diff --git a/include/haproxy/quic_cc.h b/include/haproxy/quic_cc.h
index edc22ee98..b16996d0c 100644
--- a/include/haproxy/quic_cc.h
+++ b/include/haproxy/quic_cc.h
@@ -117,5 +117,7 @@ static inline size_t quic_cc_path_prep_data(struct quic_cc_path *path)
 	return path->cwnd - path->prep_in_flight;
 }
 
+int quic_cwnd_may_increase(const struct quic_cc_path *path);
+
 #endif /* USE_QUIC */
 #endif /* _PROTO_QUIC_CC_H */
diff --git a/src/quic_cc.c b/src/quic_cc.c
index 811a766dd..603914cd4 100644
--- a/src/quic_cc.c
+++ b/src/quic_cc.c
@@ -65,3 +65,22 @@ uint quic_cc_default_pacing_burst(const struct quic_cc *cc)
 	struct quic_cc_path *path = container_of(cc, struct quic_cc_path, cc);
 	return path->pacing_burst;
 }
+
+/* Returns true if congestion window on path ought to be increased. */
+int quic_cwnd_may_increase(const struct quic_cc_path *path)
+{
+	/* RFC 9002 7.8. Underutilizing the Congestion Window
+	 *
+	 * When bytes in flight is smaller than the congestion window and
+	 * sending is not pacing limited, the congestion window is
+	 * underutilized. This can happen due to insufficient application data
+	 * or flow control limits. When this occurs, the congestion window
+	 * SHOULD NOT be increased in either slow start or congestion avoidance.
+	 */
+
+	/* Consider that congestion window can be increased if it is at least
+	 * half full or window size is less than 16k. These conditions should
+	 * not be restricted too much to prevent slow window growing.
+	 */
+	return 2 * path->in_flight >= path->cwnd  || path->cwnd < 16384;
+}
diff --git a/src/quic_cc_cubic.c b/src/quic_cc_cubic.c
index a1768a151..9783d8f85 100644
--- a/src/quic_cc_cubic.c
+++ b/src/quic_cc_cubic.c
@@ -382,9 +382,11 @@ static inline void quic_cubic_update(struct quic_cc *cc, uint32_t acked)
 			inc = W_est_inc;
 	}
 
-	path->cwnd += inc;
-	path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
-	path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+	if (quic_cwnd_may_increase(path)) {
+		path->cwnd += inc;
+		path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
+		path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+	}
  leave:
 	TRACE_LEAVE(QUIC_EV_CONN_CC, cc->qc);
 }
@@ -453,8 +455,10 @@ static void quic_cc_cubic_ss_cb(struct quic_cc *cc, struct quic_cc_event *ev)
 			if (path->cwnd >= QUIC_CC_INFINITE_SSTHESH - acked)
 				goto out;
 
-			path->cwnd += acked;
-			path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+			if (quic_cwnd_may_increase(path)) {
+				path->cwnd += acked;
+				path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+			}
 			quic_cc_hystart_track_min_rtt(cc, h, path->loss.latest_rtt);
 			if (ev->ack.pn >= h->wnd_end)
 				h->wnd_end = UINT64_MAX;
@@ -465,8 +469,10 @@ static void quic_cc_cubic_ss_cb(struct quic_cc *cc, struct quic_cc_event *ev)
 			}
 		}
 		else if (path->cwnd < QUIC_CC_INFINITE_SSTHESH - ev->ack.acked) {
-			path->cwnd += ev->ack.acked;
-			path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
+			if (quic_cwnd_may_increase(path)) {
+				path->cwnd += ev->ack.acked;
+				path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
+			}
 		}
 		/* Exit to congestion avoidance if slow start threshold is reached. */
 		if (path->cwnd >= c->ssthresh)
@@ -544,8 +550,10 @@ static void quic_cc_cubic_cs_cb(struct quic_cc *cc, struct quic_cc_event *ev)
 		if (path->cwnd >= QUIC_CC_INFINITE_SSTHESH - acked)
 			goto out;
 
-		path->cwnd += acked;
-		path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+		if (quic_cwnd_may_increase(path)) {
+			path->cwnd += acked;
+			path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+		}
 		quic_cc_hystart_track_min_rtt(cc, h, path->loss.latest_rtt);
 		if (quic_cc_hystart_may_reenter_ss(h)) {
 			/* Exit to slow start */
diff --git a/src/quic_cc_newreno.c b/src/quic_cc_newreno.c
index ec63bc3e8..989a7c105 100644
--- a/src/quic_cc_newreno.c
+++ b/src/quic_cc_newreno.c
@@ -86,9 +86,11 @@ static void quic_cc_nr_ss_cb(struct quic_cc *cc, struct quic_cc_event *ev)
 	path = container_of(cc, struct quic_cc_path, cc);
 	switch (ev->type) {
 	case QUIC_CC_EVT_ACK:
-		path->cwnd += ev->ack.acked;
-		path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
-		path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+		if (quic_cwnd_may_increase(path)) {
+			path->cwnd += ev->ack.acked;
+			path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
+			path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+		}
 		/* Exit to congestion avoidance if slow start threshold is reached. */
 		if (path->cwnd > nr->ssthresh)
 			nr->state = QUIC_CC_ST_CA;
@@ -124,9 +126,11 @@ static void quic_cc_nr_ca_cb(struct quic_cc *cc, struct quic_cc_event *ev)
 		 */
 		acked = ev->ack.acked * path->mtu + nr->remain_acked;
 		nr->remain_acked = acked % path->cwnd;
-		path->cwnd += acked / path->cwnd;
-		path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
-		path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+		if (quic_cwnd_may_increase(path)) {
+			path->cwnd += acked / path->cwnd;
+			path->cwnd = QUIC_MIN(path->max_cwnd, path->cwnd);
+			path->mcwnd = QUIC_MAX(path->cwnd, path->mcwnd);
+		}
 		break;
 	}
 
diff --git a/src/quic_rx.c b/src/quic_rx.c
index c3510975c..27eebec0d 100644
--- a/src/quic_rx.c
+++ b/src/quic_rx.c
@@ -439,7 +439,6 @@ static void qc_notify_cc_of_newly_acked_pkts(struct quic_conn *qc,
 	list_for_each_entry_safe(pkt, tmp, newly_acked_pkts, list) {
 		pkt->pktns->tx.in_flight -= pkt->in_flight_len;
 		p->prep_in_flight -= pkt->in_flight_len;
-		p->in_flight -= pkt->in_flight_len;
 		if (pkt->flags & QUIC_FL_TX_PACKET_ACK_ELICITING)
 			p->ifae_pkts--;
 		/* If this packet contained an ACK frame, proceed to the
@@ -455,6 +454,7 @@ static void qc_notify_cc_of_newly_acked_pkts(struct quic_conn *qc,
 		ev.ack.pn = pkt->pn_node.key;
 		/* Note that this event is not emitted for BBR. */
 		quic_cc_event(&p->cc, &ev);
+		p->in_flight -= pkt->in_flight_len;
 		if (drs && (pkt->flags & QUIC_FL_TX_PACKET_ACK_ELICITING))
 			quic_cc_drs_update_rate_sample(drs, pkt, time_ns);
 		LIST_DEL_INIT(&pkt->list);