Add an output_plugin_libraries GUC to bless trusted output plugins

REPLICATION users were not previously subject to restrictions on output
plugin paths, so they were able to bypass LOAD-time protections during
logical decoding. Unfortunately, adding the standard LOAD restrictions
now would retroactively require all third-party output plugins to be
installed under the $libdir/plugins directory. This would prevent the
use of dynamic_library_path, introduce a wire incompatibility for
clients, and require all plugin authors to check that their libraries
are safe for use by any unprivileged user; we want to avoid that.

Instead, introduce an output_plugin_libraries GUC so that DBAs can
specify the output plugins that are trusted for use in logical decoding.
For simplicity, superusers are subject to the restriction as well
(though they're free to modify the GUC at will during a session, so no
power is actually lost).

The default setting is 'pgoutput, test_decoding'. If other third-party
plugins are in use, DBAs will need to modify this parameter after they
update. Some pointers have been added to the documentation to assist
with this.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Reported-by: Vladimir Tokarev <vladimirelitokarev@gmail.com>
Reported-by: Yu Kunpeng <yu443940816@live.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Backpatch-through: 14
Security: CVE-2026-6471

contrib/test_decoding/expected/permissions.out

@@ -29,6 +29,17 @@ SELECT pg_drop_replication_slot('regression_slot');
 (1 row)
 
 RESET ROLE;
+-- no users can load an untrusted plugin
+SET output_plugin_libraries = pgoutput;
+SET ROLE regress_lr_replication;
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+ERROR:  library "test_decoding" may not be used as an output plugin
+HINT:  If it is safe for all REPLICATION users to use this library as an output plugin, add it to "output_plugin_libraries" and reload the server configuration.
+RESET ROLE;
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+ERROR:  library "test_decoding" may not be used as an output plugin
+HINT:  If it is safe for all REPLICATION users to use this library as an output plugin, add it to "output_plugin_libraries" and reload the server configuration.
+RESET output_plugin_libraries;
 -- replication user can control replication
 SET ROLE regress_lr_replication;
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');

contrib/test_decoding/expected/slot.out

@@ -30,8 +30,11 @@ SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_t2', 'tes
  init
 (1 row)
 
+BEGIN;
+SET LOCAL output_plugin_libraries = nonexistent;
 SELECT pg_create_logical_replication_slot('foo', 'nonexistent');
 ERROR:  could not access file "nonexistent": No such file or directory
+ROLLBACK;
 -- here we want to start a new session and wait till old one is gone
 select pg_backend_pid() as oldpid \gset
 \c -

contrib/test_decoding/sql/permissions.sql

@@ -15,6 +15,14 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'inc
 SELECT pg_drop_replication_slot('regression_slot');
 RESET ROLE;
 
+-- no users can load an untrusted plugin
+SET output_plugin_libraries = pgoutput;
+SET ROLE regress_lr_replication;
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+RESET ROLE;
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+RESET output_plugin_libraries;
+
 -- replication user can control replication
 SET ROLE regress_lr_replication;
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');

contrib/test_decoding/sql/slot.sql

@@ -9,7 +9,10 @@ SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_p', 'test
 
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_t2', 'test_decoding', true);
 
+BEGIN;
+SET LOCAL output_plugin_libraries = nonexistent;
 SELECT pg_create_logical_replication_slot('foo', 'nonexistent');
+ROLLBACK;
 
 -- here we want to start a new session and wait till old one is gone
 select pg_backend_pid() as oldpid \gset

doc/src/sgml/config.sgml

@@ -4612,6 +4612,63 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
        </listitem>
       </varlistentry>
 
