GHSA-wf6j-7hvr-gr93: Arbitrary Code Execution via PyYAML Target Blocklist Bypass in hydra-core

Vulnerability ID: GHSA-WF6J-7HVR-GR93

CVSS Score: 7.8

Published: 2026-10-27

A high-severity target blocklist omission in the hydra-core Python package (versions 1.3.6 through 1.3.7 and 1.4.0.dev9 through 1.4.0rc2) allows arbitrary code execution when instantiating untrusted configuration dictionaries. Because the execution policy failed to restrict the PyYAML (yaml.) namespace, an attacker can specify functions such as yaml.unsafe_load as the instantiation target and supply a crafted YAML stream containing Python object construction tags to execute arbitrary system commands within the host process.

TL;DR

Hydra's dynamic object instantiation blocklist omitted the PyYAML namespace (yaml.), enabling attackers who control configuration metadata passed to hydra.utils.instantiate() or hydra.utils.call() to invoke yaml.unsafe_load and achieve arbitrary code execution.

⚠️ Exploit Status: POC

Technical…