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File access control rules not applied to image previews (NC-SA-2018-002)

21st June 2018

Risk level: Low

CVSS v3 Base Score: 3.5 (AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:N/A:N)

CWE: Improper Check for Dropped Privileges (CWE-273)

HackerOne report: 358339


A missing check for read permissions allowed users that received an incomming share containing files tagged so they should be denied access to still request a preview for those files.

Affected Software

  • Nextcloud Server < 13.0.3 (CVE-2018-3762)
  • Nextcloud Server < 12.0.8 (CVE-2018-3762)

Action Taken

The error has been fixed and regression tests been added.


It is recommended that all instances are upgraded to Nextcloud 13.0.3.


The Nextcloud team thanks the following people for their research and responsible disclosure of the above advisory:

  • Reinis Martinsons ( - Vulnerability discovery and disclosure.

This advisory is licensed CC BY-SA 4.0.

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