diff --git a/src/backend/distributed/commands/multi_copy.c b/src/backend/distributed/commands/multi_copy.c
index 8887513cc..ee8cd5700 100644
--- a/src/backend/distributed/commands/multi_copy.c
+++ b/src/backend/distributed/commands/multi_copy.c
@@ -2609,6 +2609,7 @@ ProcessCopyStmt(CopyStmt *copyStatement, char *completionTag, const char *queryS
 			Query *query = NULL;
 			Node *queryNode = copyStatement->query;
 			List *queryTreeList = NIL;
+			StringInfo userFilePath = makeStringInfo();
 
 #if (PG_VERSION_NUM >= 100000)
 			RawStmt *rawStmt = makeNode(RawStmt);
@@ -2625,6 +2626,14 @@ ProcessCopyStmt(CopyStmt *copyStatement, char *completionTag, const char *queryS
 			}
 
 			query = (Query *) linitial(queryTreeList);
+
+			/*
+			 * Add a user ID suffix to prevent other users from reading/writing
+			 * the same file. We do this consistently in all functions that interact
+			 * with task files.
+			 */
+			appendStringInfo(userFilePath, "%s.%u", filename, GetUserId());
+
 			tuplesSent = WorkerExecuteSqlTask(query, filename, binaryCopyFormat);
 
 			snprintf(completionTag, COMPLETION_TAG_BUFSIZE,
diff --git a/src/backend/distributed/commands/transmit.c b/src/backend/distributed/commands/transmit.c
index 95c90bba9..fa976661c 100644
--- a/src/backend/distributed/commands/transmit.c
+++ b/src/backend/distributed/commands/transmit.c
@@ -359,6 +359,32 @@ IsTransmitStmt(Node *parsetree)
 }
 
 
+/*
+ * TransmitStatementUser extracts the user attribute from a
+ * COPY ... (format 'transmit', user '...') statement.
+ */
+char *
+TransmitStatementUser(CopyStmt *copyStatement)
+{
+	ListCell *optionCell = NULL;
+	char *userName = NULL;
+
+	AssertArg(IsTransmitStmt((Node *) copyStatement));
+
+	foreach(optionCell, copyStatement->options)
+	{
+		DefElem *defel = (DefElem *) lfirst(optionCell);
+
+		if (strncmp(defel->defname, "user", NAMEDATALEN) == 0)
+		{
+			userName = defGetString(defel);
+		}
+	}
+
+	return userName;
+}
+
+
 /*
  * VerifyTransmitStmt checks that the passed in command is a valid transmit
  * statement. Raise ERROR if not.
diff --git a/src/backend/distributed/commands/utility_hook.c b/src/backend/distributed/commands/utility_hook.c
index b247e4952..f7699a94e 100644
--- a/src/backend/distributed/commands/utility_hook.c
+++ b/src/backend/distributed/commands/utility_hook.c
@@ -222,17 +222,28 @@ multi_ProcessUtility(PlannedStmt *pstmt,
 	if (IsTransmitStmt(parsetree))
 	{
 		CopyStmt *copyStatement = (CopyStmt *) parsetree;
+		char *userName = TransmitStatementUser(copyStatement);
+		bool missingOK = false;
+		StringInfo transmitPath = makeStringInfo();
 
 		VerifyTransmitStmt(copyStatement);
 
 		/* ->relation->relname is the target file in our overloaded COPY */
+		appendStringInfoString(transmitPath, copyStatement->relation->relname);
+
+		if (userName != NULL)
+		{
+			Oid userId = get_role_oid(userName, missingOK);
+			appendStringInfo(transmitPath, ".%d", userId);
+		}
+
 		if (copyStatement->is_from)
 		{
-			RedirectCopyDataToRegularFile(copyStatement->relation->relname);
+			RedirectCopyDataToRegularFile(transmitPath->data);
 		}
 		else
 		{
-			SendRegularFile(copyStatement->relation->relname);
+			SendRegularFile(transmitPath->data);
 		}
 
