CVE-2026-115921: ONNX shape inference crash from malicious model scan attributes

Summary

ONNX versions 1.4.0 through 1.22.0 contain a flaw in ScanInferenceFunction, ScanInferenceFunction_opset8 and ScanInferenceFunction_opset9, which do not safely validate the required num_scan_inputs attribute. A missing attribute is dereferenced as a null pointer, and a negative value is converted to size_t and can reach an oversized allocation path. An untrusted model can terminate or disrupt shape inference, including a negative-value model that passes onnx.checker.check_model().

Mitigation

Fixed in version 1.22.0.

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