Versions 2.0.12 of Mosquitto has been released. This is a security and bugfix release.

Security

An MQTT v5 client connecting with a large number of user-property properties could cause excessive CPU usage, leading to a loss of performance and possible denial of service. This has been fixed.
Fix max_keepalive not applying to MQTT v3.1.1 and v3.1 connections. These clients are now rejected if their keepalive value exceeds max_keepalive. This option allows CVE-2020-13849, which is for the MQTT v3.1.1 protocol itself rather than an implementation, to be addressed.
Using certain listener related configuration options e.g. cafile, that apply to the default listener without defining any listener would cause a remotely accessible listener to be opened that was not confined to the local machine but did have anonymous access enabled, contrary to the documentation. This has been fixed. Closes [#2283].
CVE-2021-34434: If a plugin had granted ACL subscription access to a durable/non-clean-session client, then removed that access, the client would keep its existing subscription. This has been fixed.
Incoming QoS 2 messages that had not completed the QoS flow were not being checked for ACL access when a clean session=False client was reconnecting. This has been fixed.

Broker

Fix possible out of bounds memory reads when reading a corrupt/crafted configuration file. Unless your configuration file is writable by untrusted users this is not a risk. Closes #567213.
Fix max_connections option not being correctly counted.
Fix TLS certificates and TLS-PSK not being able to be configured at the same time.
Disable TLS v1.3 when using TLS-PSK, because it isn't correctly configured.
Fix max_keepalive not applying to MQTT v3.1.1 and v3.1 connections. These clients are now rejected if their keepalive value exceeds max_keepalive. This option allows CVE-2020-13849, which is for the MQTT v3.1.1 protocol itself rather than an implementation, to be addressed.
Fix broker not quitting if e.g. the password_file is specified as a directory. Closes #2241.
Fix listener mount_point not being removed on outgoing messages. Closes #2244.
Strict protocol compliance fixes, plus test suite.
Fix $share subscriptions not being recovered for durable clients that reconnect.
Update plugin configuration documentation. Closes #2286.

Client library

If a client uses TLS-PSK then force the default cipher list to use "PSK" ciphers only. This means that a client connecting to a broker configured with x509 certificates only will now fail. Prior to this, the client would connect successfully without verifying certificates, because they were not configured.
Disable TLS v1.3 when using TLS-PSK, because it isn't correctly configured.
Threaded mode is deconfigured when the mosquitto_loop_start() thread ends, which allows mosquitto_loop_start() to be called again. Closes #2242.
Fix MOSQ_OPT_SSL_CTX not being able to be set to NULL. Closes #2289.
Fix reconnecting failing when MOSQ_OPT_TLS_USE_OS_CERTS was in use, but none of capath, cafile, psk, nor MOSQ_OPT_SSL_CTX were set, and MOSQ_OPT_SSL_CTX_WITH_DEFAULTS was set to the default value of true. Closes #2288.

Apps

Fix mosquitto_ctrl dynsec setDefaultACLAccess command not working.

Clients

mosquitto_sub and mosquitto_rr now open stdout in binary mode on Windows so binary payloads are not modified when printing.
Document TLS certificate behaviour when using -p 8883.

Build

Fix installation using WITH_TLS=no. Closes #2281.
Fix builds with libressl 3.4.0. Closes #2198.
Remove some unnecessary code guards related to libressl.
Fix printf format build warning on MIPS. Closes #2271.