 		/* Don't execute the faux copy statement */
diff --git a/src/backend/distributed/executor/multi_task_tracker_executor.c b/src/backend/distributed/executor/multi_task_tracker_executor.c
index 724359b57..ae4dcd682 100644
--- a/src/backend/distributed/executor/multi_task_tracker_executor.c
+++ b/src/backend/distributed/executor/multi_task_tracker_executor.c
@@ -2590,9 +2590,11 @@ ManageTransmitTracker(TaskTracker *transmitTracker)
 		int32 connectionId = transmitTracker->connectionId;
 		StringInfo jobDirectoryName = JobDirectoryName(transmitState->jobId);
 		StringInfo taskFilename = TaskFilename(jobDirectoryName, transmitState->taskId);
+		char *userName = CurrentUserName();
 
 		StringInfo fileTransmitQuery = makeStringInfo();
-		appendStringInfo(fileTransmitQuery, TRANSMIT_REGULAR_COMMAND, taskFilename->data);
+		appendStringInfo(fileTransmitQuery, TRANSMIT_WITH_USER_COMMAND,
+						 taskFilename->data, quote_literal_cstr(userName));
 
 		fileTransmitStarted = MultiClientSendQuery(connectionId, fileTransmitQuery->data);
 		if (fileTransmitStarted)
diff --git a/src/backend/distributed/worker/worker_data_fetch_protocol.c b/src/backend/distributed/worker/worker_data_fetch_protocol.c
index b90e19f9c..d0533158a 100644
--- a/src/backend/distributed/worker/worker_data_fetch_protocol.c
+++ b/src/backend/distributed/worker/worker_data_fetch_protocol.c
@@ -108,7 +108,7 @@ worker_fetch_partition_file(PG_FUNCTION_ARGS)
 
 	/* local filename is <jobId>/<upstreamTaskId>/<partitionTaskId> */
 	StringInfo taskDirectoryName = TaskDirectoryName(jobId, upstreamTaskId);
-	StringInfo taskFilename = TaskFilename(taskDirectoryName, partitionTaskId);
+	StringInfo taskFilename = UserTaskFilename(taskDirectoryName, partitionTaskId);
 
 	/*
 	 * If we are the first function to fetch a file for the upstream task, the
@@ -154,7 +154,7 @@ worker_fetch_query_results_file(PG_FUNCTION_ARGS)
 
 	/* local filename is <jobId>/<upstreamTaskId>/<queryTaskId> */
 	StringInfo taskDirectoryName = TaskDirectoryName(jobId, upstreamTaskId);
-	StringInfo taskFilename = TaskFilename(taskDirectoryName, queryTaskId);
+	StringInfo taskFilename = UserTaskFilename(taskDirectoryName, queryTaskId);
 
 	/*
 	 * If we are the first function to fetch a file for the upstream task, the
@@ -191,6 +191,21 @@ TaskFilename(StringInfo directoryName, uint32 taskId)
 }
 
 
+/*
+ * UserTaskFilename returns a full file path for a task file including the
+ * current user ID as a suffix.
+ */
+StringInfo
+UserTaskFilename(StringInfo directoryName, uint32 taskId)
+{
+	StringInfo taskFilename = TaskFilename(directoryName, taskId);
+
+	appendStringInfo(taskFilename, ".%u", GetUserId());
+
+	return taskFilename;
+}
+
+
 /*
  * FetchRegularFileAsSuperUser copies a file from a remote node in an idempotent
  * manner. It connects to the remote node as superuser to give file access.
@@ -203,6 +218,7 @@ FetchRegularFileAsSuperUser(const char *nodeName, uint32 nodePort,
 	char *nodeUser = NULL;
 	StringInfo attemptFilename = NULL;
 	StringInfo transmitCommand = NULL;
+	char *userName = CurrentUserName();
 	uint32 randomId = (uint32) random();
 	bool received = false;
 	int renamed = 0;
@@ -217,7 +233,8 @@ FetchRegularFileAsSuperUser(const char *nodeName, uint32 nodePort,
 					 MIN_TASK_FILENAME_WIDTH, randomId, ATTEMPT_FILE_SUFFIX);
 
 	transmitCommand = makeStringInfo();
-	appendStringInfo(transmitCommand, TRANSMIT_REGULAR_COMMAND, remoteFilename->data);
+	appendStringInfo(transmitCommand, TRANSMIT_WITH_USER_COMMAND, remoteFilename->data,
+					 quote_literal_cstr(userName));
 
