diff --git a/video/encoder_bitrate_adjuster.cc b/video/encoder_bitrate_adjuster.cc
index 10c73343ea2..d65688a432d 100644
--- a/video/encoder_bitrate_adjuster.cc
+++ b/video/encoder_bitrate_adjuster.cc
@@ -382,6 +382,19 @@ void EncoderBitrateAdjuster::OnEncodedFrame(DataSize size,
                                             int stream_index,
                                             int temporal_index) {
   ++frames_since_layout_change_;
+  // `temporal_index` can originate from the temporal_idx field of an
+  // EncodedImage which is set from external (Mojo IPC) input via
+  // BitstreamBufferMetadata; reject out-of-range values to avoid OOB
+  // indexing into `overshoot_detectors_`.
+  if (stream_index < 0 ||
+      stream_index >= static_cast<int>(kMaxSpatialLayers) ||
+      temporal_index < 0 ||
+      temporal_index >= static_cast<int>(kMaxTemporalStreams)) {
+    RTC_LOG(LS_WARNING) << "EncoderBitrateAdjuster::OnEncodedFrame ignoring "
+                           "out-of-range indices si="
+                        << stream_index << " ti=" << temporal_index;
+    return;
+  }
   // Detectors may not exist, for instance if ScreenshareLayers is used.
   auto& detector = overshoot_detectors_[stream_index][temporal_index];
   if (detector) {