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Mail app not verifying TLS host of mail servers (NC-SA-2020-020)

24th March 2020

Risk level: Low

CVSS v3 Base Score: 5.9 (AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:L/A:L)

CWE: Improper Certificate Validation (CWE-295)

HackerOne report: 803734


A missing verification of the TLS host in Nextcloud Mail 1.1.3 allowed a man in the middle attack.

Affected Software

  • Nextcloud Mail < 1.1.4 (CVE-2020-8156)

Action Taken

The error has been fixed.


It is recommended that the Nextcloud Mail app is upgraded to 1.1.4.


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

  • Frank Isemann ( - Vulnerability discovery and disclosure.

This advisory is licensed CC BY-SA 4.0.

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