PYSEC-2026-3937

Vulnerability from pysec - Published: 2026-09-10 09:45 - Updated: 2026-09-10 11:02
VLAI
Details

Summary

When the vLLM API receives a malformed request (e.g., invalid JSON or missing required fields), FastAPI raises a Pydantic RequestValidationError. The validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts this exception to a string via str(exc), which includes the internal file path and line number of the handler function. The existing sanitize_message() function in vllm/entrypoints/utils.py strips memory addresses (e.g., 0x7f...) but does not strip File "...", line X patterns. The result is a user-facing HTTP response that leaks internal system information.

Impact

An unauthenticated attacker can extract the following with a single malformed request:

  • OS username running the vLLM process (e.g., ubuntu)
  • Home directory path (e.g., /home/ubuntu/)
  • Virtual environment path (e.g., vllm-env/)
  • Python version (e.g., 3.12)
  • Internal package structure and line numbers (e.g., vllm/entrypoints/openai/chat_completion/api_router.py)
  • Handler function names per endpoint, enabling precise version fingerprinting

This information aids attackers in constructing targeted exploits: environment paths narrow the attack surface, and handler function names + line numbers enable exact version identification even when the /version endpoint is disabled.

All POST endpoints that accept JSON bodies are affected, including /v1/chat/completions, /v1/completions, /tokenize, and /detokenize.

Workarounds

Deploying vLLM behind a reverse proxy that rewrites error response bodies to strip file paths would mitigate this, though it is fragile.

Remediation Recommendation

Two possible fixes (either suffices):

Option A — Fix validation_exception_handler: Construct the error message from exc.errors() (the structured Pydantic error list) rather than str(exc). This avoids the traceback-style string entirely.

Option B — Fix sanitize_message: Add a regex to strip File "...", line \d+ patterns, similar to how memory addresses are already stripped:

import re
msg = re.sub(r'File ".*?", line \d+, in \w+', '[internal]', msg)

Option A is preferred as it addresses the root cause rather than filtering symptoms.

Environment Tested

  • vLLM 0.20.1 (pip install, latest stable as of May 2026)
  • Python 3.12
  • Ubuntu 22.04
  • Model: Qwen/Qwen2-0.5B (text-only; bug is model-independent)

