CVE-2025-61620

Date: October 17, 2025

Summary A resource-exhaustion (denial-of-service) vulnerability exists in multiple endpoints of the OpenAI-Compatible Server due to the ability to specify Jinja templates via the "chat_template" and "chat_template_kwargs" parameters. If an attacker can supply these parameters to the API, they can cause a service outage by exhausting CPU and/or memory resources. Details When using an LLM as a chat model, the conversation history must be rendered into a text input for the model. In "hf/transformer", this rendering is performed using a Jinja template. The OpenAI-Compatible Server launched by vllm serve exposes a "chat_template" parameter that lets users specify that template. In addition, the server accepts a "chat_template_kwargs" parameter to pass extra keyword arguments to the rendering function. Because Jinja templates support programming-language-like constructs (loops, nested iterations, etc.), a crafted template can consume extremely large amounts of CPU and memory and thereby trigger a denial-of-service condition. Importantly, simply forbidding the "chat_template" parameter does not fully mitigate the issue. The implementation constructs a dictionary of keyword arguments for "apply_hf_chat_template" and then updates that dictionary with the user-supplied "chat_template_kwargs" via "dict.update". Since "dict.update" can overwrite existing keys, an attacker can place a "chat_template" key inside "chat_template_kwargs" to replace the template that will be used by "apply_hf_chat_template". vllm/entrypoints/openai/serving_engine.py#L794-L816 _chat_template_kwargs: dict[str, Any] = dict( chat_template=chat_template, add_generation_prompt=add_generation_prompt, continue_final_message=continue_final_message, tools=tool_dicts, documents=documents, ) _chat_template_kwargs.update(chat_template_kwargs or {}) request_prompt: Union[str, list[int]] if isinstance(tokenizer, MistralTokenizer): ... else: request_prompt = apply_hf_chat_template( tokenizer=tokenizer, conversation=conversation, model_config=model_config, **_chat_template_kwargs, ) Impact If an OpenAI-Compatible Server exposes endpoints that accept "chat_template" or "chat_template_kwargs" from untrusted clients, an attacker can submit a malicious Jinja template (directly or by overriding "chat_template" inside "chat_template_kwargs") that consumes excessive CPU and/or memory. This can result in a resource-exhaustion denial-of-service that renders the server unresponsive to legitimate requests. Fixes * https://github.com/vllm-project/vllm/pull/25794