CVE-2026-45267: Nextcloud Form Submission Authorization Bypass – Patch & Detection Guide

Technical summary

CVE-2026-45267 is a broken access control vulnerability in Nextcloud prior to version 5.2.6 stemming from insufficient authorization validation in form submission handling. The application fails to enforce CWE-862 (Missing Authorization) checks, allowing any authenticated principal to read form responses across organizational boundaries. The flaw maps to CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor). With a CVSS v3.1 score of 6.5 (MEDIUM), the vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N reflects network accessibility, low attack complexity, requirement for valid credentials, absence of user interaction, and high confidentiality impact isolated to the attacked scope.

Business impact

This vulnerability poses a data confidentiality risk to organizations relying on Nextcloud forms for internal information collection. Depending on form content, exposed submissions could include employee feedback, survey responses, health information, project details, or other sensitive data. The breach does not enable modification or deletion of data, but unauthorized disclosure could violate data protection policies, breach employee privacy, or expose competitive insights. Organizations should assess what information their forms collect and determine exposure scope if a user account has been compromised or misused.

Affected systems

Nextcloud versions prior to 5.2.6 are affected. The vulnerability requires authenticated access; anonymous or unauthenticated users cannot exploit it. Any Nextcloud deployment using form functionality in versions below 5.2.6 should be considered at risk if form submissions contain sensitive data. The patch is available in Nextcloud 5.2.6 and later.

Exploitability

Exploitation is straightforward for any user with valid Nextcloud credentials. No special privileges, complex attack chains, or user interaction are required. An attacker needs only to authenticate and attempt to access form submissions through the API or user interface. The low attack complexity and network-accessible nature of Nextcloud deployments make this readily exploitable if an insider account is compromised or if an organization has permissive user provisioning practices. Exploitation does not require knowledge of specific submission IDs if the application exposes directories or lists of available forms.

Remediation

Organizations should prioritize upgrading Nextcloud to version 5.2.6 or later immediately. Patching closes the authorization bypass entirely. During the patching window, administrators should review form-based data classification and consider temporarily restricting form access to trusted users or disabling form functionality if forms contain highly sensitive information. Access logs should be reviewed for suspicious cross-user form submission requests to detect possible exploitation.

Patch guidance

Upgrade Nextcloud to version 5.2.6 or any later stable release. The patch introduces explicit permission checks on form submission access, ensuring users can only read their own submissions or those they have explicit authorization to view. Follow Nextcloud's standard upgrade procedures: back up the database and file directory, download the latest version, run the upgrade, and verify functionality post-deployment. Test form functionality in a staging environment before production rollout if the organization has custom form workflows.

Detection guidance

Monitor Nextcloud audit logs for unusual form submission access patterns, particularly cross-user or cross-group access. Web application firewalls or network monitoring can flag repeated attempts to access form endpoints with different user contexts. Endpoint Detection and Response tools should flag processes attempting bulk export or exfiltration of form data. Look for API calls to form submission endpoints by accounts that do not own the forms or lack documented authorization. Consider enabling verbose logging of form access if available in your Nextcloud configuration.

Why prioritize this

Although the CVSS score is MEDIUM, this vulnerability warrants timely attention because it enables direct confidentiality breach of user-submitted data in a collaboration platform where forms often collect sensitive organizational information. The low barrier to exploitation (any valid user) and high-sensitivity nature of form data justify rapid patching. However, it does not enable system-wide compromise or data manipulation, so it should not override critical vulnerabilities in network infrastructure or authentication systems.

Risk score, explained

CVSS v3.1 assigns a score of 6.5 (MEDIUM) reflecting the following factors: the vulnerability is network-accessible (AV:N) with low attack complexity (AC:L), requires a valid user login (PR:L), does not require user interaction (UI:N), and impacts only the confidentiality of data within the affected scope (C:H/I:N/A:N). The score does not account for context such as form data sensitivity, insider threat vectors, or organizational trust models—factors that may elevate the practical risk in specific deployments.

Frequently asked questions