 	/* connect as superuser to give file access */
 	nodeUser = CitusExtensionOwnerName();
diff --git a/src/backend/distributed/worker/worker_merge_protocol.c b/src/backend/distributed/worker/worker_merge_protocol.c
index f95d19d16..74331f984 100644
--- a/src/backend/distributed/worker/worker_merge_protocol.c
+++ b/src/backend/distributed/worker/worker_merge_protocol.c
@@ -23,6 +23,7 @@
 #include "catalog/pg_namespace.h"
 #include "commands/copy.h"
 #include "commands/tablecmds.h"
+#include "common/string.h"
 #include "distributed/metadata_cache.h"
 #include "distributed/worker_protocol.h"
 #include "distributed/version_compat.h"
@@ -42,7 +43,7 @@ static List * ArrayObjectToCStringList(ArrayType *arrayObject);
 static void CreateTaskTable(StringInfo schemaName, StringInfo relationName,
 							List *columnNameList, List *columnTypeList);
 static void CopyTaskFilesFromDirectory(StringInfo schemaName, StringInfo relationName,
-									   StringInfo sourceDirectoryName);
+									   StringInfo sourceDirectoryName, Oid userId);
 
 
 /* exports for SQL callable functions */
@@ -78,6 +79,7 @@ worker_merge_files_into_table(PG_FUNCTION_ARGS)
 	List *columnTypeList = NIL;
 	Oid savedUserId = InvalidOid;
 	int savedSecurityContext = 0;
+	Oid userId = GetUserId();
 
 	/* we should have the same number of column names and types */
 	int32 columnNameCount = ArrayObjectCount(columnNameObject);
@@ -112,7 +114,8 @@ worker_merge_files_into_table(PG_FUNCTION_ARGS)
 	GetUserIdAndSecContext(&savedUserId, &savedSecurityContext);
 	SetUserIdAndSecContext(CitusExtensionOwner(), SECURITY_LOCAL_USERID_CHANGE);
 
-	CopyTaskFilesFromDirectory(jobSchemaName, taskTableName, taskDirectoryName);
+	CopyTaskFilesFromDirectory(jobSchemaName, taskTableName, taskDirectoryName,
+							   userId);
 
 	SetUserIdAndSecContext(savedUserId, savedSecurityContext);
 
@@ -155,6 +158,7 @@ worker_merge_files_and_run_query(PG_FUNCTION_ARGS)
 	int createMergeTableResult = 0;
 	int createIntermediateTableResult = 0;
 	int finished = 0;
+	Oid userId = GetUserId();
 
 	CheckCitusVersion(ERROR);
 
@@ -196,7 +200,8 @@ worker_merge_files_and_run_query(PG_FUNCTION_ARGS)
 
 	appendStringInfo(mergeTableName, "%s%s", intermediateTableName->data,
 					 MERGE_TABLE_SUFFIX);
-	CopyTaskFilesFromDirectory(jobSchemaName, mergeTableName, taskDirectoryName);
+	CopyTaskFilesFromDirectory(jobSchemaName, mergeTableName, taskDirectoryName,
+							   userId);
 
