[SECURITY] YAML Deserialization RCE via Unsafe yaml.Loader in generator.py

[xiaodao00266]

SUMMARY
YAML Deserialization Remote Code Execution via Unsafe yaml.Loader in generator.py

SEVERITY
High (CVSS 7.8) — AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

VULNERABLE CODE
File: qface/generator.py, lines 31-35 and 406-415

The FileSystem.load_yaml() method uses the unsafe yaml.Loader (or yaml.CLoader)
to parse YAML documents, allowing arbitrary Python code execution.

Lines 31-35:

try:
    from yaml import CLoader as Loader, CDumper as Dumper
except ImportError:
    from yaml import Loader, Dumper

Lines 406-415:

@staticmethod
def load_yaml(document: Path, required=False):
    ...
    return yaml.load(document.read_text(), Loader=Loader)

The Loader is the unsafe PyYAML loader that supports !!python/object/apply tags,
allowing arbitrary Python function calls during YAML parsing.

PROOF OF CONCEPT
A malicious YAML file with the following content triggers code execution:

!!python/object/apply:os.system ["echo EXPLOITED"]

When qface processes this file via --rules, os.system() executes:

qface --rules malicious.yaml --target output/

PoC verified locally: yaml.Loader executes os.system() successfully.
yaml.SafeLoader correctly blocks the same payload.

IMPACT
An attacker who can supply a malicious YAML file to qface can execute arbitrary
code with the privileges of the user running qface. Attack vectors include:

1. A malicious pull request containing a YAML rule file processed by CI/CD
2. A developer cloning a repository that contains a crafted YAML configuration
3. Any application using qface as a library and loading YAML from untrusted sources

FIX
Replace yaml.Loader/yaml.CLoader with yaml.SafeLoader/yaml.CSafeLoader:

try:
    from yaml import CSafeLoader as Loader, CSafeDumper as Dumper
except ImportError:
    from yaml import SafeLoader as Loader, SafeDumper as Dumper

AFFECTED VERSIONS
All versions of qface using the unsafe yaml.Loader.

RESEARCHER
Found during automated security assessment.