diff --git a/src/transport/xqc_conn.h b/src/transport/xqc_conn.h
index ed9206715..a9dd16e38 100644
--- a/src/transport/xqc_conn.h
+++ b/src/transport/xqc_conn.h
@@ -39,6 +39,16 @@
 
 #define XQC_MAX_RECV_WINDOW (16 * 1024 * 1024)
 
+/* RFC 9000: variable-length integer max value, used as flow control upper bound */
+#define XQC_MAX_FLOW_CONTROL_WINDOW (((uint64_t)1 << 62) - 1)
+
+/* clamp a flow control value to the RFC 9000 variable-length integer maximum */
+static inline uint64_t
+xqc_clamp_to_max_flow_ctl(uint64_t value)
+{
+    return value > XQC_MAX_FLOW_CONTROL_WINDOW ? XQC_MAX_FLOW_CONTROL_WINDOW : value;
+}
+
 #define XQC_MP_SETTINGS_STR_LEN (30)
 
 static const uint32_t MAX_RSP_CONN_CLOSE_CNT = 3;
diff --git a/src/transport/xqc_frame.c b/src/transport/xqc_frame.c
index 33d5619a3..5f8833445 100644
--- a/src/transport/xqc_frame.c
+++ b/src/transport/xqc_frame.c
@@ -1268,7 +1268,7 @@ xqc_process_data_blocked_frame(xqc_connection_t *conn, xqc_packet_in_t *packet_i
         return XQC_OK;
     }
 
-    new_limit = conn->conn_flow_ctl.fc_data_read + conn->conn_flow_ctl.fc_recv_windows_size;
+    new_limit = xqc_clamp_to_max_flow_ctl(conn->conn_flow_ctl.fc_data_read + conn->conn_flow_ctl.fc_recv_windows_size);
 
     if (new_limit > conn->conn_flow_ctl.fc_max_data_can_recv) {
         conn->conn_flow_ctl.fc_max_data_can_recv = new_limit;
@@ -1324,7 +1324,7 @@ xqc_process_stream_data_blocked_frame(xqc_connection_t *conn, xqc_packet_in_t *p
         return XQC_OK;
     }
 
-    new_limit = stream->stream_data_in.next_read_offset + stream->stream_flow_ctl.fc_stream_recv_window_size;
+    new_limit = xqc_clamp_to_max_flow_ctl(stream->stream_data_in.next_read_offset + stream->stream_flow_ctl.fc_stream_recv_window_size);
 
     if (new_limit > stream->stream_flow_ctl.fc_max_stream_data_can_recv) {
         stream->stream_flow_ctl.fc_max_stream_data_can_recv = new_limit;
diff --git a/src/transport/xqc_packet_out.c b/src/transport/xqc_packet_out.c
index b564cc947..08b09003f 100644
--- a/src/transport/xqc_packet_out.c
+++ b/src/transport/xqc_packet_out.c
@@ -1215,6 +1215,7 @@ xqc_write_stream_frame_to_packet(xqc_connection_t *conn,
 
         if (stream->stream_flow_ctl.fc_stream_recv_window_size > available_window) {        
             stream->stream_flow_ctl.fc_max_stream_data_can_recv += (stream->stream_flow_ctl.fc_stream_recv_window_size - available_window);
+            stream->stream_flow_ctl.fc_max_stream_data_can_recv = xqc_clamp_to_max_flow_ctl(stream->stream_flow_ctl.fc_max_stream_data_can_recv);
             xqc_log(conn->log, XQC_LOG_DEBUG,
                     "|initial_fc_credit_update|stream:%ui|new_max_data:%ui|stream_max_recv_offset:%ui|next_read_offset:%ui|window_size:%ui|pkt_type:%d|",
                     stream->stream_id,
diff --git a/src/transport/xqc_stream.c b/src/transport/xqc_stream.c
index 479a6fcd4..feae94a44 100644
--- a/src/transport/xqc_stream.c
+++ b/src/transport/xqc_stream.c
@@ -416,6 +416,7 @@ xqc_stream_do_recv_flow_ctl(xqc_stream_t *stream)
 
         if (stream->stream_flow_ctl.fc_stream_recv_window_size > available_window) {        
             stream->stream_flow_ctl.fc_max_stream_data_can_recv += (stream->stream_flow_ctl.fc_stream_recv_window_size - available_window);
+            stream->stream_flow_ctl.fc_max_stream_data_can_recv = xqc_clamp_to_max_flow_ctl(stream->stream_flow_ctl.fc_max_stream_data_can_recv);
             xqc_log(conn->log, XQC_LOG_DEBUG,
                     "|xqc_write_max_stream_data_to_packet|new_max_data:%ui|stream_max_recv_offset:%ui|next_read_offset:%ui|window_size:%ui|",
                     stream->stream_flow_ctl.fc_max_stream_data_can_recv, stream->stream_max_recv_offset,
@@ -462,6 +463,7 @@ xqc_stream_do_recv_flow_ctl(xqc_stream_t *stream)
 
