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Improper validation on OAuth2 token endpoint (NC-SA-2018-003)

21st June 2018

Risk level: Medium

CVSS v3 Base Score: 6.4 (AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:N)

CWE: Improper Input Validation (CWE-20)

HackerOne report: 343111


Improper validation of input allowed an attacker with access to the OAuth2 refresh token to obtain new tokens.

Affected Software

  • Nextcloud Server < 13.0.3 (CVE-2018-3761)
  • Nextcloud Server < 12.0.8 (CVE-2018-3761)

Action Taken

The error has been fixed according to RFC6749.


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:

  • Mikael Karlsson - Vulnerability discovery and disclosure.

This advisory is licensed CC BY-SA 4.0.

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