CVE-2026-41523: vLLM Security Check Bypass Vulnerability

HIGH
Advisory Identifier: GHSA-q8gq-377p-jq3r
Share
CVSS 7.5
CWE-617 CWE-94
Published
Advisory Summary

CVE-2026-41523 is a HIGH (CVSS 7.5) security check bypass vulnerability (CWE-617: Reachable Assertion) affecting vLLM libraries in the PyPI ecosystem. The issue involves security check bypass via assert statement in activation function loading allows arbitrary code execution. Remediation is available by upgrading affected packages to version 0.22.0.

CVE-2026-41523 at a Glance

CVE Identifier CVE-2026-41523
GHSA Identifier GHSA-q8gq-377p-jq3r
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-617: Reachable Assertion
Affected Product vLLM
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction Required (UI:R)
Scope Unchanged (S:U)
Confidentiality High (C:H)
Integrity High (I:H)
Availability High (A:H)

How to Fix CVE-2026-41523

To remediate this vulnerability, upgrade all affected vLLM dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

vllm fixed: 0.22.0 
pip install vllm>=0.22.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show vllm

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-41523, inspect your installed packages using pip:

pip show vllm

Analyze the terminal output to locate any version of vLLM in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.22.0), your application is affected.

Note: The pip show vllm command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-617: Reachable Assertion, where the product contains an assert() or similar statement that can be triggered by an attacker, which leads to an application exit or other behavior that is more severe than necessary. In vLLM, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Summary An `assert`-based security check in vLLM's activation function loading allows any unauthenticated attacker to achieve arbitrary code execution on the server by publishing a malicious HuggingFace model, when vLLM runs in Python optimized mode (`python -O` or `PYTHONOPTIMIZE=1`). ### Details vLLM uses an `assert` statement at [`vllm/model_executor/layers/pooler/activations.py:48`](https://github.com/vllm-project/vllm/blob/main/vllm/model_executor/layers/pooler/activations.py#L48) as its sole security control to restrict which activation functions can be loaded from a HuggingFace model's `config.json`: ```python # vllm/model_executor/layers/pooler/activations.py:35-53 function_name: str | None = None if ( hasattr(config, "sentence_transformers") and "activation_fn" in config.sentence_transformers ): function_name = config.sentence_transformers["activation_fn"] elif ( hasattr(config, "sbert_ce_default_activation_function") and config.sbert_ce_default_activation_function is not None ): function_name = config.sbert_ce_default_activation_function if function_name is not None: assert function_name.startswith("torch.nn.modules."), ( "Loading of activation functions is restricted to " "torch.nn.modules for security reasons" ) fn = resolve_obj_by_qualname(function_name)() ``` Python's `assert` statements are stripped at compile time when running in optimized mode (`python -O` or `PYTHONOPTIMIZE=1`). When the assert is absent, the attacker-controlled `function_name` from the model's `config.json` is passed directly to [`resolve_obj_by_qualname()`](https://github.com/vllm-project/vllm/blob/main/vllm/utils/import_utils.py#L106) — an unrestricted import gadget: ```python def resolve_obj_by_qualname(qualname: str) -> Any: module_name, obj_name = qualname.rsplit(".", 1) module = importlib.import_module(module_name) return getattr(module, obj_name) ``` This is the same vulnerability class as **CVE-2017-1000433** (pysaml2 assert-based auth bypass), flagged by Bandit B101 and Ruff S101, and the reason Django proactively replaced all assert-based security checks (ticket #32508). **Attacker-controlled input sources:** - `config.sentence_transformers["activation_fn"]` (line 40) - `config.sbert_ce_default_activation_function` (line 45) **Affected call sites** — `get_act_fn()` is called via `resolve_classifier_act_fn()` from: - `vllm/model_executor/layers/pooler/seqwise/poolers.py:122` — SequencePooler - `vllm/model_executor/layers/pooler/tokwise/poolers.py:130` — TokenPooler **Broader systemic risk:** `resolve_obj_by_qualname` is called from ~20 locations across the codebase with no validation of its own. Any future caller feeding user-controlled input to it without validation creates the same vulnerability class. **Suggested fix:** Replace the `assert` with an explicit conditional raise: ```python if not function_name.startswith("torch.nn.modules."): raise ValueError( "Loading of activation functions is restricted to " "torch.nn.modules for security reasons" ) ``` ### Impact **Arbitrary code execution.** A malicious model author publishes a HuggingFace model with a crafted `config.json`. When a victim loads this model with vLLM running under `python -O` or `PYTHONOPTIMIZE=1`, arbitrary code executes during model initialization with the privileges of the vLLM process. The attack requires: 1. Victim loads a malicious model from HuggingFace (user interaction) 2. vLLM runs under `python -O` or `PYTHONOPTIMIZE=1` (documented in production use) 3. Model uses a cross-encoder architecture (e.g. BERT or RoBERTa with sequence classification) **Coordinated disclosure note:** This vulnerability was also reported via huntr.com on April 2, 2026 (https://huntr.com/bounties/dcb05b04-e625-41e7-adbc-bbae0cc2d64c). A GitHub Security Advisory was also filed because it is vLLM's stated preferred disclosure channel per SECURITY.md. ### Fix A fix for this was introduced in this commit: https://github.com/vllm-project/vllm/commit/b3c7ffcab82c2439726f8cb213800f6f38c023d3

