CVE-2026-41523

### Summary An `assert`-based security check in vLLM's activation function loading allows any unauthenticated attacker to achieve arbitrary code execution on the server by publishing a malicious HuggingFace model, when vLLM runs in Python optimized mode (`python -O` or `PYTHONOPTIMIZE=1`). ### Details vLLM uses an `assert` statement at [`vllm/model_executor/layers/pooler/activations.py:48`](https://github.com/vllm-project/vllm/blob/main/vllm/model_executor/layers/pooler/activations.py#L48) as its sole security control to restrict which activation functions can be loaded from a HuggingFace model's `config.json`: ```python # vllm/model_executor/layers/pooler/activations.py:35-53 function_name: str | None = None if ( hasattr(config, "sentence_transformers") and "activation_fn" in config.sentence_transformers ): function_name = config.sentence_transformers["activation_fn"] elif ( hasattr(config, "sbert_ce_default_activation_function") and config.sbert_ce_default_activation_function is not None ): function_name = config.sbert_ce_default_activation_function if function_name is not None: assert function_name.startswith("torch.nn.modules."), ( "Loading of activation functions is restricted to " "torch.nn.modules for security reasons" ) fn = resolve_obj_by_qualname(function_name)() ``` Python's `assert` statements are stripped at compile time when running in optimized mode (`python -O` or `PYTHONOPTIMIZE=1`). When the assert is absent, the attacker-controlled `function_name` from the model's `config.json` is passed directly to [`resolve_obj_by_qualname()`](https://github.com/vllm-project/vllm/blob/main/vllm/utils/import_utils.py#L106) — an unrestricted import gadget: ```python def resolve_obj_by_qualname(qualname: str) -> Any: module_name, obj_name = qualname.rsplit(".", 1) module = importlib.import_module(module_name) return getattr(module, obj_name) ``` This is the same vulnerability class as **CVE-201