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CVE-2026-45281— Nextcloud: Cross-Account Calendar Takeover via Unauthorized Group-Member-Set Update

Quick assessment

Affected
nextcloud security-advisories
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Nextcloud Server是Nextcloud开源的一个Nextcloud服务器程序。 Nextcloud Server 32.0.0版本至32.0.9之前版本和33.0.0版本至33.0.3之前版本存在安全漏洞，该漏洞源于日历后端授权控制不当，可能导致已认证用户利用其他用户的主体URL发送请求以完全访问其日历。

CVSS 8.1 · High EPSS 0.50% · P40

Affected Version Matrix 2

VendorProduct Version RangeStatus
nextcloud security-advisories >= 32.0.0, < 32.0.9 affected
>= 33.0.0, < 33.0.3 affected
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I. Basic Information for CVE-2026-45281

Vulnerability Information
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Vulnerability Title
Nextcloud: Cross-Account Calendar Takeover via Unauthorized Group-Member-Set Update
Source: CVE Program / CVE List V5
Vulnerability Description
Nextcloud is an open source content collaboration platform. In Nextcloud Server from versions 32.0.0 to before 32.0.9, and 33.0.0 to before 33.0.3, with the knowledge of other users’ principal URL an attacker could possibly send a request to gain full access to their calendar. Therefore, the attacker must be an authenticated user. This is because of improper authorization controls in the backend of the calendar. If the attacker had access to the calendar, they would be able to view and modify it. It is recommended that the Nextcloud Server is upgraded to 33.0.3 or 32.0.9. It is recommended that the Nextcloud Enterprise Server is upgraded to 33.0.3, 32.0.9, 31.0.14.5, 30.0.17.9, 29.0.16.16, 28.0.14.17, 27.1.11.26, 26.0.13.26, 25.0.13.29, 24.0.12.34, 23.0.12.35, 22.2.10.39, or 21.0.9.23
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
通过用户控制密钥绕过授权机制
Source: CVE Program / CVE List V5
Vulnerability Title
Nextcloud Server 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Nextcloud Server是Nextcloud开源的一个Nextcloud服务器程序。 Nextcloud Server 32.0.0版本至32.0.9之前版本和33.0.0版本至33.0.3之前版本存在安全漏洞，该漏洞源于日历后端授权控制不当，可能导致已认证用户利用其他用户的主体URL发送请求以完全访问其日历。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
nextcloud security-advisories >= 32.0.0, < 32.0.9 -

II. Public POCs for CVE-2026-45281

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III. Intelligence Information for CVE-2026-45281

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Patches & Fixes for CVE-2026-45281 (1)

Vendor Advisories for CVE-2026-45281 (1)

Same Patch Batch · nextcloud · 2026-06-01 · 26 CVEs total

CVE-2026-45545 8.2 HIGH Nextcloud: SQL Injection in Column Type Parameter Allows Arbitrary SQL Execution
CVE-2026-45156 8.1 HIGH Nextcloud: Authentication Bypass in ID4me handling via Missing JWT Signature Verification
CVE-2026-45722 7.1 HIGH Nextcloud: Tables app allows limited SQLi in ORDER BY with malicious sort order argument f
CVE-2026-45810 6.8 MEDIUM Nextcloud: Propfind requests for file comments allowed to load comments for other files
CVE-2026-45275 6.5 MEDIUM Nextcloud: Authorization bypass in approval feature allows unauthorized file sharing with
CVE-2026-45282 6.5 MEDIUM Nextcloud: Logged-in user bypasses share password and download restrictions on Text attach
CVE-2026-45267 6.5 MEDIUM Nextcloud: Missing permission check for from submissions
CVE-2026-45285 6.4 MEDIUM Nextcloud: Hidden Public Link creation when sharing to a Team External Member
CVE-2026-45283 6.3 MEDIUM Nextcloud: Files Lock app allows users to lock and unlock files of other users
CVE-2026-45157 6.3 MEDIUM Nextcloud: Valid share tokens allow to access tempory upload files of share owner
CVE-2026-45690 5.9 MEDIUM Nextcloud: Two-Factor Authentication Bypass via Pending Session Token Replay
CVE-2026-45691 5.9 MEDIUM Nextcloud: Bypass of second factor authentication on DAV endpoints
CVE-2026-45543 5.3 MEDIUM Nextcloud: Deleting a Forms collaborator share leaves uploaded response files accessible t
CVE-2026-45153 4.6 MEDIUM Nextcloud: PIN bypass in PassCodeActivity via back button
CVE-2026-45284 4.6 MEDIUM Nextcloud: Wrong condition in the User OIDC app's LdapService allowed deleted LDAP users t
CVE-2026-45279 4.4 MEDIUM Nextcloud: Limited path traversal via template API if using `{lang}` in config
CVE-2026-45264 4.3 MEDIUM Nextcloud: ACL Rename Permission Bypass in Team Folders Allows Unauthorized File Renames
CVE-2026-45286 4.3 MEDIUM Nextcloud: Calendar app leaked user identifiers via attendee suggestion endpoint
CVE-2026-45544 4.3 MEDIUM Nextcloud: Information Disclosure of view filter metdata via Broken Sensitive Data Masking
CVE-2026-45159 3.5 LOW Nextcloud: Files drop share links for end-to-end encrypted folders allowed to drop files i

Showing top 20 of 26 CVEs. View all on vendor page &rarr; →

IV. Related Vulnerabilities

Same product: security-advisories
Same vendor: nextcloud
Same weakness: CWE-639

V. Comments for CVE-2026-45281

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