diff --git a/onnxruntime/contrib_ops/cpu/bert/group_query_attention.cc b/onnxruntime/contrib_ops/cpu/bert/group_query_attention.cc
index debda282eb4f1..1080498831474 100644
--- a/onnxruntime/contrib_ops/cpu/bert/group_query_attention.cc
+++ b/onnxruntime/contrib_ops/cpu/bert/group_query_attention.cc
@@ -145,6 +145,7 @@ Status GroupQueryAttention<T>::Compute(OpKernelContext* context) const {
   // Validate seqlens_k values before they are used as GEMM dimensions to prevent OOB access.
   {
     const int32_t* seqlens_k_data = seqlens_k->Data<int32_t>();
+    const int past_kv_seqlen = parameters.seqlen_past_kv_cache;
     for (int b = 0; b < batch_size; b++) {
       if (seqlens_k_data[b] < 0 || seqlens_k_data[b] >= present_kv_seqlen) {
         return ORT_MAKE_STATUS(ONNXRUNTIME, INVALID_ARGUMENT,
@@ -156,6 +157,25 @@ Status GroupQueryAttention<T>::Compute(OpKernelContext* context) const {
                                "seqlens_k[", b, "] = ", seqlens_k_data[b],
                                " is too small for sequence_length ", sequence_length);
       }
+      // Bound the number of past KV rows copied out of the past key/value buffers during
+      // token generation (decode). ConcatStateChunkGQA copies (seqlens_k + 1 - sequence_length)
+      // rows from the past buffer (sized past_kv_seqlen). The present-buffer check above does
+      // not bound this past-side read, because the present buffer can be larger than the past
+      // buffer when total_sequence_length exceeds the past sequence length. A large seqlens_k
+      // combined with a small past buffer would otherwise read past the end of the past tensors.
+      // Shared KV (kv_sequence_length == 0) appends no new KV and its past read is already
+      // bounded by the present-buffer check together with the total_sequence_length <=
+      // seqlen_past_kv_cache enforcement in the apply-attention paths, so it needs no check here.
+      if (past_key != nullptr && past_value != nullptr && parameters.kv_sequence_length != 0 &&
+          !parameters.is_first_prompt) {
+        const int64_t past_rows = static_cast<int64_t>(seqlens_k_data[b]) + 1 - sequence_length;
+        if (past_rows > past_kv_seqlen) {
+          return ORT_MAKE_STATUS(ONNXRUNTIME, INVALID_ARGUMENT,
+                                 "seqlens_k[", b, "] = ", seqlens_k_data[b], " requires ", past_rows,
+                                 " past KV rows, which exceeds the past buffer sequence length ",
+                                 past_kv_seqlen, ".");
+        }
+      }
     }
   }
   int q_hidden_size = parameters.hidden_size;
diff --git a/onnxruntime/test/contrib_ops/group_query_attention_op_test.cc b/onnxruntime/test/contrib_ops/group_query_attention_op_test.cc
index ef7a04bc67d4f..e17aff159e6cc 100644
--- a/onnxruntime/test/contrib_ops/group_query_attention_op_test.cc
+++ b/onnxruntime/test/contrib_ops/group_query_attention_op_test.cc
@@ -369,6 +369,35 @@ TEST(GroupQueryAttentionTest, NonPromptSeqlensKUnderflow_OOB) {
       /*past_seq_len=*/4);
 }
 
+// Regression: present buffer large enough (total_seq_len passes the present-buffer check),
+// but the past buffer is much smaller. ConcatStateChunkGQA would copy
+// (seqlens_k + 1 - sequence_length) rows out of the small past buffer, reading past its end.
+TEST(GroupQueryAttentionTest, SeqlensKExceedsPastBuffer_OOBRead) {
+  // present_kv_seqlen = max(total_seq_len=100, past_seq_len=2) = 100, so seqlens_k=50 passes the
+  // present-buffer check, but past_seqlen = 51 - 1 = 50 rows >> past buffer (2 rows) => OOB read.
+  RunGQASeqlensKTest(
+      /*seqlens_k_data=*/{50},
+      /*total_seq_len=*/100,
+      /*batch_size=*/1,
+      /*sequence_length=*/1,
+      OpTester::ExpectResult::kExpectFailure,
+      "exceeds the past buffer sequence length",
+      /*provide_past=*/true,
+      /*past_seq_len=*/2);
+}
+
+TEST(GroupQueryAttentionTest, SeqlensKExceedsEmptyPastBuffer_OOBRead) {
+  RunGQASeqlensKTest(
+      /*seqlens_k_data=*/{50},
+      /*total_seq_len=*/100,
+      /*batch_size=*/1,
+      /*sequence_length=*/1,
+      OpTester::ExpectResult::kExpectFailure,
+      "exceeds the past buffer sequence length",
+      /*provide_past=*/true,
+      /*past_seq_len=*/0);
+}
+
 // INT32_MAX seqlens_k: rejected by the >= present_kv_seqlen check.
 TEST(GroupQueryAttentionTest, Int32MaxSeqlensK_OOB) {
   RunGQASeqlensKTest(