The SMTP server in Postfix before 2.5.13 2.6.x before 2.6.10 2.7.x before 2.7.4 and 2.8.x before 2.8.3 when certain Cyrus SASL authentication methods are enabled does not create a new server handle after client authentication fails which allows remote attackers to cause a denial of service (heap memory corruption and daemon crash) or possibly execute arbitrary code via an invalid AUTH command with one method followed by an AUTH command with a different method.