diff --git a/include/zephyr/bluetooth/gatt.h b/include/zephyr/bluetooth/gatt.h
index 3597c7c5fdb..2c7d36a6758 100644
--- a/include/zephyr/bluetooth/gatt.h
+++ b/include/zephyr/bluetooth/gatt.h
@@ -918,6 +918,7 @@ ssize_t bt_gatt_attr_read_service(struct bt_conn *conn,
  *  Read include service attribute value from local database storing the result
  *  into buffer after encoding it.
  *  @note Only use this with attributes which user_data is a ``bt_gatt_include``.
+ *  The function returns EINVAL if @p attr or @p attr->user_data is NULL.
  *
  *  @param conn Connection object.
  *  @param attr Attribute to read.
diff --git a/include/zephyr/bluetooth/l2cap.h b/include/zephyr/bluetooth/l2cap.h
index f8781d791f0..380b52ad508 100644
--- a/include/zephyr/bluetooth/l2cap.h
+++ b/include/zephyr/bluetooth/l2cap.h
@@ -111,6 +111,20 @@ extern "C" {
  */
 #define BT_L2CAP_ECRED_MIN_MPS 64
 
+/** @brief L2CAP maximum MTU
+ *
+ *  The maximum MTU for an L2CAP Based Connection. This is the same with or without ECRED. This
+ *  requirement is taken from text in Core 3.A.4.22 and 3.A.4.26 v6.0.
+ */
+#define BT_L2CAP_MAX_MTU UINT16_MAX
+
+/** @brief L2CAP maximum MPS
+ *
+ *  The maximum MPS for an L2CAP Based Connection. This is the same with or without ECRED. This
+ *  requirement is taken from text in Core 3.A.4.22 and 3.A.4.26 v6.0.
+ */
+#define BT_L2CAP_MAX_MPS 65533
+
 /** @brief The maximum number of channels in ECRED L2CAP signaling PDUs
  *
  *  Currently, this is the maximum number of channels referred to in the
diff --git a/subsys/bluetooth/host/gatt.c b/subsys/bluetooth/host/gatt.c
index 3092801554a..be642fec9fe 100644
--- a/subsys/bluetooth/host/gatt.c
+++ b/subsys/bluetooth/host/gatt.c
@@ -1917,6 +1917,10 @@ ssize_t bt_gatt_attr_read_included(struct bt_conn *conn,
 				   const struct bt_gatt_attr *attr,
 				   void *buf, uint16_t len, uint16_t offset)
 {
+	if ((attr == NULL) || (attr->user_data == NULL)) {
+		return -EINVAL;
+	}
+
 	struct bt_gatt_attr *incl = attr->user_data;
 	uint16_t handle = bt_gatt_attr_get_handle(incl);
 	struct bt_uuid *uuid = incl->user_data;
diff --git a/subsys/bluetooth/host/l2cap.c b/subsys/bluetooth/host/l2cap.c
index 9fa5638d961..caf5424717b 100644
--- a/subsys/bluetooth/host/l2cap.c
+++ b/subsys/bluetooth/host/l2cap.c
@@ -46,6 +46,7 @@ LOG_MODULE_REGISTER(bt_l2cap, CONFIG_BT_L2CAP_LOG_LEVEL);
 #define CHAN_RX(_w) CONTAINER_OF(_w, struct bt_l2cap_le_chan, rx_work)
 
 #define L2CAP_LE_MIN_MTU		23
+#define L2CAP_LE_MIN_MPS		23
 
 #define L2CAP_LE_MAX_CREDITS		(BT_BUF_ACL_RX_COUNT - 1)
 
@@ -1478,11 +1479,6 @@ static void le_conn_req(struct bt_l2cap *l2cap, uint8_t ident,
 
 	LOG_DBG("psm 0x%02x scid 0x%04x mtu %u mps %u credits %u", psm, scid, mtu, mps, credits);
 
-	if (mtu < L2CAP_LE_MIN_MTU || mps < L2CAP_LE_MIN_MTU) {
-		LOG_ERR("Invalid LE-Conn Req params: mtu %u mps %u", mtu, mps);
-		return;
-	}
-
 	buf = l2cap_create_le_sig_pdu(BT_L2CAP_LE_CONN_RSP, ident,
 				      sizeof(*rsp));
 	if (!buf) {
@@ -1492,6 +1488,16 @@ static void le_conn_req(struct bt_l2cap *l2cap, uint8_t ident,
 	rsp = net_buf_add(buf, sizeof(*rsp));
 	(void)memset(rsp, 0, sizeof(*rsp));
 
+	/* Validate parameters. Requirements are from Core Spec v6.0, Vol 3.A.4.22. Valid credit
+	 * range is from 0 to UINT16_MAX, thus no credit validation is needed.
+	 */
+	if (!IN_RANGE(mtu, L2CAP_LE_MIN_MTU, BT_L2CAP_MAX_MTU) ||
+	    !IN_RANGE(mps, L2CAP_LE_MIN_MPS, BT_L2CAP_MAX_MPS)) {
+		LOG_ERR("Invalid le conn req params: mtu %u mps %u", mtu, mps);
+		result = BT_L2CAP_LE_ERR_UNACCEPT_PARAMS;
+		goto rsp;
+	}
+
 	/* Check if there is a server registered */
 	server = bt_l2cap_server_lookup_psm(psm);
 	if (!server) {
@@ -1577,8 +1583,12 @@ static void le_ecred_conn_req(struct bt_l2cap *l2cap, uint8_t ident,
 