CVE-2026-41523 Affected vLLM Versions

1 component(s)
Fixed Versions:
0.22.0
Product / Package Ecosystem Affected Versions Fixed Version
vllm
pypi/vllm
PyPI
< 0.22.0
0.22.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/vllm-project/vllm/security/advisories/GHSA-q8gq-377p-jq3r
NVD — CVE-2026-41523 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-41523
Security Fix Commit FIX COMMIT
https://github.com/vllm-project/vllm/commit/b3c7ffcab82c2439726f8cb213800f6f38c023d3
vLLM Security Information SECURITY
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-41523.json
huntr.com/bounties/dcb05b04-e... WEB
https://huntr.com/bounties/dcb05b04-e625-41e7-adbc-bbae0cc2d64c
vLLM Source Repository REPOSITORY
https://github.com/vllm-project/vllm
vLLM Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/vllm/PYSEC-2026-2300.yaml
bugzilla.redhat.com/show_bug.cgi WEB
https://bugzilla.redhat.com/show_bug.cgi?id=2491582
vLLM Security Information SECURITY
https://access.redhat.com/security/cve/CVE-2026-41523
access.redhat.com/errata/rhsa-2026:62... WEB
https://access.redhat.com/errata/RHSA-2026:62336
access.redhat.com/errata/rhsa-2026:62... WEB
https://access.redhat.com/errata/RHSA-2026:62335
access.redhat.com/errata/rhsa-2026:61... WEB
https://access.redhat.com/errata/RHSA-2026:61629
access.redhat.com/errata/rhsa-2026:61... WEB
https://access.redhat.com/errata/RHSA-2026:61627
access.redhat.com/errata/rhsa-2026:59... WEB
https://access.redhat.com/errata/RHSA-2026:59151
access.redhat.com/errata/rhsa-2026:59... WEB
https://access.redhat.com/errata/RHSA-2026:59144
access.redhat.com/errata/rhsa-2026:59... WEB
https://access.redhat.com/errata/RHSA-2026:59139
access.redhat.com/errata/rhsa-2026:59... WEB
https://access.redhat.com/errata/RHSA-2026:59138
access.redhat.com/errata/rhsa-2026:57... WEB
https://access.redhat.com/errata/RHSA-2026:57390
access.redhat.com/errata/rhsa-2026:57... WEB
https://access.redhat.com/errata/RHSA-2026:57389
access.redhat.com/errata/rhsa-2026:57... WEB
https://access.redhat.com/errata/RHSA-2026:57387
access.redhat.com/errata/rhsa-2026:57... WEB
https://access.redhat.com/errata/RHSA-2026:57380
access.redhat.com/errata/rhsa-2026:36... WEB
https://access.redhat.com/errata/RHSA-2026:36006
access.redhat.com/errata/rhsa-2026:36... WEB
https://access.redhat.com/errata/RHSA-2026:36005

