Submit #956250: vLLM Project vLLM v0.26.0 Denial of Service

vLLM's Rust-based Gemma4UnifiedParser contains an uncontrolled recursion vulnerability when parsing nested tool-call arguments. The parser implements Gemma4's object and array grammar using mutually recursive parsing functions without enforcing a maximum nesting depth.

A sufficiently deeply nested tool-call argument can exhaust the stack of the worker thread performing the parse. In Rust, stack overflow results in a fatal runtime error and process abort rather than a recoverable parser error or ordinary panic. Consequently, triggering the vulnerable parser can terminate the vLLM serving process and cause denial of service.

The parser processes model-generated tool-call output. An attacker who can submit prompts to a vLLM deployment can attempt to cause the model to emit the nested structure required to exercise the vulnerable parser.

Once sufficient nesting reaches the parser, stack exhaustion causes a fatal Rust runtime abort. This is not limited to returning a parser error for the malicious request: the process executing the parser terminates.

Therefore, an attacker capable of influencing generated tool-call output may be able to terminate an affected vLLM serving process and disrupt availability for other users of that process.

The exploit requires:

A vLLM deployment using the affected Gemma4 unified parser.
The attacker being able to submit inference requests.
The attacker successfully steering model output toward a sufficiently deeply nested Gemma4 tool-call argument.

Fix

The issue was fixed by vLLM PR #54303:

https://github.com/vllm-project/vllm/pull/54303

• Source: ⚠️ https://github.com/vllm-project/vllm/issues/50927
• User: Zyz3366 (UID 97230)
• Submission: 09/02/2026 00:19 (29 days ago)
• Moderation: 09/30/2026 12:37 (29 days later)
• Status: Accepted
• VulDB entry: 411965 [vllm-project vLLM up to 0.26.0 Gemma4UnifiedParser gemma4.rs denial of service]
• Points: 20