A Dovecot security update has been released for Ubuntu Linux 20.04 LTS, 20.10, and 21.04.

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Ubuntu Security Notice USN-4993-1
June 21, 2021

dovecot vulnerabilities
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 21.04
- Ubuntu 20.10
- Ubuntu 20.04 LTS

Summary:

Several security issues were fixed in Dovecot.

Software Description:
- dovecot: IMAP and POP3 email server

Details:

Kirin discovered that Dovecot incorrectly escaped kid and azp fields in JWT
tokens. A local attacker could possibly use this issue to validate tokens
using arbitrary keys. This issue only affected Ubuntu 20.10 and Ubuntu
21.04. (CVE-2021-29157)

Fabian Ising and Damian Poddebniak discovered that Dovecot incorrectly
handled STARTTLS when using the SMTP submission service. A remote attacker
could possibly use this issue to inject plaintext commands before
STARTTLS negotiation. (CVE-2021-33515)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 21.04:
dovecot-core 1:2.3.13+dfsg1-1ubuntu1.1

Ubuntu 20.10:
dovecot-core 1:2.3.11.3+dfsg1-2ubuntu0.2

Ubuntu 20.04 LTS:
dovecot-core 1:2.3.7.2-1ubuntu3.4

In general, a standard system update will make all the necessary changes.

References:
  https://ubuntu.com/security/notices/USN-4993-1
CVE-2021-29157, CVE-2021-33515

Package Information:
  https://launchpad.net/ubuntu/+source/dovecot/1:2.3.13+dfsg1-1ubuntu1.1
  https://launchpad.net/ubuntu/+source/dovecot/1:2.3.11.3+dfsg1-2ubuntu0.2
  https://launchpad.net/ubuntu/+source/dovecot/1:2.3.7.2-1ubuntu3.4