+      <varlistentry id="guc-output-plugin-libraries" xreflabel="output_plugin_libraries">
+       <term><varname>output_plugin_libraries</varname> (<type>string</type>)
+       <indexterm>
+        <primary><varname>output_plugin_libraries</varname> configuration parameter</primary>
+       </indexterm>
+       </term>
+       <listitem>
+        <para>
+         Lists the libraries installed in <xref linkend="guc-dynamic-library-path"/>
+         that are also trusted for use as logical output plugins by replication
+         clients. Any <link linkend="logicaldecoding-example">logical decoding</link>
+         or <link linkend="protocol-replication">replication</link> requests for
+         other libraries will be refused. All users are subject to this
+         restriction. The default is <literal>'pgoutput, test_decoding'</literal>,
+         which are the two logical output plugins included in the standard
+         <productname>PostgreSQL</productname> distribution.
+        </para>
+        <para>
+         The format is a comma-separated list of library names, where each name
+         is interpreted as for the <link linkend="sql-load"><command>LOAD</command></link>
+         command (but logical decoding clients must specify a plugin name that
+         <emphasis>exactly</emphasis> matches an entry in the list, without
+         variations in case or path structure). Whitespace between entries is
+         ignored; surround a library name with double quotes if you need to
+         include whitespace or commas in the name.
+        </para>
+        <para>
+         It is the responsibility of the server administrator to ensure that
+         libraries added to this list do not unintentionally give additional
+         privileges to non-superusers when they are loaded into the server.
+        </para>
+        <note>
+         <para>
+          When updating the server from a version that does not have the
+          <literal>output_plugin_libraries</literal> parameter, the following
+          query can help construct the list of plugins that are required by all
+          persistent logical replication slots:
+<programlisting>
+SELECT DISTINCT plugin FROM pg_replication_slots WHERE plugin IS NOT NULL;
+</programlisting>
+          Review the list carefully for safety before adjusting
+          <literal>output_plugin_libraries</literal>.
+         </para>
+         <para>
+          The above query can only display plugins which were successfully
+          added to replication slots at some point in the past. Newly refused
+          requests will appear in the logs with a message similar to
+<programlisting>
+ERROR:  library "..." may not be used as an output plugin
+DETAIL:  The configuration parameter "output_plugin_libraries" (currently 'pgoutput, test_decoding') does not name this library as a trusted output plugin.
+HINT:  If it is safe for all REPLICATION users to use this library as an output plugin, add it to "output_plugin_libraries" and reload the server configuration.
+</programlisting>
+         </para>
+        </note>
+       </listitem>
+      </varlistentry>
+
       <varlistentry id="guc-wal-keep-size" xreflabel="wal_keep_size">
        <term><varname>wal_keep_size</varname> (<type>integer</type>)
        <indexterm>

doc/src/sgml/logical-replication.sgml

@@ -2271,6 +2271,15 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
    and it must have the <literal>LOGIN</literal> attribute.
   </para>
 
+  <para>
+   The name of the output plugin used by the replication connection must be
+   included in the server's <xref linkend="guc-output-plugin-libraries"/>. (For
+   subscriptions, the plugin name that is used is <literal>pgoutput</literal>.)
+   Superusers may modify the trusted list per-connection, by including
+   <literal>options=-coutput_plugin_libraries=...</literal> in the connection
+   string.
+  </para>
+
   <para>
    In order to be able to copy the initial table data, the role used for the
    replication connection must have the <literal>SELECT</literal> privilege on
@@ -2487,8 +2496,10 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
     </listitem>
     <listitem>
      <para>
-      The output plugins referenced by the slots on the old cluster must be
-      installed in the new PostgreSQL executable directory.
+      The output plugins referenced by the slots in the old cluster must be
+      installed in the new PostgreSQL executable directory. They must also be
+      included in the new cluster's <xref linkend="guc-output-plugin-libraries"/>;
+      see that parameter's documentation for safety information.
      </para>
     </listitem>
     <listitem>

src/backend/replication/logical/logical.c

@@ -42,9 +42,11 @@
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/builtins.h"
+#include "utils/guc.h"
 #include "utils/injection_point.h"
 #include "utils/inval.h"
 #include "utils/memutils.h"
+#include "utils/varlena.h"
 
