hydra-core: Hydra instantiate target blacklist bypasses permit code execution

Summary

Hydra's instantiate() API resolves and invokes Python callables named by the _target_ field in configuration. The target blacklist introduced for CVE-2026-68508 was incomplete.

Representative bypasses included execution wrappers such as timeit.timeit, executable deserialization through pickle.loads, and generic dispatch or wrapper targets that obscured the effective callable. Target aliases, callable-returning helpers, and deferred dispatch could similarly bypass name-based checks.

An attacker who can cause an application to instantiate an untrusted Hydra configuration can use these gaps to execute code with the application's privileges.

Fix

Hydra 1.3.6 expands and hardens the blacklist used by the 1.3 compatibility line. It blocks the reported execution and deserialization surfaces, checks canonical callable identities and aliases, mediates callable results, and blocks generic dispatch and wrapper targets that bypass immediate target checks.

The Hydra 1.3 blacklist is a best-effort, defense-in-depth measure. It is not a complete security boundary and does not make untrusted configuration safe to instantiate.

Hydra 1.4.0.dev9 applies the same hardening and introduces the execution whitelist as the recommended primary security boundary. When a whitelist is supplied, targets are rejected unless trusted Python code authorizes them. If no whitelist is supplied, Hydra preserves legacy behavior with a deprecation warning and the blacklist as a best-effort compatibility fallback. Targets that permit configuration to select or supply executable behavior remain non-whitelistable.

Remediation

Upgrade to Hydra 1.3.6, or to Hydra 1.4.0.dev9 or later when testing the 1.4 prerelease line.

On Hydra 1.3, do not instantiate configuration from untrusted sources. On Hydra 1.4 and later, constrain untrusted configuration with a narrow execution whitelist supplied by trusted Python code.

For Hydra 1.4 execution-whitelist configuration, see: https://hydra.cc/docs/advanced/execution_whitelist/