Frequently Asked Questions

What is CVE-2026-41523?
CVE-2026-41523 is a security vulnerability in vLLM involving Security Check Bypass via assert Statement in Activation Function Loading Allows Arbitrary Code Execution under CWE-617: Reachable Assertion.
Which vLLM versions are affected?
Affected versions include releases prior to 0.22.0 across affected packages: vllm.
What version fixes CVE-2026-41523?
Upgrading to 0.22.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is vllm affected?
Yes, vllm is affected in releases prior to 0.22.0. Upgrade your dependency to 0.22.0 or later.
What is the CVSS score of CVE-2026-41523?
CVE-2026-41523 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H.
What CWE is associated with CVE-2026-41523?
CVE-2026-41523 is classified under CWE-617: Reachable Assertion (The product contains an assert() or similar statement that can be triggered by an attacker, which leads to an application exit or other behavior that is more severe than necessary.).
How can I check PyPI dependencies for CVE-2026-41523?
Run the dependency tree command: 'pip show vllm'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-q8gq-377p-jq3r",
  "modified" : "2026-09-03T15:31:58Z",
  "published" : "2026-06-16T17:34:49Z",
  "aliases" : [ "CVE-2026-41523" ],
  "summary" : "vLLM: Security Check Bypass via assert Statement in Activation Function Loading Allows Arbitrary Code Execution",
  "details" : "### Summary\n\nAn `assert`-based security check in vLLM's activation function loading allows any unauthenticated attacker to achieve arbitrary code execution on the server by publishing a malicious HuggingFace model, when vLLM runs in Python optimized mode (`python -O` or `PYTHONOPTIMIZE=1`).\n\n### Details\n\nvLLM uses an `assert` statement at [`vllm/model_executor/layers/pooler/activations.py:48`](https://github.com/vllm-project/vllm/blob/main/vllm/model_executor/layers/pooler/activations.py#L48) as its sole security control to restrict which activation functions can be loaded from a HuggingFace model's `config.json`:\n\n```python\n# vllm/model_executor/layers/pooler/activations.py:35-53\nfunction_name: str | None = None\nif (\n    hasattr(config, \"sentence_transformers\")\n    and \"activation_fn\" in config.sentence_transformers\n):\n    function_name = config.sentence_transformers[\"activation_fn\"]\nelif (\n    hasattr(config, \"sbert_ce_default_activation_function\")\n    and config.sbert_ce_default_activation_function is not None\n):\n    function_name = config.sbert_ce_default_activation_function\n\nif function_name is not None:\n    assert function_name.startswith(\"torch.nn.modules.\"), (\n        \"Loading of activation functions is restricted to \"\n        \"torch.nn.modules for security reasons\"\n    )\n    fn = resolve_obj_by_qualname(function_name)()\n```\n\nPython's `assert` statements are stripped at compile time when running in optimized mode (`python -O` or `PYTHONOPTIMIZE=1`). When the assert is absent, the attacker-controlled `function_name` from the model's `config.json` is passed directly to [`resolve_obj_by_qualname()`](https://github.com/vllm-project/vllm/blob/main/vllm/utils/import_utils.py#L106) — an unrestricted import gadget:\n\n```python\ndef resolve_obj_by_qualname(qualname: str) -> Any:\n    module_name, obj_name = qualname.rsplit(\".\", 1)\n    module = importlib.import_module(module_name)\n    return getattr(module, obj_name)\n```\n\nThis is the same vulnerability class as **CVE-2017-1000433** (pysaml2 assert-based auth bypass), flagged by Bandit B101 and Ruff S101, and the reason Django proactively replaced all assert-based security checks (ticket #32508).\n\n**Attacker-controlled input sources:**\n- `config.sentence_transformers[\"activation_fn\"]` (line 40)\n- `config.sbert_ce_default_activation_function` (line 45)\n\n**Affected call sites** — `get_act_fn()` is called via `resolve_classifier_act_fn()` from:\n- `vllm/model_executor/layers/pooler/seqwise/poolers.py:122` — SequencePooler\n- `vllm/model_executor/layers/pooler/tokwise/poolers.py:130` — TokenPooler\n\n**Broader systemic risk:** `resolve_obj_by_qualname` is called from ~20 locations across the codebase with no validation of its own. Any future caller feeding user-controlled input to it without validation creates the same vulnerability class.\n\n**Suggested fix:** Replace the `assert` with an explicit conditional raise:\n\n```python\nif not function_name.startswith(\"torch.nn.modules.\"):\n    raise ValueError(\n        \"Loading of activation functions is restricted to \"\n        \"torch.nn.modules for security reasons\"\n    )\n```\n\n### Impact\n\n**Arbitrary code execution.** A malicious model author publishes a HuggingFace model with a crafted `config.json`. When a victim loads this model with vLLM running under `python -O` or `PYTHONOPTIMIZE=1`, arbitrary code executes during model initialization with the privileges of the vLLM process.\n\nThe attack requires:\n1. Victim loads a malicious model from HuggingFace (user interaction)\n2. vLLM runs under `python -O` or `PYTHONOPTIMIZE=1` (documented in production use)\n3. Model uses a cross-encoder architecture (e.g. BERT or RoBERTa with sequence classification)\n\n**Coordinated disclosure note:** This vulnerability was also reported via huntr.com on April 2, 2026 (https://huntr.com/bounties/dcb05b04-e625-41e7-adbc-bbae0cc2d64c). A GitHub Security Advisory was also filed because it is vLLM's stated preferred disclosure channel per SECURITY.md.\n\n### Fix\n\nA fix for this was introduced in this commit: https://github.com/vllm-project/vllm/commit/b3c7ffcab82c2439726f8cb213800f6f38c023d3",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "vllm"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.22.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/security/advisories/GHSA-q8gq-377p-jq3r"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-41523"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/commit/b3c7ffcab82c2439726f8cb213800f6f38c023d3"
  }, {
    "type" : "WEB",
    "url" : "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-41523.json"
  }, {
    "type" : "WEB",
    "url" : "https://huntr.com/bounties/dcb05b04-e625-41e7-adbc-bbae0cc2d64c"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/vllm-project/vllm"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/vllm/PYSEC-2026-2300.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://bugzilla.redhat.com/show_bug.cgi?id=2491582"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/security/cve/CVE-2026-41523"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:62336"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:62335"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:61629"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:61627"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:59151"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:59144"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:59139"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:59138"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:57390"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:57389"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:57387"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:57380"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:36006"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/errata/RHSA-2026:36005"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-617", "CWE-94" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-16T17:34:49Z",
    "nvd_published_at" : "2026-06-22T23:16:30Z"
  }
}