Background

This vulnerability is found in the DiffusionPipeline.from_pretrained flow, which is used to load a pipeline from the HuggingFace Hub.

This function accepts an optional custom_pipeline keyword argument: the name of a Python file in the repo that contains a custom class inheriting from DiffusionPipeline. An equivalent flow is triggered when the _class_name field in model_index.json (the repo config file) is set to a custom class.

Any attempt to use a custom pipeline throws the following exception, requesting that trust_remote_code is also passed:

DiffusionPipeline.from_pretrained(
    pretrained_model_name_or_path='ido-shani/custom-pipeline',
    custom_pipeline="custom"
)

ValueError: The repository for ido-shani/custom-pipeline contains custom code in
custom.py which must be executed to correctly load the model. You can inspect the
repository content at https://hf.co/ido-shani/custom-pipeline/blob/main/custom.py.
Please pass the argument `trust_remote_code=True` to allow custom code to be run.

The vulnerability is a silent RCE - it allows arbitrary code to be loaded through the custom_pipeline flow from a Hub repo, with no custom_pipeline or trust_remote_code kwargs and nothing suspicious in the config. The from_pretrained call succeeds and returns a functional pipeline.

Naive Flow

First, all relevant arguments are popped from kwargs and stored in local variables.

Given a pretrained_model_name_or_path that is a Hub repo ID, DiffusionPipeline.download() is called. This function serves two roles: it orchestrates downloading relevant model files, and it is the security gatekeeper for trust_remote_code. It is called even if the model is already cached; in that case it exits early. If the repo contains custom code, it checks whether trust_remote_code was passed and raises otherwise:

# pipeline_utils.py:1645-1652
load_pipe_from_hub = custom_pipeline is not None and f"{custom_pipeline}.py" in filenames

...

if load_pipe_from_hub and not trust_remote_code:
    raise ValueError(...)

It then runs _get_pipeline_class, which returns the class object of the pipeline in order to inspect its __init__ signature and determine which component files need to be downloaded. As part of building the allow_patterns list used to filter the snapshot download to necessary files only, the custom pipeline file is explicitly included if present:

# pipeline_utils.py:1707
allow_patterns += [f"{custom_pipeline}.py"] if f"{custom_pipeline}.py" in filenames else []

The function then checks if all expected files are already present, and either exits early or triggers a snapshot download with those patterns.

The next step in from_pretrained is loading the pipeline class a second time, this time to actually instantiate it. Before calling _get_pipeline_class again, _resolve_custom_pipeline_and_cls is called to translate the custom_pipeline name into a local path, since the files have already been downloaded:

# pipeline_loading_utils.py:965-974
def _resolve_custom_pipeline_and_cls(folder, config, custom_pipeline):
    custom_class_name = None
    if os.path.isfile(os.path.join(folder, f"{custom_pipeline}.py")):
        custom_pipeline = os.path.join(folder, f"{custom_pipeline}.py")
    elif isinstance(config["_class_name"], (list, tuple)) and os.path.isfile(
        os.path.join(folder, f"{config['_class_name'][0]}.py")
    ):
        custom_pipeline = os.path.join(folder, f"{config['_class_name'][0]}.py")
        custom_class_name = config["_class_name"][1]

    return custom_pipeline, custom_class_name

When custom_class_name is None (i.e. custom_pipeline was given as a kwarg rather than via the config), _get_pipeline_class will scan the file and automatically identify the class that subclasses DiffusionPipeline.

Once this is done, _get_pipeline_class is invoked with the resolved local path, which loads the custom code, retrieves the class object, and proceeds with instantiation.

The Vulnerability

_resolve_custom_pipeline_and_cls receives custom_pipeline from the kwargs - when not supplied it defaults to None. That None is used in string formatting: f"{None}.py" = "None.py".

If the repo contains a file with this name, it will be detected as a custom pipeline.