         if (conn->conn_flow_ctl.fc_recv_windows_size > available_window) {
             conn->conn_flow_ctl.fc_max_data_can_recv += (conn->conn_flow_ctl.fc_recv_windows_size - available_window);
+            conn->conn_flow_ctl.fc_max_data_can_recv = xqc_clamp_to_max_flow_ctl(conn->conn_flow_ctl.fc_max_data_can_recv);
             xqc_log(conn->log, XQC_LOG_DEBUG,
                     "|xqc_write_max_data_to_packet|new_max_data:%ui|fc_data_recved:%ui|fc_data_read:%ui|window_size:%ui|",
                     conn->conn_flow_ctl.fc_max_data_can_recv, conn->conn_flow_ctl.fc_data_recved,
@@ -1052,7 +1054,7 @@ xqc_stream_update_settings(xqc_stream_t *stream,
             xqc_log(conn->log, XQC_LOG_DEBUG, 
                     "|fc_win_update|old_fc_win:%ui|fc_win:%ui|", 
                     old_fc_win, stream->stream_flow_ctl.fc_stream_recv_window_size);
-            new_offset = stream->stream_data_in.next_read_offset + stream->stream_flow_ctl.fc_stream_recv_window_size;
+            new_offset = xqc_clamp_to_max_flow_ctl(stream->stream_data_in.next_read_offset + stream->stream_flow_ctl.fc_stream_recv_window_size);
 
             if(new_offset > stream->stream_flow_ctl.fc_max_stream_data_can_recv) {
                 stream->stream_flow_ctl.fc_max_stream_data_can_recv = new_offset;
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index f72fb508a..f2a7fd448 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -116,6 +116,7 @@ if(HAVE_CUNIT)
         ${UNIT_TEST_DIR}/xqc_send_ctl_test.c
         ${UNIT_TEST_DIR}/xqc_vn_test.c
         ${UNIT_TEST_DIR}/xqc_frame_type_bit_test.c
+        ${UNIT_TEST_DIR}/xqc_flow_ctl_test.c
     )
 
     if(XQC_ENABLE_FEC)
diff --git a/tests/unittest/main.c b/tests/unittest/main.c
index b6b4a4e86..0b99ac79e 100644
--- a/tests/unittest/main.c
+++ b/tests/unittest/main.c
@@ -44,6 +44,7 @@
 #include "xqc_send_ctl_test.h"
 #include "xqc_vn_test.h"
 #include "xqc_frame_type_bit_test.h"
+#include "xqc_flow_ctl_test.h"
 
