CVE-2026-55832: tract: Arbitrary file read via unsanitized ONNX external_data `location` (path traversal) on model load in tract-onnx

tract (the tract-onnx crate) resolves an ONNX tensor’s external-data location by joining it onto the model directory without any sanitization. Because location comes from the (untrusted) .onnx file, a malicious model can make tract open and read an arbitrary local file at load time, with the file’s contents flowing into the model’s tensors / inference output (read-only file disclosure). This is the ONNX external-data path-traversal class that the reference onnx library hardened over several CVEs; tract resolves location itself and was never hardened.