diff --git a/mysql-test/suite/plugins/r/mdev38550.result b/mysql-test/suite/plugins/r/mdev38550.result
new file mode 100644
index 0000000000000..1265fb7dc9964
--- /dev/null
+++ b/mysql-test/suite/plugins/r/mdev38550.result
@@ -0,0 +1,51 @@
+#
+# Setup
+#
+INSTALL PLUGIN IF NOT EXISTS cleartext_plugin_server SONAME '';
+Warnings:
+Note	1968	Plugin 'cleartext_plugin_server' already installed
+CREATE DATABASE mdev38550_db;
+#
+# Test 1: COM_CHANGE_USER with short password
+# Verify mysql_change_user() works correctly with database parameter
+#
+CREATE USER changeusertest IDENTIFIED VIA cleartext_plugin_server USING 'changepwd';
+GRANT ALL ON *.* TO changeusertest;
+SELECT DATABASE() AS db;
+db
+mdev38550_db
+#
+# Test 2: COM_CHANGE_USER with long password (260 bytes)
+# This works because auth plugin switching sends password in a SECOND packet
+# (via ma_net_write), bypassing the 255-byte limit in send_change_user_packet()
+#
+# Note: If connection used cleartext directly (no auth switch), it would fail
+# due to libmariadb's 255-byte limit in send_change_user_packet()
+#
+CREATE USER changeuserlongpwd IDENTIFIED VIA cleartext_plugin_server USING 'cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc';
+GRANT ALL ON *.* TO changeuserlongpwd;
+SELECT DATABASE() AS db;
+db
+mdev38550_db
+#
+# Test 3: COM_CHANGE_USER with long password - no auth switch
+# Connection uses cleartext directly, password goes in first packet
+# Requires LENENC support in both client and server for COM_CHANGE_USER
+#
+connect  cleartext_con, 127.0.0.1, changeuserlongpwd, $change_pwd, mdev38550_db, $MASTER_MYPORT, , , mysql_clear_password;
+SELECT DATABASE() AS db;
+db
+mdev38550_db
+SELECT CURRENT_USER() AS user;
+user
+changeuserlongpwd@%
+SELECT DATABASE() AS db;
+db
+mdev38550_db
+connection default;
+disconnect cleartext_con;
+#
+# Cleanup
+#
+DROP USER changeusertest, changeuserlongpwd;
+DROP DATABASE mdev38550_db;
diff --git a/mysql-test/suite/plugins/t/mdev38431.test b/mysql-test/suite/plugins/t/mdev38431.test
index 21b60078f4abf..c494790bb660c 100644
--- a/mysql-test/suite/plugins/t/mdev38431.test
+++ b/mysql-test/suite/plugins/t/mdev38431.test
@@ -56,4 +56,3 @@ GRANT ALL ON *.* TO verylonguser;
 --echo #
 DROP USER shortuser, longuser, verylonguser;
 DROP DATABASE mdev38431_db;
-# Note: Do not uninstall cleartext_plugin_server as it was pre-loaded by MTR
diff --git a/mysql-test/suite/plugins/t/mdev38550.test b/mysql-test/suite/plugins/t/mdev38550.test
new file mode 100644
index 0000000000000..6ea90b2c3ec33
--- /dev/null
+++ b/mysql-test/suite/plugins/t/mdev38550.test
@@ -0,0 +1,66 @@
+#
+# MDEV-38550: COM_CHANGE_USER with Long Password Corrupts Database Name
+#
+# When password > 255 bytes with COM_CHANGE_USER, the password length was
+# truncated to a single byte, causing the server to read garbage as the
+# database name.
+#
+# Fix: Add LENENC support for COM_CHANGE_USER password field in both
+# client (libmariadb) and server (sql_acl.cc).