This was fixed here: https://github.com/vllm-project/vllm/commit/e87521626f

Severity
5.3 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Impacted products
Name purl
vllm pkg:pypi/vllm
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "vllm",
        "purl": "pkg:pypi/vllm"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.26.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.0.1",
        "0.1.0",
        "0.1.1",
        "0.1.2",
        "0.1.3",
        "0.1.4",
        "0.1.5",
        "0.1.6",
        "0.1.7",
        "0.10.0",
        "0.10.1",
        "0.10.1.1",
        "0.10.2",
        "0.11.0",
        "0.11.1",
        "0.11.2",
        "0.12.0",
        "0.13.0",
        "0.14.0",
        "0.14.1",
        "0.15.0",
        "0.15.1",
        "0.16.0",
        "0.17.0",
        "0.17.1",
        "0.18.0",
        "0.18.1",
        "0.19.0",
        "0.19.1",
        "0.2.0",
        "0.2.1",
        "0.2.1.post1",
        "0.2.2",
        "0.2.3",
        "0.2.4",
        "0.2.5",
        "0.2.6",
        "0.2.7",
        "0.20.0",
        "0.20.1",
        "0.20.2",
        "0.21.0",
        "0.22.0",
        "0.22.1",
        "0.23.0",
        "0.24.0",
        "0.25.0",
        "0.25.1",
        "0.3.0",
        "0.3.1",
        "0.3.2",
        "0.3.3",
        "0.4.0",
        "0.4.0.post1",
        "0.4.1",
        "0.4.2",
        "0.4.3",
        "0.5.0",
        "0.5.0.post1",
        "0.5.1",
        "0.5.2",
        "0.5.3",
        "0.5.3.post1",
        "0.5.4",
        "0.5.5",
        "0.6.0",
        "0.6.1",
        "0.6.1.post1",
        "0.6.1.post2",
        "0.6.2",
        "0.6.3",
        "0.6.3.post1",
        "0.6.4",
        "0.6.4.post1",
        "0.6.5",
        "0.6.6",
        "0.6.6.post1",
        "0.7.0",
        "0.7.1",
        "0.7.2",
        "0.7.3",
        "0.8.0",
        "0.8.1",
        "0.8.2",
        "0.8.3",
        "0.8.4",
        "0.8.5",
        "0.8.5.post1",
        "0.9.0",
        "0.9.0.1",
        "0.9.1",
        "0.9.2"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-73555",
    "GHSA-hwrm-c4cx-rf4j"
  ],
  "details": "## Summary\n\nWhen the vLLM API receives a malformed request (e.g., invalid JSON or missing required fields), FastAPI raises a Pydantic `RequestValidationError`. The `validation_exception_handler` in `vllm/entrypoints/openai/server_utils.py` converts this exception to a string via `str(exc)`, which includes the internal file path and line number of the handler function. The existing `sanitize_message()` function in `vllm/entrypoints/utils.py` strips memory addresses (e.g., `0x7f...`) but does not strip `File \"...\", line X` patterns. The result is a user-facing HTTP response that leaks internal system information.\n\n## Impact\n\nAn unauthenticated attacker can extract the following with a single malformed request:\n\n- **OS username** running the vLLM process (e.g., `ubuntu`)\n- **Home directory path** (e.g., `/home/ubuntu/`)\n- **Virtual environment path** (e.g., `vllm-env/`)\n- **Python version** (e.g., `3.12`)\n- **Internal package structure and line numbers** (e.g., `vllm/entrypoints/openai/chat_completion/api_router.py`)\n- **Handler function names per endpoint**, enabling precise version fingerprinting\n\nThis information aids attackers in constructing targeted exploits: environment paths narrow the attack surface, and handler function names + line numbers enable exact version identification even when the `/version` endpoint is disabled.\n\nAll POST endpoints that accept JSON bodies are affected, including `/v1/chat/completions`, `/v1/completions`, `/tokenize`, and `/detokenize`.\n\n## Workarounds\n\nDeploying vLLM behind a reverse proxy that rewrites error response bodies to strip file paths would mitigate this, though it is fragile.\n\n## Remediation Recommendation\n\nTwo possible fixes (either suffices):\n\n**Option A \u2014 Fix `validation_exception_handler`:** Construct the error message from `exc.errors()` (the structured Pydantic error list) rather than `str(exc)`. This avoids the traceback-style string entirely.\n\n**Option B \u2014 Fix `sanitize_message`:** Add a regex to strip `File \"...\", line \\d+` patterns, similar to how memory addresses are already stripped:\n\n```python\nimport re\nmsg = re.sub(r\u0027File \".*?\", line \\d+, in \\w+\u0027, \u0027[internal]\u0027, msg)\n```\n\nOption A is preferred as it addresses the root cause rather than filtering symptoms.\n\n## Environment Tested\n\n- vLLM 0.20.1 (pip install, latest stable as of May 2026)\n- Python 3.12\n- Ubuntu 22.04\n- Model: Qwen/Qwen2-0.5B (text-only; bug is model-independent)\n\nThis was fixed here: https://github.com/vllm-project/vllm/commit/e87521626f",
  "id": "PYSEC-2026-3937",
  "modified": "2026-09-10T11:02:35.773647Z",
  "published": "2026-09-10T09:45:00.131177Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/vllm-project/vllm/security/advisories/GHSA-hwrm-c4cx-rf4j"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-73555"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vllm-project/vllm/pull/46415"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vllm-project/vllm/commit/e87521626febe2763f997691d1599de4175f4324"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/vllm-project/vllm"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vllm-project/vllm/releases/tag/v0.26.0"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/vllm"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-hwrm-c4cx-rf4j"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "vLLM: Unauthenticated Internal Path and Username Disclosure via Validation Error Messages"
}



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