Diffusers: TOCTOU Trust Remote Code Bypass

Background

This vulnerability is found in the diffusers package - the transformers-equivalent library for diffusion models.

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

This function has a trust_remote_code guard: if the repository’s model_index.json references a custom pipeline class defined in a .py file in the repo, the load is blocked unless trust_remote_code=True is explicitly passed:

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

The vulnerability allows arbitrary code execution through the custom pipeline flow from a Hub repo, with no custom_pipeline or trust_remote_code kwargs passed. The from_pretrained call succeeds and returns a functional pipeline.

---

Naive Flow

DiffusionPipeline.from_pretrained begins by popping all relevant arguments from kwargs into local variables, then calls DiffusionPipeline.download() to fetch the repo files:

# pipeline_utils.py:853
cached_folder = cls.download(
    pretrained_model_name_or_path,
    ...
    custom_pipeline=custom_pipeline,
    trust_remote_code=trust_remote_code,
    ...
)

Inside download(), model_index.json is fetched first as a standalone file via hf_hub_download:

# pipeline_utils.py:1636
config_file = hf_hub_download(
    pretrained_model_name,
    cls.config_name,
    ...
)
config_dict = cls._dict_from_json_file(config_file)

This config is used to detect custom pipeline code and enforce the trust check:

# pipeline_utils.py:1672
if custom_pipeline is None and isinstance(config_dict["_class_name"], (list, tuple)):
    custom_pipeline = config_dict["_class_name"][0]

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(...)

After the check passes, snapshot_download then fetches all files and saves them to disk:

# pipeline_utils.py:1778
cached_folder = snapshot_download(
    pretrained_model_name,
    ...
    revision=revision,
    allow_patterns=allow_patterns,
    ...
)

Back in from_pretrained, the config is read a second time from the downloaded snapshot, and_resolve_custom_pipeline_and_cls reads the config to re-check if custom code needs to be loaded:

# pipeline_loading_utils.py: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

If the config points to a .py file, it is imported.

---

The Vulnerability

hf_hub_download and snapshot_download are two independent HTTP calls to the Hub, both resolving the repository’s default branch (if revision=None) to its current HEAD at call time. There is no atomicity guarantee between them - if the repository is updated between the two calls, they will resolve to different commits and download different content, with no warning displayed to the user.

The trust check in download() operates on the content fetched by hf_hub_download (commit A). The snapshot_download call that immediately follows can silently fetch a newer commit (commit B). The config in the newer commit will be the one parsed by _resolve_custom_pipeline_and_cls.

Therefore, it’s possible to introduce remote code into the repo between the two calls, bypassing the trust check.

The race window is everything between the two Hub calls inside download():

# pipeline_utils.py:1636
config_file = hf_hub_download(...)   # ← sees commit A, trust check passes

# ... filenames processing, pattern building, pipeline_is_cached check ...
# ~~~ ATTACKER PUSHES COMMIT B HERE ~~~

# pipeline_utils.py:1778
cached_folder = snapshot_download(...)  # ← sees commit B, downloads pipeline.py

For the exploit, commit A carries a clean config with _class_name as a plain string, which causes load_pipe_from_hub to be False and the trust check to pass. Commit B changes _class_name to a list and adds pipeline.py:

Commit A - model_index.json:

{
  "_class_name": "FluxPipeline",
  "_diffusers_version": "0.31.0"
}

Commit B - model_index.json:

{
  "_class_name": ["pipeline", "FluxPipeline"],
  "_diffusers_version": "0.31.0"
}

When from_pretrained reads the snapshot after download() returns, config["_class_name"] is now a list, pipeline.py exists on disk (fetched by snapshot_download), and _resolve_custom_pipeline_and_cls resolves custom_pipeline to the local path of that file. _get_pipeline_class then imports it - with no trust check at this point in the code.

---

PoC

1. Create a Hub repo with commit A’s model_index.json (plain string _class_name).
2. Run DiffusionPipeline.from_pretrained("attacker/repo") with a breakpoint set at pipeline_utils.py:1778 (the snapshot_download call). This is for the window to be large enough to manually respond to it.
3. When execution pauses at the breakpoint, push commit B: update model_index.json to use a list _class_name and add pipeline.py.
4. Resume execution.
5. snapshot_download fetches commit B; /tmp/pwned is written during the subsequent _get_pipeline_class call.

---

Constraints

• Does not apply when revision is pinned to a specific commit hash - both Hub calls resolve to the same content.
• Does not apply when loading from a local directory.
• If all expected files are already present in the local HF cache, download() returns early before reaching snapshot_download (line 1767 early-return), closing the race window. The exploit therefore requires a first (or forced) download.

---

Exploitability

The window between the two calls is very short. Local testing resulted in a window of approximately ~0.5 seconds for the attacker to push the change. This is, of course, unfeasible to accomplish for each and every new download. However, given a popular repo with many downloads per day, one may achieve statistical success by changing the repo’s state every once in a while or every few seconds, with some percentage of downloaders falling on the exact window.

---

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. The from_pretrained call succeeds and returns a fully functional pipeline.