diff --git a/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.cc b/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.cc
index 63e750b5fc3..8dbd3bc6855 100644
--- a/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.cc
+++ b/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.cc
@@ -143,6 +143,9 @@ OutstandingData::AckInfo OutstandingData::HandleSack(
     UnwrappedTSN cumulative_tsn_ack,
     webrtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
     bool is_in_fast_recovery) {
+  bool cumulative_tsn_ack_advanced =
+      cumulative_tsn_ack > last_cumulative_tsn_ack_;
+
   OutstandingData::AckInfo ack_info(cumulative_tsn_ack);
   // Erase all items up to cumulative_tsn_ack.
   RemoveAcked(cumulative_tsn_ack, ack_info);
@@ -152,7 +155,7 @@ OutstandingData::AckInfo OutstandingData::HandleSack(
 
   // NACK and possibly mark for retransmit chunks that weren't acked.
   NackBetweenAckBlocks(cumulative_tsn_ack, gap_ack_blocks, is_in_fast_recovery,
-                       ack_info);
+                       cumulative_tsn_ack_advanced, ack_info);
 
   RTC_DCHECK(IsConsistent());
   return ack_info;
@@ -225,6 +228,7 @@ void OutstandingData::NackBetweenAckBlocks(
     UnwrappedTSN cumulative_tsn_ack,
     webrtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
     bool is_in_fast_recovery,
+    bool cumulative_tsn_acked_advanced,
     OutstandingData::AckInfo& ack_info) {
   // Mark everything between the blocks as NACKED/TO_BE_RETRANSMITTED.
   // https://tools.ietf.org/html/rfc4960#section-7.2.4
@@ -237,7 +241,7 @@ void OutstandingData::NackBetweenAckBlocks(
   // in-flight and between gaps should be nacked. This means that SCTP relies on
   // the T3-RTX-timer to re-send packets otherwise.
   UnwrappedTSN max_tsn_to_nack = ack_info.highest_tsn_acked;
-  if (is_in_fast_recovery && cumulative_tsn_ack > last_cumulative_tsn_ack_) {
+  if (is_in_fast_recovery && cumulative_tsn_acked_advanced) {
     // https://tools.ietf.org/html/rfc4960#section-7.2.4
     // "If an endpoint is in Fast Recovery and a SACK arrives that advances
     // the Cumulative TSN Ack Point, the miss indications are incremented for
@@ -252,7 +256,8 @@ void OutstandingData::NackBetweenAckBlocks(
     UnwrappedTSN cur_block_first_acked =
         UnwrappedTSN::AddTo(cumulative_tsn_ack, block.start);
     for (UnwrappedTSN tsn = prev_block_last_acked.next_value();
-         tsn < cur_block_first_acked && tsn <= max_tsn_to_nack;
+         tsn < cur_block_first_acked && tsn <= max_tsn_to_nack &&
+         tsn < next_tsn();
          tsn = tsn.next_value()) {
       ack_info.has_packet_loss |=
           NackItem(tsn, /*retransmit_now=*/false,
diff --git a/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.h b/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.h
index 512385e6fef..bfe668fbc54 100644
--- a/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.h
+++ b/third_party/libwebrtc/net/dcsctp/tx/outstanding_data.h
@@ -325,6 +325,7 @@ class OutstandingData {
       UnwrappedTSN cumulative_tsn_ack,
       webrtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
       bool is_in_fast_recovery,
+      bool cumulative_tsn_acked_advanced,
       OutstandingData::AckInfo& ack_info);
 
   // Process the acknowledgement of the chunk referenced by `iter` and updates
diff --git a/third_party/libwebrtc/net/dcsctp/tx/retransmission_queue.cc b/third_party/libwebrtc/net/dcsctp/tx/retransmission_queue.cc
index 9557a9be226..e3ed0e6da19 100644
--- a/third_party/libwebrtc/net/dcsctp/tx/retransmission_queue.cc
+++ b/third_party/libwebrtc/net/dcsctp/tx/retransmission_queue.cc
@@ -251,6 +251,14 @@ bool RetransmissionQueue::IsSackValid(const SackChunk& sack) const {
   } else if (cumulative_tsn_ack > outstanding_data_.highest_outstanding_tsn()) {
     return false;
   }
+
+  for (const auto& block : sack.gap_ack_blocks()) {
+    if (UnwrappedTSN::AddTo(cumulative_tsn_ack, block.end) >
+        outstanding_data_.highest_outstanding_tsn()) {
+      return false;
+    }
+  }
+
   return true;
 }