 	createIntermediateTableResult = SPI_exec(createIntermediateTableQuery, 0);
 	if (createIntermediateTableResult < 0)
@@ -482,14 +487,20 @@ CreateStatement(RangeVar *relation, List *columnDefinitionList)
  * CopyTaskFilesFromDirectory finds all files in the given directory, except for
  * those having an attempt suffix. The function then copies these files into the
  * database table identified by the given schema and table name.
+ *
+ * The function makes sure all files were generated by the current user by checking
+ * whether the filename ends with the username, since this is added to local file
+ * names by functions such as worker_fetch_partition-file. Files that were generated
+ * by other users calling worker_fetch_partition_file directly are skipped.
  */
 static void
 CopyTaskFilesFromDirectory(StringInfo schemaName, StringInfo relationName,
-						   StringInfo sourceDirectoryName)
+						   StringInfo sourceDirectoryName, Oid userId)
 {
 	const char *directoryName = sourceDirectoryName->data;
 	struct dirent *directoryEntry = NULL;
 	uint64 copiedRowTotal = 0;
+	StringInfo expectedFileSuffix = makeStringInfo();
 
 	DIR *directory = AllocateDir(directoryName);
 	if (directory == NULL)
@@ -498,6 +509,8 @@ CopyTaskFilesFromDirectory(StringInfo schemaName, StringInfo relationName,
 						errmsg("could not open directory \"%s\": %m", directoryName)));
 	}
 
+	appendStringInfo(expectedFileSuffix, ".%u", userId);
+
 	directoryEntry = ReadDir(directory, directoryName);
 	for (; directoryEntry != NULL; directoryEntry = ReadDir(directory, directoryName))
 	{
@@ -516,6 +529,18 @@ CopyTaskFilesFromDirectory(StringInfo schemaName, StringInfo relationName,
 			continue;
 		}
 
+		if (!pg_str_endswith(baseFilename, expectedFileSuffix->data))
+		{
+			/*
+			 * Someone is trying to tamper with our results. We don't throw an error
+			 * here because we don't want to allow users to prevent each other from
+			 * running queries.
+			 */
+			ereport(WARNING, (errmsg("Task file \"%s\" does not have expected suffix "
+									 "\"%s\"", baseFilename, expectedFileSuffix->data)));
+			continue;
+		}
+
 		fullFilename = makeStringInfo();
 		appendStringInfo(fullFilename, "%s/%s", directoryName, baseFilename);
 
diff --git a/src/backend/distributed/worker/worker_partition_protocol.c b/src/backend/distributed/worker/worker_partition_protocol.c
index 386e7fd28..d5ca9b44f 100644
--- a/src/backend/distributed/worker/worker_partition_protocol.c
+++ b/src/backend/distributed/worker/worker_partition_protocol.c
@@ -16,6 +16,7 @@
 
 #include "postgres.h"
 #include "funcapi.h"
+#include "miscadmin.h"
 #include "pgstat.h"
 
 #include <arpa/inet.h>
@@ -80,6 +81,7 @@ static void OutputBinaryFooters(FileOutputStream *partitionFileArray, uint32 fil
 static uint32 RangePartitionId(Datum partitionValue, const void *context);
 static uint32 HashPartitionId(Datum partitionValue, const void *context);
 static uint32 HashPartitionIdViaDeprecatedAPI(Datum partitionValue, const void *context);
+static StringInfo UserPartitionFilename(StringInfo directoryName, uint32 partitionId);
 static bool FileIsLink(char *filename, struct stat filestat);
 
 
@@ -509,7 +511,7 @@ OpenPartitionFiles(StringInfo directoryName, uint32 fileCount)
 
 	for (fileIndex = 0; fileIndex < fileCount; fileIndex++)
 	{
-		StringInfo filePath = PartitionFilename(directoryName, fileIndex);
+		StringInfo filePath = UserPartitionFilename(directoryName, fileIndex);
 
 		fileDescriptor = PathNameOpenFilePerm(filePath->data, fileFlags, fileMode);
 		if (fileDescriptor < 0)
@@ -606,7 +608,14 @@ TaskDirectoryName(uint64 jobId, uint32 taskId)
 }
 
