diff --git a/ChangeLog.txt b/ChangeLog.txt
index f708d66c..eb4f9889 100644
--- a/ChangeLog.txt
+++ b/ChangeLog.txt
@@ -1,6 +1,15 @@
 1.4.12 - 201703xx
 =================
 
+Security:
+- Fix CVE-2017-7650, which allows clients with username or client id set to
+  '#' or '+' to bypass pattern based ACLs or third party plugins. The fix
+  denies message sending or receiving of messages for clients with a '#' or
+  '+' in their username or client id and if the message is subject to a
+  pattern ACL check or plugin check.
+  Patches for other versions are available at
+  https://mosquitto.org/files/cve/2017-7650/
+
 Broker:
 - Fix mosquitto.db from becoming corrupted due to client messages being
   persisted with no stored message. Closes #424.
@@ -11,6 +20,7 @@ Broker:
 - Fixes to readme.md.
 - Fix deprecation warning for OpenSSL 1.1. PR #416.
 - Don't segfault on duplicate bridge names. Closes #446.
+- Fix CVE-2017-7650.
 
 
 1.4.11 - 20170220
diff --git a/src/security.c b/src/security.c
index 6ae9fb9c..d0c376bd 100644
--- a/src/security.c
+++ b/src/security.c
@@ -233,6 +233,21 @@ int mosquitto_acl_check(struct mosquitto_db *db, struct mosquitto *context, cons
 		{
 			username = context->username;
 		}
+
+		/* Check whether the client id or username contains a +, # or / and if
+		 * so deny access.
+		 *
+		 * Do this check for every message regardless, we have to protect the
+		 * plugins against possible pattern based attacks.
+		 */
+		if(username && strpbrk(username, "+#/")){
+			_mosquitto_log_printf(NULL, MOSQ_LOG_NOTICE, "ACL denying access to client with dangerous username \"%s\"", username);
+			return MOSQ_ERR_ACL_DENIED;
+		}
+		if(context->id && strpbrk(context->id, "+#/")){
+			_mosquitto_log_printf(NULL, MOSQ_LOG_NOTICE, "ACL denying access to client with dangerous client id \"%s\"", context->id);
+			return MOSQ_ERR_ACL_DENIED;
+		}
 		return db->auth_plugin.acl_check(db->auth_plugin.user_data, context->id, username, topic, access);
 	}
 }
diff --git a/src/security_default.c b/src/security_default.c
index 64ca846b..a41c21f4 100644
--- a/src/security_default.c
+++ b/src/security_default.c
@@ -261,6 +261,26 @@ int mosquitto_acl_check_default(struct mosquitto_db *db, struct mosquitto *conte
 	}
 
 	acl_root = db->acl_patterns;
+
+	if(acl_root){
+		/* We are using pattern based acls. Check whether the username or
+		 * client id contains a +, # or / and if so deny access.
+		 *
+		 * Without this, a malicious client may configure its username/client
+		 * id to bypass ACL checks (or have a username/client id that cannot
+		 * publish or receive messages to its own place in the hierarchy).
+		 */
+		if(context->username && strpbrk(context->username, "+#/")){
+			_mosquitto_log_printf(NULL, MOSQ_LOG_NOTICE, "ACL denying access to client with dangerous username \"%s\"", context->username);
+			return MOSQ_ERR_ACL_DENIED;
+		}
+
+		if(context->id && strpbrk(context->id, "+#/")){
+			_mosquitto_log_printf(NULL, MOSQ_LOG_NOTICE, "ACL denying access to client with dangerous client id \"%s\"", context->id);
+			return MOSQ_ERR_ACL_DENIED;
+		}
+	}
+
 	/* Loop through all pattern ACLs. */
 	clen = strlen(context->id);
 	while(acl_root){