 /* data for errcontext callback */
 typedef struct LogicalErrorCallbackState
@@ -54,6 +56,9 @@ typedef struct LogicalErrorCallbackState
 	XLogRecPtr	report_location;
 } LogicalErrorCallbackState;
 
+/* GUC variables */
+char	   *output_plugin_libraries_string;
+
 /* wrappers around output plugin callbacks */
 static void output_plugin_error_callback(void *arg);
 static void startup_cb_wrapper(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
@@ -181,7 +186,69 @@ StartupDecodingContext(List *output_plugin_options,
 	 * now.
 	 */
 	if (!fast_forward)
-		LoadOutputPlugin(&ctx->callbacks, NameStr(slot->data.plugin));
+	{
+		/*
+		 * Before loading this library, make sure it's been blessed for
+		 * logical decoding.
+		 */
+		const char *plugin = NameStr(slot->data.plugin);
+		bool		plugin_allowed = false;
+
+		if (output_plugin_libraries_string && output_plugin_libraries_string[0])
+		{
+			/* Check this plugin against output_plugin_libraries. */
+			char	   *rawstring;
+			List	   *elemlist = NIL;
+
+			/* Need a modifiable copy */
+			rawstring = pstrdup(output_plugin_libraries_string);
+
+			if (!SplitGUCList(rawstring, ',', &elemlist))
+			{
+				/* syntax error in list */
+				ereport(LOG,
+						(errcode(ERRCODE_SYNTAX_ERROR),
+						 errmsg("invalid list syntax in parameter \"%s\"",
+								"output_plugin_libraries")));
+
+				list_free(elemlist);
+				elemlist = NIL;
+			}
+
+			foreach_ptr(char, allowed, elemlist)
+			{
+				if (strcmp(allowed, plugin) == 0)
+				{
+					plugin_allowed = true;
+					break;
+				}
+			}
+
+			list_free(elemlist);
+			pfree(rawstring);
+		}
+
+		if (!plugin_allowed)
+		{
+			/*
+			 * Use the same error message as check_restricted_library_name(),
+			 * but provide additional context for the DBA in the logs. (The
+			 * HINT will be sent to the client, but that's not a secret.)
+			 */
+			ereport(ERROR,
+					errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					errmsg("library \"%s\" may not be used as an output plugin",
+						   plugin),
+			/*- translator: The second %s is the value of the output_plugin_libraries GUC, which may contain whitespace, commas, and double-quotes (") */
+					errdetail_log("The configuration parameter \"%s\" (currently '%s') does not name this library as a trusted output plugin.",
+								  "output_plugin_libraries",
+								  output_plugin_libraries_string),
+					errhint("If it is safe for all REPLICATION users to use this library as an output plugin, add it to \"%s\" and reload the server configuration.",
+							"output_plugin_libraries"));
+		}
+
+		LoadOutputPlugin(&ctx->callbacks, plugin);
+	}
 
 	/*
 	 * Now that the slot's xmin has been set, we can announce ourselves as a
@@ -729,7 +796,9 @@ OutputPluginUpdateProgress(struct LogicalDecodingContext *ctx,
 
 /*
  * Load the output plugin, lookup its output plugin init function, and check
- * that it provides the required callbacks.
+ * that it provides the required callbacks. The caller must have checked that
+ * the current user has the necessary privileges to load the given plugin;
+ * standard LOAD restrictions are not applied here.
  */
 static void
 LoadOutputPlugin(OutputPluginCallbacks *callbacks, const char *plugin)

