CVE-2021-32066: A StartTLS stripping vulnerability in Net::IMAP

A StartTLS stripping vulnerability was discovered in Net::IMAP. This vulnerability has been assigned the CVE identifier CVE-2021-32066. We strongly recommend upgrading Ruby.

net-imap is a default gem in Ruby 3.0.1 but it has a packaging issue, so please upgrade Ruby itself.


Net::IMAP does not raise an exception when StartTLS fails with an unknown response, which might allow man-in-the-middle attackers to bypass the TLS protections by leveraging a network position between the client and the registry to block the StartTLS command, aka a “StartTLS stripping attack.”

Affected Versions

  • Ruby 2.6 series: 2.6.7 and earlier
  • Ruby 2.7 series: 2.7.3 and earlier
  • Ruby 3.0 series: 3.0.1 and earlier


Thanks to Alexandr Savca for reporting the issue.


  • Originally published at 2021-07-07 09:00:00 UTC