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Medium, CVSS 6.5amended

vLLM: Unauthenticated OOM Denial of Service via Unbounded n Parameter in OpenAI API Server

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

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (vllm 0.19.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI vllm

    Affected
    >= 0.1.0, < 0.19.0
    Fixed in
    0.19.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
6.5 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
EPSS
  • CVE-2026-34756: 0.44% estimated exploitation activity in the next 30 days; percentile 38.09 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary A Denial of Service vulnerability exists in the vLLM OpenAI-compatible API server. Due to the lack of an upper bound validation on the n parameter in the ChatCompletionRequest and CompletionRequest Pydantic models, an unauthenticated attacker can send a single HTTP request with an astronomically large n value. This completely blocks the Python asyncio event loop and causes immediate Out-Of-Memory crashes by allocating millions of request object copies in the heap before the request even reaches the scheduling queue. Details The root cause of this vulnerability lies in the missing upper bound checks across the request parsing and asynchronous scheduling layers: 1. Protocol Layer: In vllm/entrypoints/openai/chatcompletion/protocol.py, the n parameter is defined simply as an integer without any pydantic.Field constraints for an upper bound. 1. SamplingParams Layer (Incomplete Validation): When the API request is converted to internal SamplingParams in vllm/samplingparams.py, the…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-3mwp-wvh9-7528

  2. Revision 1 · created ·

    Published from osv record GHSA-3mwp-wvh9-7528

Agent brief

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# Patch Next advisory brief: CVE-2026-34756

Advisory: vLLM: Unauthenticated OOM Denial of Service via Unbounded `n` Parameter in OpenAI API Server
IDs: CVE-2026-34756, GHSA-3mwp-wvh9-7528
Record: https://patchnext.com/advisories/cve-2026-34756 (revision 2, source checked 26 Oct 2026, 13:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: vllm 0.19.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI vllm: affected >= 0.1.0, < 0.19.0; fixed in 0.19.0

## Suggested inspection (read-only)
- `python -m pip show vllm`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on vllm.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-3mwp-wvh9-7528 (OSV record GHSA-3mwp-wvh9-7528 lists affected ranges: PyPI vllm >=0.1.0 <0.19.0 (fixed))
- Verified: https://github.com/advisories/GHSA-3mwp-wvh9-7528 (GitHub-reviewed advisory GHSA-3mwp-wvh9-7528 lists affected ranges: PyPI vllm >=0.1.0 <0.19.0)

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