: nextcloud security-advisories · Cross-site Scripting

Affects nextcloud security-advisoriesSource CVE Database V5

Not known to be exploited · CVSS 3.5 Low : upgrade to 5.5.4 when convenient.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 3.1 base score

3.5Low

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 16% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 5.5.4

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: Required
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: 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: 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/contacts to 5.5.4.

Affected software

Product	Affected	Fixed in
GitHubnextcloud/contacts`pkg:github/nextcloud/contacts`Other threats	Affected <5.5.4	Fixed in 5.5.4

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 promptly upgrade affected Nextcloud Contacts app instances to versions 5.5.4, 6.0.6, or 7.2.5 or later to remediate this vulnerability. Additionally, organizations should review and enforce strict access controls and authentication policies to minimize the risk of unauthorized users gaining the privileges needed to exploit this issue. Implementing multi-factor authentication (MFA) can reduce the risk of account compromise. Monitoring logs for unusual modifications to contact fields may help detect exploitation attempts. Organizations should also verify that their Nextcloud instances have properly configured Content Security Policies to block unauthorized script execution. User awareness training about phishing and UI manipulation risks can further mitigate social engineering attacks that might leverage this vulnerability. Finally, regular vulnerability scanning and patch management processes should be maintained to ensure timely updates.

Description

Contacts app for Nextcloud easily syncs contacts from various devices with your Nextcloud and allows editing. Prior to 5.5.4, 6.0.6, and 7.2.5, a malicious user was able to modify their organisation and title field to load additional CSS files. Javascript and other options were correctly blocked by the content security policy of the Nextcloud Server code. This vulnerability is fixed in 5.5.4, 6.0.6, and 7.2.5.

Impact

For European organizations using Nextcloud, especially those leveraging the Contacts app for internal or external collaboration, this vulnerability poses a low but non-negligible risk. The ability to inject additional CSS could allow attackers to subtly alter the user interface, potentially misleading users or facilitating phishing attempts by changing visual elements such as buttons or links. While the impact on confidentiality and availability is minimal, the integrity of the user interface could be compromised, which may erode user trust and lead to indirect security consequences. Organizations with strict compliance requirements around data integrity and user authentication workflows should be aware of this risk. Since exploitation requires authenticated access and user interaction, insider threats or compromised accounts could be vectors. The limited scope and absence of JavaScript execution reduce the likelihood of widespread damage, but targeted attacks against high-value users or administrators remain a concern.

Summary

CVE-2025-66554 is a cross-site scripting (CWE-79) vulnerability identified in the Contacts app component of Nextcloud, a widely used open-source file sharing and collaboration platform. The flaw exists in versions prior to 5.5.4, 6.0.6, and 7.2.5, where a malicious user with at least limited privileges can manipulate the 'organization' and 'title' fields of contact entries to inject additional CSS files into the web page generation process. This improper neutralization of input allows the attacker to influence the styling of the Nextcloud interface for other users viewing the compromised contact data. Despite this, the Nextcloud server enforces a strict content security policy (CSP) that blocks JavaScript and other potentially more dangerous payloads, mitigating the risk of script-based attacks such as full XSS. The vulnerability does not allow direct execution of arbitrary scripts or access to sensitive data, but it could be used for UI redressing or visual spoofing attacks that may facilitate phishing or social engineering. Exploitation requires the attacker to have at least some authenticated access and user interaction, limiting the attack surface. The vulnerability was publicly disclosed on December 5, 2025, with a CVSS v3.1 base score of 3.5 (low severity), reflecting its limited impact on confidentiality, integrity, and availability. No known exploits have been reported in the wild, and patches are available in the specified versions.

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #1OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Implementation · Architecture and Design
Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages.
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide…
OWASP Cheat Sheets
Cross Site Scripting Prevention (opens in a new tab)
DOM based XSS Prevention (opens in a new tab)
Injection Prevention (opens in a new tab)
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
OWASP ASVS 4.0.3
V5.3.3 (opens in a new tab)
V1.5.1 (opens in a new tab)
Attack patterns
XSS Using MIME Type Mismatch (opens in a new tab)
DOM-Based XSS (opens in a new tab)
Reflected XSS (opens in a new tab)
Stored XSS (opens in a new tab)
Cross-Site Scripting (XSS) (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, 19:08 UTC

WeaknessesCWE-79

Technical details

References

Community reviews

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