CVE-2026-106439: Hydra Mutable Instantiate Policy Sets Allow Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mwj6-rfh8-7qf4",
  "modified" : "2026-10-07T20:23:58Z",
  "published" : "2026-10-07T20:23:57Z",
  "aliases" : [ "CVE-2026-106439" ],
  "summary" : "Hydra: Mutable instantiate policy sets allow target blocklist bypass",
  "details" : "## Summary\n\nHydra's legacy `instantiate()` target blocklists and related execution-policy\ncollections are stored in mutable module-level state. Because `_locate()` can\nresolve attributes on imported objects, a configuration can resolve a mutation\nmethod such as `.discard()`, modify the active policy, and then instantiate a\ntarget that would otherwise be blocked.\n\n## Impact\n\nA configuration controlling multiple sibling `_target_` entries can first\nremove an entry from a target blocklist and then invoke the removed target.\nSibling nodes are processed in insertion order and consult the same mutable\nmodule-level policy.\n\nThis affects the legacy/default path without an execution whitelist. The 1.3\nblocklist is a defense-in-depth measure rather than a complete security\nboundary, and applications must not treat arbitrary untrusted configuration as\nsafe to instantiate or use for Python logging configuration.\n\nReleased `hydra-core` versions 1.3.4 through 1.3.6 and 1.4.0.dev4 through\n1.4.0.dev9 are affected. Fixed releases are 1.3.7 and 1.4.0.dev10.\nThe reported direct mutation path does not bypass an execution whitelist\nrestricted to intended application targets and supplied by trusted Python\ncode. During remediation, Hydra additionally hardened generic discovery,\ndispatch, introspection, alias, callable-result, and deferred-callable paths\nthat could otherwise undermine name-only authorization.\n\n## Technical details\n\nIn `hydra-core` 1.3.4 and 1.3.5, the mutable blocklist is reachable as:\n\n`hydra._internal.instantiate._instantiate2.DEFAULT_BLOCKLISTED_MODULES`\n\nIn `hydra-core` 1.3.6, the expanded policy includes mutable collections in:\n\n`hydra._internal.target_policy`\n\nFor example:\n\n`hydra._internal.target_policy.UNCONTROLLED_EXECUTION_TARGETS.discard`\n\nresolves to the bound `set.discard` method. The mutation target itself is not\nblocked on the legacy path. Once an entry is removed, subsequent authorization\nchecks observe the modified set.\n\nOther runtime policy collections can be attacked similarly by removing denied\nentries or adding entries to exception sets. Because the collections are\nmodule-level state, a successful mutation persists for the lifetime of the\nPython process unless explicitly reversed.\n\n## Safe reproduction\n\nThe behavior in `hydra-core` 1.3.6 can be demonstrated without invoking a shell\ncommand. The `finally` block restores the modified process-global state:\n\n```python\nfrom omegaconf import OmegaConf\n\nfrom hydra._internal.target_policy import UNCONTROLLED_EXECUTION_TARGETS\nfrom hydra.utils import instantiate\n\ntarget = \"builtins.eval\"\nassert target in UNCONTROLLED_EXECUTION_TARGETS\n\ntry:\n    result = instantiate(\n        OmegaConf.create(\n            {\n                \"disarm\": {\n                    \"_target_\": (\n                        \"hydra._internal.target_policy.\"\n                        \"UNCONTROLLED_EXECUTION_TARGETS.discard\"\n                    ),\n                    \"_args_\": [target],\n                },\n                \"proof\": {\n                    \"_target_\": target,\n                    \"_args_\": [\"40 + 2\"],\n                },\n            }\n        )\n    )\n\n    assert result[\"proof\"] == 42\n    assert target not in UNCONTROLLED_EXECUTION_TARGETS\nfinally:\n    UNCONTROLLED_EXECUTION_TARGETS.add(target)\n```\n\n## Remediation\n\nThe fix makes runtime policy state immutable and integrity checked, and\nprevents declarative configuration from accessing or mutating Hydra internals\nand protected Python implementation state. Target authorization now covers\ncanonical resolved identities, aliases, discovery results, callable results,\ndeferred callables, and runtime arguments.\n\nThe patch also rejects configuration-driven code, policy, and process-\nenvironment mutation, along with unsafe introspection and formatting traversal\nthat can expose protected runtime capabilities.\n\nHydra 1.3.7 receives these protections as defense in depth; it does not make\nuntrusted configuration sandboxed. Hydra 1.4 additionally uses a trusted,\nnarrowly scoped execution whitelist as the supported security boundary for\ndeclarative instantiation and Hydra-controlled Python logging configuration.\n\nUsers should upgrade to `hydra-core` 1.3.7 on the stable line or\n1.4.0.dev10 on the development line.\n\n## Workarounds\n\nDo not pass configuration from untrusted sources to `instantiate()` or to\nHydra-controlled Python logging configuration. The 1.3 release line has no\nexecution-whitelist facility, so users who cannot upgrade immediately must\nrestrict configuration input to trusted sources.\n\nOn affected 1.4 development releases, applications can also supply a trusted,\nnarrowly scoped execution whitelist from Python code. The whitelist itself must\nnot be derived from untrusted configuration.\n\nRestart any long-running process that may already have instantiated untrusted\nconfiguration, because a policy mutation persists in process-global state.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "hydra-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.3.4"
      }, {
        "fixed" : "1.3.7"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "hydra-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.4.0.dev4"
      }, {
        "fixed" : "1.4.0.dev10"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/hydra-ecosystem/hydra/security/advisories/GHSA-mwj6-rfh8-7qf4"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-106439"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/hydra-ecosystem/hydra/commit/0dd18084589a3d3e577d1f1a8a48fb485c94a5e6"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/hydra-ecosystem/hydra/commit/4720dfca2bde27fa140ec287a05709668fbdf168"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/hydra-ecosystem/hydra"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/hydra-ecosystem/hydra/releases/tag/v1.3.7"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-470", "CWE-693" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-07T20:23:57Z",
    "nvd_published_at" : "2026-10-06T19:18:12Z"
  }
}