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Message Authentication Codes calculated by the Default Encryption Module allow an attacker to silently overwrite blocks in a file (NC-SA-2020-038)

26th August 2020

Risk level: Low

CVSS v3 Base Score: 1.8 (AV:L/AC:H/PR:H/UI:R/S:U/C:N/I:L/A:N)

CWE: Violation of Secure Design Principles (CWE-657)

HackerOne report: 661051


A wrong generation of the passphrase for the encrypted block in Nextcloud Server 19.0.1 allowed an attacker to overwrite blocks in a file.

Affected Software

  • Nextcloud Server < 19.0.2 (CVE-2020-8133)
  • Nextcloud Server < 18.0.8 (CVE-2020-8133)
  • Nextcloud Server < 17.0.10 (CVE-2020-8133)

Action Taken

The error has been fixed.


It is recommended that the Nextcloud Server is upgraded to 19.0.2.


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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