irc: limit size of data received from the server to prevent memory exhaustion

A malicious or compromised IRC server could send data with no end-of-line
(or a flood of "005" messages), making WeeChat accumulate it in a buffer
that grew without limit, until all memory was exhausted.

The unterminated received message and the accumulated "005" (ISUPPORT)
data are now bounded by IRC_SERVER_RECV_MSG_MAX_LENGTH and
IRC_SERVER_ISUPPORT_MAX_LENGTH: extra data is ignored once the limit is
reached.

CHANGELOG.md

@@ -6,6 +6,12 @@ SPDX-License-Identifier: GPL-3.0-or-later
 
 # WeeChat ChangeLog
 
+## Version 4.9.2 (under dev)
+
+### Fixed
+
+- irc: limit size of data received from the server to prevent memory exhaustion
+
 ## Version 4.9.1 (2026-05-31)
 
 ### Fixed

src/plugins/irc/irc-protocol.c

@@ -4164,16 +4164,25 @@ IRC_PROTOCOL_CALLBACK(005)
         if (ctxt->server->isupport)
         {
             length_isupport = strlen (ctxt->server->isupport);
-            isupport2 = realloc (ctxt->server->isupport,
-                                 length_isupport +  /* existing */
-                                 1 +  /* space */
-                                 length +  /* new */
-                                 1);
-            if (isupport2)
+            /*
+             * limit the size of the accumulated ISUPPORT data: once the
+             * maximum is reached, ignore the extra data (protection against a
+             * server flooding "005" messages, which would consume all the
+             * memory)
+             */
+            if (length_isupport + 1 + length < IRC_SERVER_ISUPPORT_MAX_LENGTH)
             {
-                ctxt->server->isupport = isupport2;
-                strcat (ctxt->server->isupport, " ");
-                strcat (ctxt->server->isupport, str_info);
+                isupport2 = realloc (ctxt->server->isupport,
+                                     length_isupport +  /* existing */
+                                     1 +  /* space */
+                                     length +  /* new */
+                                     1);
+                if (isupport2)
+                {
+                    ctxt->server->isupport = isupport2;
+                    strcat (ctxt->server->isupport, " ");
+                    strcat (ctxt->server->isupport, str_info);
+                }
             }
         }
         else

src/plugins/irc/irc-server.c

@@ -3409,6 +3409,14 @@ irc_server_msgq_add_unterminated (struct t_irc_server *server,
 
     if (server->unterminated_message)
     {
+        /*
+         * limit the size of the unterminated message: once the maximum is
+         * reached, ignore the extra data (protection against a server sending
+         * a very long line without end-of-line, which would consume all the
+         * memory)
+         */
+        if (strlen (server->unterminated_message) >= IRC_SERVER_RECV_MSG_MAX_LENGTH)
+            return;
         unterminated_message2 =
             realloc (server->unterminated_message,
                      (strlen (server->unterminated_message) +

src/plugins/irc/irc-server.h

@@ -144,6 +144,15 @@ enum t_irc_server_option
 #define IRC_SERVER_MULTILINE_DEFAULT_MAX_BYTES 4096
 #define IRC_SERVER_MULTILINE_DEFAULT_MAX_LINES 24
 
+/*
+ * maximum length of an unterminated message (a received line without
+ * end-of-line) and of the accumulated "005" (ISUPPORT) data; these limits
+ * protect against a server sending a huge amount of data without end-of-line
+ * (or a flood of "005" messages), which would consume all the memory
+ */
+#define IRC_SERVER_RECV_MSG_MAX_LENGTH (64 * 1024)
+#define IRC_SERVER_ISUPPORT_MAX_LENGTH (64 * 1024)
+
 /* casemapping (string comparisons for nicks/channels) */
 enum t_irc_server_casemapping
 {

tests/unit/plugins/irc/test-irc-protocol.cpp

@@ -3866,6 +3866,44 @@ TEST(IrcProtocolWithServer, 005_full)
     STRCMP_EQUAL(IRC_MSG_005 " " IRC_MSG_005, ptr_server->isupport);
 }
 
+/*
+ * Test functions:
+ *   irc_protocol_cb_005 (accumulated ISUPPORT is bounded)
+ */
+
+TEST(IrcProtocolWithServer, 005_limit)
+{
+    char str_msg[4096], str_value[3500];
+    size_t length1, length2;
+    int i;
+
+    SRV_INIT;
+
+    memset (str_value, 'X', sizeof (str_value) - 1);
+    str_value[sizeof (str_value) - 1] = '\0';
+    snprintf (str_msg, sizeof (str_msg),
+              ":server 005 alice TEST=%s :are supported", str_value);
+
+    /* flood the server with "005" messages */
+    for (i = 0; i < 100; i++)
+    {
+        server_recv (str_msg);
+    }
+    CHECK(ptr_server->isupport);
+    length1 = strlen (ptr_server->isupport);
+
+    /* the accumulated ISUPPORT data must be bounded */
+    CHECK(length1 <= IRC_SERVER_ISUPPORT_MAX_LENGTH + sizeof (str_value));
+
+    /* receiving more "005" messages must not grow it any further */
+    for (i = 0; i < 100; i++)
+    {
+        server_recv (str_msg);
+    }
+    length2 = strlen (ptr_server->isupport);
+    LONGS_EQUAL(length1, length2);
+}
+
 /*
  * Test functions:
  *   irc_protocol_cb_005 (infos from server, multiple messages)

tests/unit/plugins/irc/test-irc-server.cpp

@@ -65,6 +65,49 @@ TEST(IrcServer, Valid)
     irc_server_free (server);
 }
 
+/*
+ * Test functions:
+ *   irc_server_msgq_add_unterminated (via irc_server_msgq_add_buffer)
+ *
+ * Check that data received without any end-of-line does not grow the
+ * unterminated message buffer without limit.
+ */
+
+TEST(IrcServer, MsgqAddBufferLimit)
+{
+    struct t_irc_server *server;
+    char chunk[4097];
+    int i;
+    size_t length1, length2;
+
+    server = irc_server_alloc ("server_msgq");
+    CHECK(server);
+
+    memset (chunk, 'a', sizeof (chunk) - 1);
+    chunk[sizeof (chunk) - 1] = '\0';
+
+    /* feed a lot of data with no end-of-line */
+    for (i = 0; i < 100; i++)
+    {
+        irc_server_msgq_add_buffer (server, chunk);
+    }
+    CHECK(server->unterminated_message);
+    length1 = strlen (server->unterminated_message);
+
+    /* the buffer must be bounded (not ~400 KB) */
+    CHECK(length1 <= IRC_SERVER_RECV_MSG_MAX_LENGTH + sizeof (chunk));
+
+    /* feeding more data must not grow the buffer any further */
+    for (i = 0; i < 100; i++)
+    {
+        irc_server_msgq_add_buffer (server, chunk);
+    }
+    length2 = strlen (server->unterminated_message);
+    LONGS_EQUAL(length1, length2);
+
+    irc_server_free (server);
+}
+
 /*
  * Test functions:
  *   irc_server_search