# CVE-2026-34756: vLLM Denial of Service via unbounded n parameter in OpenAI API server

Source: https://junglewise.ai/threats/cve-2026-34756-vllm-denial-of-service-via-unbounded-n-parameter-in-openai-api  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-06. Updated: 2026-07-17  
License: CC BY 4.0 with a link back

CVE-2026-34756 · Severity: medium · CVSS 6.5 · Published 2026-04-06

vLLM is a high-throughput engine for serving AI models via an OpenAI-compatible interface. A vulnerability in how it handles certain request parameters allows an attacker to crash the server by sending a single specially crafted request. This results in a complete service outage, potentially impacting AI-powered applications and customer-facing services.

Technologies: [vllm (PyPI)](https://junglewise.ai/threats/technologies/pypi-vllm), [Red Hat AI Inference Server](https://junglewise.ai/threats/technologies/ai-inference-server), [Vllm](https://junglewise.ai/threats/technologies/vllm). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi), [Red Hat](https://junglewise.ai/threats/vendors/red-hat), [Vllm](https://junglewise.ai/threats/vendors/vllm).

## Quick answers

**Is CVE-2026-34756 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-34756 being exploited in the wild.

**How severe is CVE-2026-34756?** Medium severity, with a CVSS score of 6.5, published 6 April 2026.

**Which products and versions are affected?** vllm (PyPI) ECOSYSTEM:introduced=0.1.0,fixed=0.19.0; Red Hat AI Inference Server 3.

**Is there a fix for CVE-2026-34756?** Yes. vllm (PyPI): upgrade to 0.19.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-34756 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-34756?** Check whether you run vllm (PyPI), Red Hat AI Inference Server or Vllm at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- vllm (PyPI): upgrade to 0.19.0 or later

  ```sh
  pip install "vllm>=0.19.0"
  ```


## Technical details

A resource exhaustion vulnerability exists in vLLM's OpenAI-compatible API server due to missing upper-bound validation on the 'n' parameter in ChatCompletionRequest and CompletionRequest models. An attacker can provide an extremely large integer for 'n', which causes the server to enter a synchronous loop that clones the request object millions of times. This behavior monopolizes the Python asyncio event loop and rapidly consumes system memory (RSS), triggering the OS OOM-killer. The vulnerability is reachable via the network without authentication in default configurations. A fix is available in version 0.19.0.

## Affected products

- vllm-project vllm >= 0.1.0, < 0.19.0

## Identifiers

- CVE-2026-34756
- GHSA-3mwp-wvh9-7528
- PYSEC-2026-2298

## Timeline

- 2026-04-03: disclosed
- 2026-04-03: advisory
- 2026-04-03: patched: Fixed in version 0.19.0

## References

- [https://api.github.com/users/ez-lbz](https://api.github.com/users/ez-lbz)
- [https://github.com/ez-lbz](https://github.com/ez-lbz)
- [https://api.github.com/users/ez-lbz/gists%7B/gist_id%7D](https://api.github.com/users/ez-lbz/gists%7B/gist_id%7D)
- [https://api.github.com/users/ez-lbz/repos](https://api.github.com/users/ez-lbz/repos)
- [https://avatars.githubusercontent.com/u/161842993?v=4](https://avatars.githubusercontent.com/u/161842993?v=4)
- [https://api.github.com/users/ez-lbz/events%7B/privacy%7D](https://api.github.com/users/ez-lbz/events%7B/privacy%7D)

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