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Stored XSS in CardDAV image export (NC-SA-2016-008)

10th October 2016

Risk level: Medium

CVSS v3 Base Score: 5.4 (AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N)

CWE: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (CWE-79)

HackerOne report: 163338

Description

The CardDAV image export functionality as implemented in Nextcloud allows the download of images stored within a vCard. Due to not performing any kind of verification on the image content this is prone to a stored Cross-Site Scripting attack.Note: Nextcloud employs a very strict Content Security Policy on the DAV endpoints. This is thus only exploitable on browsers that don't support Content Security Policy.

Affected Software

Action Taken

The mimetype of the exported image is now compared with a whitelist as well as download disposition headers have been set on the response.

Resolution

It is recommended that all instances are upgraded to Nextcloud 10.0.1.

Acknowledgements

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


This advisory is licensed CC BY-SA 4.0.

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