 
-/* Constructs a standardized partition file path for given directory and id. */
+/*
+ * PartitionFilename returns a partition file path for given directory and id
+ * which is suitable for use in worker_fetch_partition_file and tranmsit.
+ *
+ * It excludes the user ID part at the end of the filename, since that is
+ * added by worker_fetch_partition_file itself based on the current user.
+ * For the full path use UserPartitionFilename.
+ */
 StringInfo
 PartitionFilename(StringInfo directoryName, uint32 partitionId)
 {
@@ -619,6 +628,21 @@ PartitionFilename(StringInfo directoryName, uint32 partitionId)
 }
 
 
+/*
+ * UserPartitionFilename returns the path of a partition file for the given
+ * partition ID and the current user.
+ */
+static StringInfo
+UserPartitionFilename(StringInfo directoryName, uint32 partitionId)
+{
+	StringInfo partitionFilename = PartitionFilename(directoryName, partitionId);
+
+	appendStringInfo(partitionFilename, ".%u", GetUserId());
+
+	return partitionFilename;
+}
+
+
 /*
  * JobDirectoryElement takes in a filename, and checks if this name lives in the
  * directory path that is used for task output files. Note that this function's
diff --git a/src/backend/distributed/worker/worker_sql_task_protocol.c b/src/backend/distributed/worker/worker_sql_task_protocol.c
index 1648a0c2c..c9246038c 100644
--- a/src/backend/distributed/worker/worker_sql_task_protocol.c
+++ b/src/backend/distributed/worker/worker_sql_task_protocol.c
@@ -82,7 +82,7 @@ worker_execute_sql_task(PG_FUNCTION_ARGS)
 
 	/* job directory is created prior to scheduling the task */
 	StringInfo jobDirectoryName = JobDirectoryName(jobId);
-	StringInfo taskFilename = TaskFilename(jobDirectoryName, taskId);
+	StringInfo taskFilename = UserTaskFilename(jobDirectoryName, taskId);
 
 	query = ParseQueryString(queryString);
 	tuplesSent = WorkerExecuteSqlTask(query, taskFilename->data, binaryCopyFormat);
diff --git a/src/include/distributed/transmit.h b/src/include/distributed/transmit.h
index 28ed0dad3..5aadafec6 100644
--- a/src/include/distributed/transmit.h
+++ b/src/include/distributed/transmit.h
@@ -28,6 +28,7 @@ extern void FreeStringInfo(StringInfo stringInfo);
 
 /* Local functions forward declarations for Transmit statement */
 extern bool IsTransmitStmt(Node *parsetree);
+extern char * TransmitStatementUser(CopyStmt *copyStatement);
 extern void VerifyTransmitStmt(CopyStmt *copyStatement);
 
 
diff --git a/src/include/distributed/worker_protocol.h b/src/include/distributed/worker_protocol.h
index b92119b7f..5dca935cb 100644
--- a/src/include/distributed/worker_protocol.h
+++ b/src/include/distributed/worker_protocol.h
@@ -44,7 +44,8 @@
 
 /* Defines used for fetching files and tables */
 /* the tablename in the overloaded COPY statement is the to-be-transferred file */
-#define TRANSMIT_REGULAR_COMMAND "COPY \"%s\" TO STDOUT WITH (format 'transmit')"
+#define TRANSMIT_WITH_USER_COMMAND \
+	"COPY \"%s\" TO STDOUT WITH (format 'transmit', user %s)"
 #define COPY_OUT_COMMAND "COPY %s TO STDOUT"
 #define COPY_SELECT_ALL_OUT_COMMAND "COPY (SELECT * FROM %s) TO STDOUT"
 #define COPY_IN_COMMAND "COPY %s FROM '%s'"
@@ -128,6 +129,7 @@ extern int64 WorkerExecuteSqlTask(Query *query, char *taskFilename,
 
 /* Function declarations shared with the master planner */
 extern StringInfo TaskFilename(StringInfo directoryName, uint32 taskId);
+extern StringInfo UserTaskFilename(StringInfo directoryName, uint32 taskId);
 extern List * ExecuteRemoteQuery(const char *nodeName, uint32 nodePort, char *runAsUser,
 								 StringInfo queryString);
 extern List * ColumnDefinitionList(List *columnNameList, List *columnTypeList);