diff --git a/onnxruntime/contrib_ops/cpu/bert/attention_helper.h b/onnxruntime/contrib_ops/cpu/bert/attention_helper.h
index 8dfcb50b916e7..be07e0663db2d 100644
--- a/onnxruntime/contrib_ops/cpu/bert/attention_helper.h
+++ b/onnxruntime/contrib_ops/cpu/bert/attention_helper.h
@@ -96,14 +96,14 @@ void PrepareMask(const int32_t* mask_index,
         // mask_index is 1D: (B) or (2B) => (Bx)T
 
         // Handle right-side padding: mask value at or after the end position will be mask_filter_value
-        int end_position = std::max(0, std::min(static_cast<int>(mask_index[b_i]), all_sequence_length));
+        int end_position = std::clamp(static_cast<int>(mask_index[b_i]), 0, all_sequence_length);
         for (int m_i = end_position; m_i < all_sequence_length; m_i++) {
           p_mask[m_i] = static_cast<T>(mask_filter_value);
         }
 
         // Handle left-side padding: mask value before the start position will be mask_filter_value
         if (has_mask_start_position) {
-          int start_position = std::max(0, std::min(static_cast<int>(mask_index[b_i + batch_size]), all_sequence_length));
+          int start_position = std::clamp(static_cast<int>(mask_index[b_i + batch_size]), 0, all_sequence_length);
           for (int m_i = 0; m_i < start_position; m_i++) {
             p_mask[m_i] = static_cast<T>(mask_filter_value);
           }
diff --git a/onnxruntime/test/contrib_ops/attention_op_test.cc b/onnxruntime/test/contrib_ops/attention_op_test.cc
index a4d059ced5d42..d0bee280d9972 100644
--- a/onnxruntime/test/contrib_ops/attention_op_test.cc
+++ b/onnxruntime/test/contrib_ops/attention_op_test.cc
@@ -1425,6 +1425,49 @@ TEST(ContribOpAttentionTest, AttentionBatch2LeftPaddingMaskIndex2) {
                    AttentionMaskType::MASK_1D_END_START);
 }
 
+// Verifies that out-of-range 1D mask_index values are clamped rather than
+// causing out-of-bounds memory access.  end_position > all_sequence_length
+// must clamp to all_sequence_length (no right-side masking), and
+// start_position < 0 must clamp to 0 (no left-side masking).  Both cases
+// should produce the same output as a fully-unmasked sequence.
+TEST(ContribOpAttentionTest, AttentionMaskIndex1DClampOOB) {
+  int batch_size = 1;
+  int sequence_length = 2;
+  int hidden_size = 4;
+  int number_of_heads = 2;
+
+  std::vector<float> input_data = {
+      0.8f, -0.5f, 0.0f, 1.f,
+      0.5f, 0.2f, 0.3f, -0.6f};
+
+  std::vector<float> weight_data = {
+      0.1f, -0.2f, 0.3f, 1.0f, 1.1f, 0.3f, 0.5f, 0.2f, 0.3f, -0.6f, 1.5f, 2.0f,
+      0.5f, 0.1f, 0.4f, 1.6f, 1.0f, 2.0f, 0.4f, 0.8f, 0.9f, 0.1f, -1.3f, 0.7f,
+      0.3f, 0.2f, 4.0f, 2.2f, 1.6f, 1.1f, 0.7f, 0.2f, 0.4f, 1.0f, 1.2f, 0.5f,
+      0.2f, 0.1f, 0.4f, 1.6f, 2.4f, 3.3f, 2.1f, 4.2f, 8.4f, 0.0f, 2.1f, 3.2f};
+
+  std::vector<float> bias_data = {
+      -0.5f, 0.6f, 1.2f, 2.1f, 0.5f, 0.7f, 0.2f, 1.2f, 0.5f, 0.4f, 0.3f, 1.2f};
+
+  // end_position=999 is well above sequence_length=2, so it must clamp to 2
+  // (no right-side masking). Expected output equals the fully-unmasked case.
+  std::vector<int32_t> mask_index_data_end_oob = {999};
+  std::vector<float> output_data = {
+      3.1495983600616455f, 0.10843668878078461f, 4.25f, 5.6499996185302734f,
+      3.9696791172027588f, 0.073143675923347473f, 4.2499995231628418f, 5.6499991416931152f};
+
+  RunAttentionTest(input_data, weight_data, bias_data, mask_index_data_end_oob, output_data,
+                   batch_size, sequence_length, hidden_size, number_of_heads);
+
+  // start_position=-5 is below zero, so it must clamp to 0 (no left-side
+  // masking). end_position=2 keeps all tokens unmasked on the right.
+  // Expected output is again the fully-unmasked case.
+  std::vector<int32_t> mask_index_data_start_neg = {2, -5};
+  RunAttentionTest(input_data, weight_data, bias_data, mask_index_data_start_neg, output_data,
+                   batch_size, sequence_length, hidden_size, number_of_heads, false, false, false, 0,
+                   nullptr, nullptr, AttentionMaskType::MASK_1D_END_START);
+}
+
 TEST(ContribOpAttentionTest, Attention3DMask) {
   int batch_size = 2;
   int sequence_length = 2;