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CVE-2026-73556 medium CVSS 5.3

Vulnerability in vllm (CVE-2026-73556)

Summary

Vulnerability in vllm (CVE-2026-73556). Risk of unauthorized operations or information disclosure. Exploitable via POST /v1/completions. Mitigation: upgrade to 0.26.0 or later.

Original description

The description as published by the source.

Summary

The fix for GHSA-rwxx-mrjm-wc2m ("ReDoS via structured_outputs.regex compiled without timeout") wrapped the regex compile in the xgrammar and outlines backends with compile_regex_with_timeout (and, for outlines, validate_regex_is_buildable). The lm-format-enforcer backend was left unguarded: it compiles the attacker-supplied regex with no timeout and no buildability check. A single request with a catastrophic regex hangs the structured-output compile step and stalls the engine worker (denial of service).

Affected code (HEAD d6d39c1)

vllm/v1/structured_output/backend_lm_format_enforcer.py:

  • line 110: character_level_parser = lmformatenforcer.RegexParser(grammar_spec) — builds an interegular FSM from the attacker regex synchronously, no timeout.
  • line 155: validate_structured_output_request_lm_format_enforcer returns immediately on if so_params.regex: — no validation.

Sibling backends that WERE patched by GHSA-rwxx:

  • backend_xgrammar.py:92 → compile_regex_with_timeout(...).
  • backend_outlines.py:65 → compile_regex_with_timeout(...) (plus validate_regex_is_buildable).

lm-format-enforcer uses the same interegular DFA-construction primitive the advisory cites for the outlines backend.

Reproduction (runtime-verified against the sink)

The sink lmformatenforcer.RegexParser() was exercised directly (this is exactly what the backend calls):

baseline  '[0-9]{3}'          -> 0.0002 s
attacker  '(a{1,30…

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-2026-73556 has been found in vllm.
Attackers can target a specific entry point like POST /v1/completions over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 5.3/10.
What to do: upgrade vllm to 0.26.0 or later.
If unsure, ask your IT team or search "vllm CVE-2026-73556" on the vendor's site.

CVE-2026-73556 (vllm) — CWE-400 / CVSS v3 5.3
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: POST /v1/completions / structured_outputs.regex / compile_regex_with_timeout / validate_regex_is_buildable
Patched: 0.26.0 — 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-400) exists in vllm. Attackers reach the vulnerable code path via POST /v1/completions without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of vllm. Attack surface: POST /v1/completions / structured_outputs.regex / compile_regex_with_timeout / validate_regex_is_buildable.

🔥 Severity

Severity: Medium (CVSS 5.3/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 0.26.0.

🛡️ 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 POST /v1/completions requests with malformed payloads or SQL meta-characters. 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
    Hunt for indicators of compromise detect 
    grep 'POST /v1/completions' /var/log/nginx/access.log | grep -E '(unusual_payload|sqli_pattern)'

    アクセスログで POST /v1/completions への異常なリクエスト (不正な認証ヘッダ・SQLメタ文字)を過去 30〜90日分捜索。WAF/SIEM があれば該当パスのアラート発火履歴を確認。

  3. 3
    Apply patch patch 
    Upgrade vllm to 0.26.0

    ステージング環境で 0.26.0 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と、Step 2 で見つけた異常検知の続報チェックを含めること。

  4. 4
    Post-deployment verification verify 
    Replay attack against POST /v1/completions on staging to confirm patch closes the vector

    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 Step 2 と同じログクエリ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":"0.26.0"}]}]

References

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