This is only reached on the second invocation of _get_pipeline_class (inside from_pretrained, after download() returns). The trust_remote_code check lives entirely in download(), which evaluated custom_pipeline is None -> False and skipped it. By the time _resolve_custom_pipeline_and_cls runs, it is no longer relevant.

As a bonus, None.py even gets downloaded automatically when the model isn't cached yet. This isn't strictly required - it is quite plausible that the victim has already run hf download <model> and has all files locally - but if they haven't, revisiting the allow_patterns line above shows it makes the same error: f"{None}.py" = "None.py" is added to allow_patterns and fetched.

What should None.py contain? To avoid breaking the pipeline load, it must define a class inheriting from DiffusionPipeline. To avoid leaving suspicious clues in the config, that class should shadow one that already exists in diffusers. The following satisfies both requirements:

from diffusers import FluxPipeline as _FluxPipeline

class FluxPipeline(_FluxPipeline):
    pass

# INSERT MALICIOUS CODE HERE
import pathlib
pathlib.Path("/tmp/pwned").write_text(":)")

With this, model_index.json can contain "_class_name": "FluxPipeline" - appearing to use the standard diffusers class - and the resulting pipeline is fully functional (it is also functional when running as a local directory). This has been verified against an extracted version of DDUF/tiny-flux-dev-pipe-dduf.

All the attacker needs the victim to run is:

from diffusers import DiffusionPipeline

pipeline = DiffusionPipeline.from_pretrained('ido-shani/none-py-trust-remote-code-bypass')

PoC

• Upload this zip as a model to the hub. https://drive.google.com/file/d/1mULARMLJJUTCi57xIv0wtDauko-JW0h7/view?usp=sharing
• Run DiffusionPipeline.from_pretrained on the uploaded model hub identifier.
• RCE occured; /tmp/pwned was created. If you are running the exploit on windows, change the path touched in None.py.

Impact

The vulnerability is a silent RCE - it allows arbitrary code to be loaded through the custom_pipeline flow from a Hub repo, with no custom_pipeline or trust_remote_code kwargs and nothing suspicious in the config. The from_pretrained call succeeds and returns a functional pipeline.

Occurrences

https://github.com/huggingface/diffusers/blob/e1b5db52bda85d47a4f8f75954f77e672a8f7f1c/src/diffusers/pipelines/pipeline_loading_utils.py#L976

Patches

Yes. Fixed in diffusers 0.38.0 via PR #13448. All users on versions < 0.38.0 should upgrade:

pip install --upgrade "diffusers>=0.38.0"

The fix moves the trust_remote_code gate out of DiffusionPipeline.download() and into get_cached_module_file in src/diffusers/utils/dynamic_modules_utils.py, which is the actual chokepoint for every dynamic module load (local, Hub, or community mirror). All three variants now raise ValueError when trust_remote_code=False instead of executing untrusted code.

Workarounds

If upgrading immediately is not possible:

• Only call from_pretrained with pretrained_model_name_or_path, custom_pipeline, and local snapshot directories from sources you fully trust and have audited.
• Do not pass custom_pipeline= pointing at a Hub repository different from the primary pretrained_model_name_or_path unless you have read its pipeline.py.
• Before calling from_pretrained on a local snapshot, inspect the snapshot for unexpected *.py files, especially under component subdirectories (unet/, scheduler/, etc.) and at the snapshot root.

Why this should have a dedicated CVE

GHSA-j7w6-vpvq-j3gm is a distinct defect from CVE-2026-44513. CVE-2026-44513 is a misplaced-security-gate bug requiring a user-supplied custom_pipeline argument or a config entry declaring custom code. GHSA-j7w6 is a string-formatting bug where the default custom_pipeline=None is interpolated into the filename None.py, allowing silent RCE on a fully default from_pretrained('repo') call with no kwargs and a model_index.json that shadows a legitimate class. The root cause root cause and trigger are different, although the fix applied to address CVE-2026-44513 also addresses this vulnerability.