CVE-2025-66548VulnerabilityPublished 5 Dec 2025Updated 8 Dec 2025

CVE-2025-66548: nextcloud security-advisories · Improper Encoding or Escaping of Output

Affects nextcloud security-advisoriesSource CVE Database V5

Not known to be exploited · CVSS 3.3 Low · fixed in 1.15.1 : update nextcloud security-advisories when convenient.

Data as of 25 Oct 2026, 19:02 UTC

CVSS 3.1 base score

3.3Low

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

EPSS: 0.1% chance of exploitation in 30 days – higher than 4% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.15.1, 1.14.4, 1.12.7

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

Attack Vector: Local
Attack Complexity: Low
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: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
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: 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 2026-10-25.

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/deck to 1.15.1, 1.14.4 or 1.12.7.

Affected software

Product	Affected	Fixed in
GitHubnextcloud/deck`pkg:github/nextcloud/deck`Other threats	Affected <1.12.7 <1.14.4 <1.15.1	Fixed in 1.12.7 1.14.4 1.15.1

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 upgrade Nextcloud Deck to versions 1.12.7, 1.14.4, or 1.15.1 or later, where the vulnerability is fixed. Additionally, implement strict file upload validation and sanitization to detect and block filenames containing RTLO or other Unicode control characters that could be used for spoofing. Educate users about the risks of opening files with suspicious or unexpected extensions, especially those received through internal file sharing platforms. Employ endpoint security solutions capable of detecting and blocking execution of potentially malicious files regardless of extension. Monitor file uploads and user activity logs within Nextcloud for unusual behavior or attempts to upload files with obfuscated names. Consider disabling or restricting file uploads from untrusted users or external sources until patches are applied. Finally, maintain a robust patch management process to ensure timely updates of collaboration tools.

Description

Nextcloud Deck is a kanban style organization tool aimed at personal planning and project organization for teams integrated with Nextcloud. Prior to 1.12.7, 1.14.4, and 1.15.1, file extension can be spoofed by using RTLO characters, tricking users into download files with a different extension than what is displayed. This vulnerability is fixed in 1.12.7, 1.14.4, and 1.15.1.

Impact

For European organizations using Nextcloud Deck, this vulnerability primarily poses a risk of social engineering attacks through file extension spoofing. While it does not directly compromise confidentiality or availability, the misleading file names could cause users to open malicious files under false pretenses, potentially leading to malware execution or data integrity issues. Organizations relying heavily on Nextcloud for collaboration and file sharing may see increased risk if attackers exploit this flaw to distribute harmful payloads disguised as benign documents. The requirement for local access and low privileges limits remote exploitation, but insider threats or compromised accounts could leverage this vulnerability. The impact is thus mostly on data integrity and user trust, with indirect risks to confidentiality if malicious files are executed. Given the widespread use of Nextcloud in European public and private sectors, especially in countries with strong open-source adoption, the vulnerability could affect a significant number of users if unpatched.

Summary

CVE-2025-66548 is a vulnerability classified under CWE-116 (Improper Encoding or Escaping of Output) found in Nextcloud Deck, a kanban-style organizational tool integrated with the Nextcloud platform. The vulnerability arises from the improper handling of file names containing Right-to-Left Override (RTLO) Unicode characters. RTLO characters can reverse the display order of characters in file names, enabling an attacker to spoof the file extension shown to the user. For example, a malicious file named 'docx‮exe' could appear as 'exe.docx' in the user interface, misleading users about the true file type. This spoofing can facilitate social engineering attacks by tricking users into opening potentially harmful files they believe to be safe. The vulnerability affects Nextcloud Deck versions prior to 1.12.7, 1.14.4, and 1.15.1. Exploitation requires local access with low privileges and does not require user interaction, but the attacker must be able to upload or place files within the Nextcloud environment. The CVSS v3.1 base score is 3.3, indicating low severity, with the vector showing local attack vector, low complexity, low privileges required, no user interaction, and impact limited to integrity only. No known exploits have been reported in the wild. The vulnerability was publicly disclosed on December 5, 2025, and patches are available in the specified versions. The root cause is the failure to properly encode or escape output filenames containing RTLO characters, allowing the visual spoofing of file extensions in the Nextcloud Deck interface.

CWE-116Improper Encoding or Escaping of Output (opens in a new tab)
Class weaknessLikelihood of exploit: HighOWASP Top 10 A05:2025: Injection (opens in a new tab)
The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.
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.
Architecture and Design · Parameterization
If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
Architecture and Design · Implementation
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.
OWASP Cheat Sheets
Content Security Policy (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
Query Parameterization (opens in a new tab)
OWASP ASVS 4.0.3
V14.4.6 (opens in a new tab)
V14.4.4 (opens in a new tab)
V14.4.2 (opens in a new tab)
Attack patterns
Cross Zone Scripting (opens in a new tab)
User-Controlled Filename (opens in a new tab)
Web Server Logs Tampering (opens in a new tab)
AJAX Footprinting (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, 19:07 UTC

WeaknessesCWE-116

Technical details

References

Community reviews

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