GHSA-Q8GQ-377P-JQ3R
Vulnerability from github – Published: 2026-06-16 17:34 – Updated: 2026-09-03 15:31Summary
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 as its sole security control to restrict which activation functions can be loaded from a HuggingFace model's config.json:
# 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() — an unrestricted import gadget:
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:
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
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "vllm"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "0.22.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-41523"
],
"database_specific": {
"cwe_ids": [
"CWE-617",
"CWE-94"
],
"github_reviewed": true,
"github_reviewed_at": "2026-06-16T17:34:49Z",
"nvd_published_at": "2026-06-22T23:16:30Z",
"severity": "HIGH"
},
"details": "### Summary\n\nAn `assert`-based security check in vLLM\u0027s 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\u0027s `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\u0027s `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\u0027s `config.json` is passed directly to [`resolve_obj_by_qualname()`](https://github.com/vllm-project/vllm/blob/main/vllm/utils/import_utils.py#L106) \u2014 an unrestricted import gadget:\n\n```python\ndef resolve_obj_by_qualname(qualname: str) -\u003e 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** \u2014 `get_act_fn()` is called via `resolve_classifier_act_fn()` from:\n- `vllm/model_executor/layers/pooler/seqwise/poolers.py:122` \u2014 SequencePooler\n- `vllm/model_executor/layers/pooler/tokwise/poolers.py:130` \u2014 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\u0027s 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",
"id": "GHSA-q8gq-377p-jq3r",
"modified": "2026-09-03T15:31:58Z",
"published": "2026-06-16T17:34:49Z",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
],
"summary": "vLLM: Security Check Bypass via assert Statement in Activation Function Loading Allows Arbitrary Code Execution"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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