+#
+
+--source include/not_embedded.inc
+--source include/have_plugin_auth.inc
+
+--echo #
+--echo # Setup
+--echo #
+eval INSTALL PLUGIN IF NOT EXISTS cleartext_plugin_server SONAME '$PLUGIN_AUTH';
+CREATE DATABASE mdev38550_db;
+
+--echo #
+--echo # Test 1: COM_CHANGE_USER with short password
+--echo # Verify mysql_change_user() works correctly with database parameter
+--echo #
+CREATE USER changeusertest IDENTIFIED VIA cleartext_plugin_server USING 'changepwd';
+GRANT ALL ON *.* TO changeusertest;
+
+# Use change_user command to switch user with password and database
+change_user changeusertest,changepwd,mdev38550_db;
+SELECT DATABASE() AS db;
+change_user root;
+
+--echo #
+--echo # Test 2: COM_CHANGE_USER with long password (260 bytes)
+--echo # This works because auth plugin switching sends password in a SECOND packet
+--echo # (via ma_net_write), bypassing the 255-byte limit in send_change_user_packet()
+--echo #
+--echo # Note: If connection used cleartext directly (no auth switch), it would fail
+--echo # due to libmariadb's 255-byte limit in send_change_user_packet()
+--echo #
+--let $change_pwd=`SELECT REPEAT('c', 260)`
+eval CREATE USER changeuserlongpwd IDENTIFIED VIA cleartext_plugin_server USING '$change_pwd';
+GRANT ALL ON *.* TO changeuserlongpwd;
+
+change_user changeuserlongpwd,$change_pwd,mdev38550_db;
+SELECT DATABASE() AS db;
+change_user root;
+
+--echo #
+--echo # Test 3: COM_CHANGE_USER with long password - no auth switch
+--echo # Connection uses cleartext directly, password goes in first packet
+--echo # Requires LENENC support in both client and server for COM_CHANGE_USER
+--echo #
+--connect (cleartext_con, 127.0.0.1, changeuserlongpwd, $change_pwd, mdev38550_db, $MASTER_MYPORT, , , mysql_clear_password)
+SELECT DATABASE() AS db;
+SELECT CURRENT_USER() AS user;
+change_user changeuserlongpwd,$change_pwd,mdev38550_db;
+SELECT DATABASE() AS db;
+--connection default
+--disconnect cleartext_con
+
+--echo #
+--echo # Cleanup
+--echo #
+DROP USER changeusertest, changeuserlongpwd;
+DROP DATABASE mdev38550_db;
diff --git a/sql/sql_acl.cc b/sql/sql_acl.cc
index 512a0b055f736..ef0a214fbdbd6 100644
--- a/sql/sql_acl.cc
+++ b/sql/sql_acl.cc
@@ -13613,10 +13613,30 @@ static bool parse_com_change_user_packet(MPVIO_EXT *mpvio, uint packet_length)
     Cast *passwd to an unsigned char, so that it doesn't extend the sign for
     *passwd > 127 and become 2**32-127+ after casting to uint.
   */
-  uint passwd_len= (thd->client_capabilities & CLIENT_SECURE_CONNECTION ?
-                    (uchar) (*passwd++) : (uint)strlen(passwd));
-
-  db+= passwd_len + 1;
+  size_t passwd_len;
+  if (!(thd->client_capabilities & CLIENT_SECURE_CONNECTION))
+  {
+    passwd_len= strlen(passwd);
+    db= passwd + passwd_len + 1;  /* +1 to skip null terminator */
+  }
+  else if (!(thd->client_capabilities & CLIENT_PLUGIN_AUTH_LENENC_CLIENT_DATA))
+  {
+    passwd_len= (uchar)(*passwd++);
+    db= passwd + passwd_len;
+  }
+  else
+  {
+    ulonglong len= safe_net_field_length_ll((uchar**)&passwd,
+                                            end - passwd);
+    if (len > packet_length)
+    {
+      my_message(ER_UNKNOWN_COM_ERROR, ER_THD(thd, ER_UNKNOWN_COM_ERROR),
+                 MYF(0));
+      DBUG_RETURN(1);
+    }
+    passwd_len= (size_t)len;
+    db= passwd + passwd_len;
+  }
   /*
     Database name is always NUL-terminated, so in case of empty database
     the packet must contain at least the trailing '\0'.