 static int xqc_init_suite(void) { return 0; }
 static int xqc_clean_suite(void) { return 0; }
@@ -85,6 +86,10 @@ main()
         || !CU_add_test(pSuite, "xqc_test_pq", xqc_test_pq)
         || !CU_add_test(pSuite, "xqc_test_common", xqc_test_common)
         || !CU_add_test(pSuite, "xqc_test_vint", xqc_test_vint)
+        /* issue #675: flow control window clamp to 2^62-1 (RFC 9000) */
+        || !CU_add_test(pSuite, "xqc_test_stream_flow_ctl_clamp", xqc_test_stream_flow_ctl_clamp)
+        || !CU_add_test(pSuite, "xqc_test_conn_flow_ctl_clamp", xqc_test_conn_flow_ctl_clamp)
+        || !CU_add_test(pSuite, "xqc_test_flow_ctl_normal_no_clamp", xqc_test_flow_ctl_normal_no_clamp)
         || !CU_add_test(pSuite, "xqc_test_recv_record", xqc_test_recv_record)
         || !CU_add_test(pSuite, "xqc_test_reno", xqc_test_reno)
         || !CU_add_test(pSuite, "xqc_test_reno_init_cwnd", xqc_test_reno_init_cwnd)
diff --git a/tests/unittest/xqc_flow_ctl_test.c b/tests/unittest/xqc_flow_ctl_test.c
new file mode 100644
index 000000000..ba27039c9
--- /dev/null
+++ b/tests/unittest/xqc_flow_ctl_test.c
@@ -0,0 +1,91 @@
+/**
+ * @copyright Copyright (c) 2022, Alibaba Group Holding Limited
+ */
+
+#include "xqc_flow_ctl_test.h"
+#include "src/transport/xqc_conn.h"
+#include "src/transport/xqc_stream.h"
+#include <CUnit/CUnit.h>
+#include <stdint.h>
+
+/**
+ * Test: stream-level fc_max_stream_data_can_recv is clamped to 2^62-1.
+ * Simulates the accumulation logic in xqc_stream_do_recv_flow_ctl and
+ * verifies the clamp prevents overflow beyond XQC_MAX_FLOW_CONTROL_WINDOW.
+ */
+void
+xqc_test_stream_flow_ctl_clamp(void)
+{
+    /* XQC_MAX_FLOW_CONTROL_WINDOW must equal 2^62 - 1 */
+    CU_ASSERT_EQUAL(XQC_MAX_FLOW_CONTROL_WINDOW, ((uint64_t)1 << 62) - 1);
+
+    xqc_stream_flow_ctl_t flow_ctl;
+    memset(&flow_ctl, 0, sizeof(flow_ctl));
+
+    /* set fc_max_stream_data_can_recv near the limit */
+    flow_ctl.fc_max_stream_data_can_recv = XQC_MAX_FLOW_CONTROL_WINDOW - 100;
+    flow_ctl.fc_stream_recv_window_size = XQC_MAX_RECV_WINDOW;
+
+    /* simulate the accumulation that happens in xqc_stream_do_recv_flow_ctl */
+    uint64_t available_window = 0; /* worst case: all consumed */
+    uint64_t increment = flow_ctl.fc_stream_recv_window_size - available_window;
+    flow_ctl.fc_max_stream_data_can_recv += increment;
+
+    /* apply clamp (mirrors the fix in xqc_stream.c) */
+    if (flow_ctl.fc_max_stream_data_can_recv > XQC_MAX_FLOW_CONTROL_WINDOW) {
+        flow_ctl.fc_max_stream_data_can_recv = XQC_MAX_FLOW_CONTROL_WINDOW;
+    }
+
+    CU_ASSERT_EQUAL(flow_ctl.fc_max_stream_data_can_recv, XQC_MAX_FLOW_CONTROL_WINDOW);
+}
+
+/**
+ * Test: connection-level fc_max_data_can_recv is clamped to 2^62-1.
+ */
+void
+xqc_test_conn_flow_ctl_clamp(void)
+{
+    xqc_conn_flow_ctl_t flow_ctl;
+    memset(&flow_ctl, 0, sizeof(flow_ctl));
+
+    /* set near the limit */
+    flow_ctl.fc_max_data_can_recv = XQC_MAX_FLOW_CONTROL_WINDOW - 50;
+    flow_ctl.fc_recv_windows_size = XQC_MAX_RECV_WINDOW;
+
+    /* simulate accumulation */
+    uint64_t available_window = 0;
+    uint64_t increment = flow_ctl.fc_recv_windows_size - available_window;
+    flow_ctl.fc_max_data_can_recv += increment;
+
+    /* apply clamp */
+    if (flow_ctl.fc_max_data_can_recv > XQC_MAX_FLOW_CONTROL_WINDOW) {
+        flow_ctl.fc_max_data_can_recv = XQC_MAX_FLOW_CONTROL_WINDOW;
+    }
+
+    CU_ASSERT_EQUAL(flow_ctl.fc_max_data_can_recv, XQC_MAX_FLOW_CONTROL_WINDOW);
+}
+
+/**
+ * Test: normal values well below 2^62-1 are not affected by the clamp.
+ */
+void
+xqc_test_flow_ctl_normal_no_clamp(void)
+{
+    xqc_stream_flow_ctl_t stream_fc;
+    memset(&stream_fc, 0, sizeof(stream_fc));
+
+    stream_fc.fc_max_stream_data_can_recv = 1024 * 1024; /* 1MB */
+    stream_fc.fc_stream_recv_window_size = XQC_MAX_RECV_WINDOW;
+
+    uint64_t expected = stream_fc.fc_max_stream_data_can_recv + stream_fc.fc_stream_recv_window_size;
+
+    /* simulate accumulation + clamp */
+    stream_fc.fc_max_stream_data_can_recv += stream_fc.fc_stream_recv_window_size;
+    if (stream_fc.fc_max_stream_data_can_recv > XQC_MAX_FLOW_CONTROL_WINDOW) {
+        stream_fc.fc_max_stream_data_can_recv = XQC_MAX_FLOW_CONTROL_WINDOW;
+    }
+
+    /* normal value should pass through unchanged */
+    CU_ASSERT_EQUAL(stream_fc.fc_max_stream_data_can_recv, expected);
+    CU_ASSERT(stream_fc.fc_max_stream_data_can_recv < XQC_MAX_FLOW_CONTROL_WINDOW);
+}
diff --git a/tests/unittest/xqc_flow_ctl_test.h b/tests/unittest/xqc_flow_ctl_test.h
new file mode 100644
index 000000000..e307935ce
--- /dev/null
+++ b/tests/unittest/xqc_flow_ctl_test.h
@@ -0,0 +1,12 @@
+/**
+ * @copyright Copyright (c) 2022, Alibaba Group Holding Limited
+ */
+
+#ifndef _XQC_TEST_FLOW_CTL_H_INCLUDED_
+#define _XQC_TEST_FLOW_CTL_H_INCLUDED_
+
+void xqc_test_stream_flow_ctl_clamp(void);
+void xqc_test_conn_flow_ctl_clamp(void);
+void xqc_test_flow_ctl_normal_no_clamp(void);
+
+#endif /* _XQC_TEST_FLOW_CTL_H_INCLUDED_ */