 	LOG_DBG("psm 0x%02x mtu %u mps %u credits %u", psm, mtu, mps, credits);
 
-	if (mtu < BT_L2CAP_ECRED_MIN_MTU || mps < BT_L2CAP_ECRED_MIN_MTU) {
-		LOG_ERR("Invalid ecred conn req params. mtu %u mps %u", mtu, mps);
+	/* Validate parameters. Requirements are from Core Spec v6.0, Vol 3.A.4.25. */
+	if (!IN_RANGE(mtu, BT_L2CAP_ECRED_MIN_MTU, BT_L2CAP_MAX_MTU) ||
+	    !IN_RANGE(mps, BT_L2CAP_ECRED_MIN_MPS, BT_L2CAP_MAX_MPS) ||
+	    !IN_RANGE(credits, BT_L2CAP_ECRED_CREDITS_MIN, BT_L2CAP_ECRED_CREDITS_MAX)) {
+		LOG_ERR("Invalid le ecred conn req params: mtu %u mps %u credits %u", mtu, mps,
+			credits);
 		result = BT_L2CAP_LE_ERR_INVALID_PARAMS;
 		goto response;
 	}
@@ -1685,7 +1695,7 @@ static void le_ecred_reconf_req(struct bt_l2cap *l2cap, uint8_t ident,
 	mtu = sys_le16_to_cpu(req->mtu);
 	mps = sys_le16_to_cpu(req->mps);
 
-	if (mps < BT_L2CAP_ECRED_MIN_MTU) {
+	if (mps < BT_L2CAP_ECRED_MIN_MPS) {
 		result = BT_L2CAP_RECONF_OTHER_UNACCEPT;
 		goto response;
 	}
@@ -1981,13 +1991,24 @@ static void le_ecred_conn_rsp(struct bt_l2cap *l2cap, uint8_t ident,
 
 			LOG_DBG("dcid 0x%04x", dcid);
 
-			/* If a Destination CID is 0x0000, the channel was not
+			/* Validate parameters before assignment. Requirements are from Core Spec
+			 * v6.0, Vol 3.A.4.26. If a Destination CID is 0x0000, the channel was not
 			 * established.
 			 */
-			if (!dcid) {
+			if (dcid == 0U) {
 				bt_l2cap_chan_remove(conn, &chan->chan);
 				bt_l2cap_chan_del(&chan->chan);
 				continue;
+			} else if (!L2CAP_LE_CID_IS_DYN(dcid) ||
+				   !IN_RANGE(mtu, BT_L2CAP_ECRED_MIN_MTU, BT_L2CAP_MAX_MTU) ||
+				   !IN_RANGE(mps, BT_L2CAP_ECRED_MIN_MPS, BT_L2CAP_MAX_MPS) ||
+				   !IN_RANGE(credits, BT_L2CAP_ECRED_CREDITS_MIN,
+					     BT_L2CAP_ECRED_CREDITS_MAX)) {
+				LOG_WRN("Invalid ecred conn rsp params: dcid 0x%04x mtu %u mps %u "
+					"credits %u. Disconnecting.",
+					dcid, mtu, mps, credits);
+				bt_conn_disconnect(conn, BT_HCI_ERR_UNACCEPT_CONN_PARAM);
+				return;
 			}
 
 			c = bt_l2cap_le_lookup_tx_cid(conn, dcid);
@@ -2085,6 +2106,20 @@ static void le_conn_rsp(struct bt_l2cap *l2cap, uint8_t ident,
 
 	switch (result) {
 	case BT_L2CAP_LE_SUCCESS:
+		/* Validate parameters on successful connection. Requirements are from Core Spec
+		 * v6.0, Vol 3.A.4.23. Valid credit range is from 0 to UINT16_MAX, thus no credit
+		 * validation is needed.
+		 */
+		if ((!L2CAP_LE_CID_IS_DYN(dcid) ||
+		     !IN_RANGE(mtu, L2CAP_LE_MIN_MTU, BT_L2CAP_MAX_MTU) ||
+		     !IN_RANGE(mps, L2CAP_LE_MIN_MPS, BT_L2CAP_MAX_MPS))) {
+			LOG_WRN("Invalid conn rsp params: dcid 0x%04x mtu %u mps %u. "
+				"Disconnecting.",
+				dcid, mtu, mps);
+			bt_conn_disconnect(conn, BT_HCI_ERR_UNACCEPT_CONN_PARAM);
+			return;
+		}
+
 		chan->tx.cid = dcid;
 		chan->tx.mtu = mtu;
 		chan->tx.mps = mps;
@@ -2157,6 +2192,17 @@ static void le_credits(struct bt_l2cap *l2cap, uint8_t ident,
 	cid = sys_le16_to_cpu(ev->cid);
 	credits = sys_le16_to_cpu(ev->credits);
 