src/backend/utils/misc/guc_tables.c

@@ -72,6 +72,7 @@
 #include "postmaster/syslogger.h"
 #include "postmaster/walsummarizer.h"
 #include "postmaster/walwriter.h"
+#include "replication/logical.h"
 #include "replication/logicallauncher.h"
 #include "replication/slot.h"
 #include "replication/slotsync.h"
@@ -4982,6 +4983,18 @@ struct config_string ConfigureNamesString[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"output_plugin_libraries", PGC_SUSET, REPLICATION_SENDING,
+			gettext_noop("Lists libraries that may be named as logical decoding output plugins."),
+			gettext_noop("Users with REPLICATION privileges may only use plugins in this list when creating logical replication slots."),
+			/* note: src/bin/pg_upgrade/check.c assumes GUC_LIST_QUOTE here */
+			GUC_LIST_INPUT | GUC_LIST_QUOTE | GUC_SUPERUSER_ONLY
+		},
+		&output_plugin_libraries_string,
+		"pgoutput, test_decoding",
+		NULL, NULL, NULL
+	},
+
 	{
 		{"log_connections", PGC_SU_BACKEND, LOGGING_WHAT,
 			gettext_noop("Logs specified aspects of connection establishment and setup."),

src/backend/utils/misc/postgresql.conf.sample

@@ -818,6 +818,7 @@
 #dynamic_library_path = '$libdir'
 #extension_control_path = '$system'
 #gin_fuzzy_search_limit = 0
+#output_plugin_libraries = 'pgoutput, test_decoding'  # approved plugins for logical decoding
 
 
 #------------------------------------------------------------------------------

src/bin/pg_dump/dumputils.c

@@ -731,6 +731,7 @@ variable_is_guc_list_quote(const char *name)
 {
 	if (pg_strcasecmp(name, "local_preload_libraries") == 0 ||
 		pg_strcasecmp(name, "oauth_validator_libraries") == 0 ||
+		pg_strcasecmp(name, "output_plugin_libraries") == 0 ||
 		pg_strcasecmp(name, "search_path") == 0 ||
 		pg_strcasecmp(name, "session_preload_libraries") == 0 ||
 		pg_strcasecmp(name, "shared_preload_libraries") == 0 ||

src/bin/pg_upgrade/check.c

@@ -2058,6 +2058,7 @@ check_new_cluster_logical_replication_slots(void)
 	int			nslots_on_old;
 	int			nslots_on_new;
 	int			max_replication_slots;
+	char	   *output_plugin_libraries;
 	char	   *wal_level;
 
 	/* Logical slots can be migrated since PG17. */
@@ -2091,10 +2092,10 @@ check_new_cluster_logical_replication_slots(void)
 	PQclear(res);
 
 	res = executeQueryOrDie(conn, "SELECT setting FROM pg_settings "
-							"WHERE name IN ('wal_level', 'max_replication_slots') "
+							"WHERE name IN ('wal_level', 'output_plugin_libraries', 'max_replication_slots') "
 							"ORDER BY name DESC;");
 
-	if (PQntuples(res) != 2)
+	if (PQntuples(res) != 3)
 		pg_fatal("could not determine parameter settings on new cluster");
 
 	wal_level = PQgetvalue(res, 0, 0);
@@ -2103,7 +2104,85 @@ check_new_cluster_logical_replication_slots(void)
 		pg_fatal("\"wal_level\" must be \"logical\" but is set to \"%s\"",
 				 wal_level);
 
