diff --git a/libavformat/mov.c b/libavformat/mov.c
index ec57a05803..6f92742e23 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -1326,6 +1326,10 @@ static int update_frag_index(MOVContext *c, int64_t offset)
         return -1;
 
     for (i = 0; i < c->fc->nb_streams; i++) {
+        // Avoid building frag index if streams lack track id.
+        if (c->fc->streams[i]->id < 0)
+            return AVERROR_INVALIDDATA;
+
         frag_stream_info[i].id = c->fc->streams[i]->id;
         frag_stream_info[i].sidx_pts = AV_NOPTS_VALUE;
         frag_stream_info[i].tfdt_dts = AV_NOPTS_VALUE;
@@ -4154,7 +4158,7 @@ static int mov_read_trak(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
     st = avformat_new_stream(c->fc, NULL);
     if (!st) return AVERROR(ENOMEM);
-    st->id = c->fc->nb_streams;
+    st->id = -1;
     sc = av_mallocz(sizeof(MOVStreamContext));
     if (!sc) return AVERROR(ENOMEM);
 
@@ -4438,6 +4442,11 @@ static int mov_read_tkhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     st = c->fc->streams[c->fc->nb_streams-1];
     sc = st->priv_data;
 
+    // Each stream (trak) should have exactly 1 tkhd. This catches bad files and
+    // avoids corrupting AVStreams mapped to an earlier tkhd.
+    if (st->id != -1)
+        return AVERROR_INVALIDDATA;
+
     version = avio_r8(pb);
     flags = avio_rb24(pb);
     st->disposition |= (flags & MOV_TKHD_FLAG_ENABLED) ? AV_DISPOSITION_DEFAULT : 0;
@@ -4704,6 +4713,7 @@ static int mov_read_trun(MOVContext *c, AVIOContext *pb, MOVAtom atom)
             break;
         }
     }
+    av_assert0(index_entry_pos <= st->nb_index_entries);
 
     avio_r8(pb); /* version */
     flags = avio_rb24(pb);