+	if (!L2CAP_LE_CID_IS_DYN(cid)) {
+		LOG_WRN("Can't add credits to non-dynamic channel %p (cid 0x%04x)", &l2cap->chan,
+			cid);
+		return;
+	}
+
+	if (credits == 0U) {
+		LOG_WRN("Ignoring zero credit packet");
+		return;
+	}
+
 	LOG_DBG("cid 0x%04x credits %u", cid, credits);
 
 	chan = bt_l2cap_le_lookup_tx_cid(conn, cid);
diff --git a/subsys/bluetooth/host/l2cap_internal.h b/subsys/bluetooth/host/l2cap_internal.h
index 07bd428334a..61fa7416dd5 100644
--- a/subsys/bluetooth/host/l2cap_internal.h
+++ b/subsys/bluetooth/host/l2cap_internal.h
@@ -125,6 +125,9 @@ struct bt_l2cap_le_credits {
 	uint16_t credits;
 } __packed;
 
+#define BT_L2CAP_ECRED_CREDITS_MIN      1
+#define BT_L2CAP_ECRED_CREDITS_MAX      UINT16_MAX
+
 #define BT_L2CAP_ECRED_CONN_REQ         0x17
 struct bt_l2cap_ecred_conn_req {
 	uint16_t psm;
diff --git a/subsys/bluetooth/host/smp.c b/subsys/bluetooth/host/smp.c
index 916bb661bb9..ff920fb0f3a 100644
--- a/subsys/bluetooth/host/smp.c
+++ b/subsys/bluetooth/host/smp.c
@@ -2555,22 +2555,35 @@ static uint8_t legacy_pairing_req(struct bt_smp *smp)
 static uint8_t legacy_pairing_random(struct bt_smp *smp)
 {
 	struct bt_conn *conn = smp->chan.chan.conn;
-	uint8_t tmp[16];
+	uint8_t tmp[16], cfm_i[16];
 	int err;
 
 	LOG_DBG("");
 
-	/* calculate confirmation */
+	/* calculate LP_CONFIRM_R */
 	err = smp_c1(smp->tk, smp->rrnd, smp->preq, smp->prsp,
 		     &conn->le.init_addr, &conn->le.resp_addr, tmp);
 	if (err) {
 		return BT_SMP_ERR_UNSPECIFIED;
 	}
 
+	/* calculate LP_CONFIRM_I */
+	err = smp_c1(smp->tk, smp->prnd, smp->preq, smp->prsp,
+		     &conn->le.init_addr, &conn->le.resp_addr, cfm_i);
+	if (err) {
+		return BT_SMP_ERR_UNSPECIFIED;
+	}
+
 	LOG_DBG("pcnf %s", bt_hex(smp->pcnf, 16));
-	LOG_DBG("cfm %s", bt_hex(tmp, 16));
+	LOG_DBG("cfm (remote) %s", bt_hex(tmp, 16));
+	LOG_DBG("cfm (local) %s", bt_hex(cfm_i, 16));
 
-	if (memcmp(smp->pcnf, tmp, sizeof(smp->pcnf))) {
+	/* Core Specification, Vol 3, Part H, section 2.3.5.5 (Errata ES-24491): If the computed
+	 * LP_CONFIRM_R value is not equal to the received LP_CONFIRM_R value, or the received
+	 * LP_CONFIRM_R value is equal to the LP_CONFIRM_I value, fail pairing.
+	 */
+	if (memcmp(smp->pcnf, tmp, sizeof(smp->pcnf)) ||
+	    !memcmp(smp->pcnf, cfm_i, sizeof(smp->pcnf))) {
 		return BT_SMP_ERR_CONFIRM_FAILED;
 	}
 
@@ -4486,7 +4499,7 @@ static uint8_t smp_public_key(struct bt_smp *smp, struct net_buf *buf)
 		}
 	} else if (!bt_pub_key_is_valid(smp->pkey)) {
 		LOG_WRN("Received invalid public key");
-		return BT_SMP_ERR_INVALID_PARAMS;
+		return BT_SMP_ERR_DHKEY_CHECK_FAILED;
 	}
 
 	if (IS_ENABLED(CONFIG_BT_CENTRAL) &&
diff --git a/tests/bsim/bluetooth/host/l2cap/many_conns/src/main.c b/tests/bsim/bluetooth/host/l2cap/many_conns/src/main.c
index ff9f3030d1e..a4f45bc1890 100644
--- a/tests/bsim/bluetooth/host/l2cap/many_conns/src/main.c
+++ b/tests/bsim/bluetooth/host/l2cap/many_conns/src/main.c
@@ -29,7 +29,7 @@ DEFINE_FLAG_STATIC(flag_l2cap_connected);
 #define NUM_PERIPHERALS CONFIG_BT_MAX_CONN
 #define L2CAP_CHANS     NUM_PERIPHERALS
 #define SDU_NUM         1
-#define SDU_LEN         10
+#define SDU_LEN         23
 
 /* Only one SDU per link will be transmitted */
 NET_BUF_POOL_DEFINE(sdu_tx_pool,