-	max_replication_slots = atoi(PQgetvalue(res, 1, 0));
+	output_plugin_libraries = PQgetvalue(res, 1, 0);
+
+	/*
+	 * Make sure the output_plugin_libraries setting covers all plugins needed
+	 * by any migrated slots.
+	 */
+	if (nslots_on_old > 0)
+	{
+		char	   *guc_copy = pg_strdup(output_plugin_libraries);
+		char	  **allowed_plugins;
+		char		output_path[MAXPGPATH];
+		FILE	   *script = NULL;
+
+		if (!SplitGUCList(guc_copy, ',', &allowed_plugins))
+		{
+			/*
+			 * Should not happen. (Frontend and backend GUC_LIST_QUOTE parsing
+			 * have to remain compatible for pg_dump at minimum.)
+			 */
+			pg_fatal("could not parse \"output_plugin_libraries\" setting '%s'",
+					 output_plugin_libraries);
+		}
+
+		snprintf(output_path, sizeof(output_path), "%s/%s",
+				 log_opts.basedir,
+				 "disallowed_output_plugins.txt");
+
+		for (int dbnum = 0; dbnum < old_cluster.dbarr.ndbs; dbnum++)
+		{
+			LogicalSlotInfoArr *slot_arr = &old_cluster.dbarr.dbs[dbnum].slot_arr;
+
+			for (int slotnum = 0; slotnum < slot_arr->nslots; slotnum++)
+			{
+				LogicalSlotInfo *slot = &slot_arr->slots[slotnum];
+				bool		allowed = false;
+
+				/*
+				 * We expect the output_plugin_libraries length to be small in
+				 * practice; O(n*m) shouldn't be a problem here.
+				 */
+				for (char **p = allowed_plugins; *p; p++)
+				{
+					if (strcmp(slot->plugin, *p) == 0)
+					{
+						allowed = true;
+						break;
+					}
+				}
+
+				if (!allowed)
+				{
+					if (script == NULL &&
+						(script = fopen_priv(output_path, "w")) == NULL)
+						pg_fatal("could not open file \"%s\": %m", output_path);
+
+					fprintf(script, "The slot \"%s\" uses plugin \"%s\"\n",
+							slot->slotname, slot->plugin);
+				}
+			}
+		}
+
+		if (script)
+		{
+			fclose(script);
+
+			pg_log(PG_REPORT, "fatal");
+			pg_fatal("Your installation contains logical replication slots with plugins\n"
+					 "that are not allowed by the new cluster's output_plugin_libraries\n"
+					 "setting. You can add trusted plugins to output_plugin_libraries\n"
+					 "and/or remove affected slots, and then restart the upgrade.\n"
+					 "A list of the problematic slots is in the file:\n"
+					 "    %s", output_path);
+		}
+
+		pg_free(allowed_plugins);
+		pg_free(guc_copy);
+	}
+
+	max_replication_slots = atoi(PQgetvalue(res, 2, 0));
 
 	if (nslots_on_old > max_replication_slots)
 		pg_fatal("\"max_replication_slots\" (%d) must be greater than or equal to the number of "

src/bin/pg_upgrade/t/003_logical_slots.pl

@@ -62,7 +62,7 @@ chdir ${PostgreSQL::Test::Utils::tmp_check};
 $oldpub->start;
 $oldpub->safe_psql(
 	'postgres', qq[
-	SELECT pg_create_logical_replication_slot('test_slot1', 'test_decoding');
+	SELECT pg_create_logical_replication_slot('test_slot1', 'pgoutput');
 	SELECT pg_create_logical_replication_slot('test_slot2', 'test_decoding');
 ]);
 $oldpub->stop();
@@ -89,6 +89,48 @@ ok(-d $newpub->data_dir . "/pg_upgrade_output.d",
 # old cluster. Both slots will be used for subsequent tests.
 $newpub->append_conf('postgresql.conf', "max_replication_slots = 2");
 
+# ------------------------------
+# TEST: Confirm pg_upgrade fails when slot plugins are prohibited by the new cluster
+
+$newpub->append_conf('postgresql.conf',
+	"output_plugin_libraries = 'pgoutput'");
+
+command_checks_all(
+	[@pg_upgrade_cmd],
+	1,
+	[
+		qr/Your installation contains logical replication slots with plugins/,
+		qr/that are not allowed by the new cluster's output_plugin_libraries/,
+	],
+	[qr//],
+	'run of pg_upgrade where the old cluster has untrusted output plugins');
+
+my $slots_filename;
+
+# Find a txt file that contains a list of logical replication slots that cannot
+# be upgraded. We cannot predict the file's path because the output directory
+# contains a milliseconds timestamp. File::Find::find must be used.
+find(
+	sub {
+		if ($File::Find::name =~ m/disallowed_output_plugins\.txt/)
+		{
+			$slots_filename = $File::Find::name;
+		}
+	},
+	$newpub->data_dir . "/pg_upgrade_output.d");
+
+# Check the report.
+my $content = slurp_file($slots_filename);
+like(
+	$content,
+	qr/The slot "test_slot2" uses plugin "test_decoding"/m,
+	'the previous test failed due to prohibited plugins');
+unlike($content, qr/test_slot1/m, 'allowed plugin is not reported');
+
+# Fix things for the next tests.
+$newpub->append_conf('postgresql.conf',
+	"output_plugin_libraries = 'pgoutput, test_decoding'");
+
 
