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CVE-2026-34756 medium CVSS 6.5

Vulnerability in vllm (CVE-2026-34756)

Summary

Vulnerability in vllm (CVE-2026-34756). Risk of unauthorized operations or information disclosure. Exploitable via ChatCompletionRequest. Mitigation: upgrade to 0.19.0 or later.

Original description

The description as published by the source.

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/chat_completion/protocol.py, the n parameter is defined simply as an integer without any pydantic.Field constraints for an upper bound.
class ChatCompletionRequest(OpenAIBaseModel):
    # Ordered by official OpenAI API documentation
    # https://platform.openai.com/docs/api/reference/chat/create
    messages: list[ChatCompletionMessageParam]
    model: str | None = None
    frequency_penalty: float | None = 0.0
    logit_bias: dict[str, float] | None = None
    logprobs: bool | None = False
    top_logprobs: int | None = 0
    max_tokens: int | None = Field(
        default=None,
        deprecated="max_tokens is deprecated in favor of "
        "the max_completion_tokens field",
    )
    max_completion_tokens: int | None…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-34756 has been found in vllm.
Attackers can target a specific entry point like ChatCompletionRequest over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 6.5/10.
What to do: upgrade vllm to 0.19.0 or later.
If unsure, ask your IT team or search "vllm CVE-2026-34756" on the vendor's site.

CVE-2026-34756 (vllm) — CWE-770 / CVSS v3 6.5
Attack vector: remote (network-reachable) / no user interaction
Attack surface: ChatCompletionRequest / CompletionRequest / asyncio / pydantic.Field
Patched: 0.19.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-770) exists in vllm. Attackers reach the vulnerable code path via ChatCompletionRequest.

📍 Affected scope

Check the vendor advisory for the affected versions of vllm. Attack surface: ChatCompletionRequest / CompletionRequest / asyncio / pydantic.Field.

🔥 Severity

Severity: Medium (CVSS 6.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 0.19.0.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'vllm' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'vllm' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で vllm を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade vllm to 0.19.0

    ステージング環境で 0.19.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

pip vllm
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.19.0"}]}]

References

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