 # ------------------------------
 # TEST: Confirm pg_upgrade fails when the slot still has unconsumed WAL records
@@ -124,11 +166,6 @@ command_checks_all(
 );
 
 # Verify the reason why the logical replication slot cannot be upgraded
-my $slots_filename;
-
-# Find a txt file that contains a list of logical replication slots that cannot
-# be upgraded. We cannot predict the file's path because the output directory
-# contains a milliseconds timestamp. File::Find::find must be used.
 find(
 	sub {
 		if ($File::Find::name =~ m/invalid_logical_slots\.txt/)

src/include/replication/logical.h

@@ -152,4 +152,7 @@ extern bool LogicalReplicationSlotHasPendingWal(XLogRecPtr end_of_wal);
 extern XLogRecPtr LogicalSlotAdvanceAndCheckSnapState(XLogRecPtr moveto,
 													  bool *found_consistent_snapshot);
 
+/* GUCs */
+extern PGDLLIMPORT char *output_plugin_libraries_string;
+
 #endif

src/test/subscription/t/100_bugs.pl

@@ -477,13 +477,13 @@ $result =
 is( $result, qq(2|f
 3|t), 'check replicated update on subscriber');
 
-# Test create and immediate drop of replication slot via replication commands
-# (this exposed a memory-management bug in v18)
 my $publisher_host = $node_publisher->host;
 my $publisher_port = $node_publisher->port;
 my $connstr_db =
   "host=$publisher_host port=$publisher_port replication=database dbname=postgres";
 
+# Test create and immediate drop of replication slot via replication commands
+# (this exposed a memory-management bug in v18)
 is( $node_publisher->psql(
 		'postgres',
 		qq[
@@ -495,6 +495,27 @@ is( $node_publisher->psql(
 	0,
 	'create and immediate drop of replication slot');
 
+# REPLICATION users should not be able to bypass LOAD restrictions.
+$node_publisher->safe_psql(
+	'postgres', qq(
+	CREATE USER repluser REPLICATION;
+));
+
+my ($ret, $stdout, $stderr) = $node_publisher->psql(
+	'postgres',
+	qq[
+		SET ROLE repluser;
+		CREATE_REPLICATION_SLOT fail_slot LOGICAL regress;',
+	],
+	timeout => $PostgreSQL::Test::Utils::timeout_default,
+	extra_params => [ '-d', $connstr_db ]);
+
+is($ret, 3, 'loading unblessed output plugin fails');
+like(
+	$stderr,
+	qr/ERROR:  library "regress" may not be used as an output plugin/,
+	'loading unblessed output plugin fails: stderr');
+
 $node_publisher->stop('fast');
 $node_subscriber->stop('fast');
 
@@ -592,7 +613,7 @@ $node_publisher->safe_psql(
 	CREATE SUBSCRIPTION regress_sub1 CONNECTION '$publisher_connstr' PUBLICATION regress_pub WITH (connect=false);
 ));
 
-my ($ret, $stdout, $stderr) =
+($ret, $stdout, $stderr) =
   $node_publisher->psql('postgres', q{DROP SUBSCRIPTION regress_sub1});
 
 isnt($ret, 0, "replication slot does not exist: exit code not 0");