GHSA-rmj7-2vxq-3g9f — jackson-databind has an array subtype allowlist bypass in BasicPolymorphicTypeValidator (allowIfSubTypeIsArray)

Affected: Maven: com.fasterxml.jackson.core:jackson-databind, Maven: tools.jackson.core:jackson-databind

## Summary `BasicPolymorphicTypeValidator.Builder.allowIfSubTypeIsArray()` allowlists any array type based only on `clazz.isArray()`, without validating the array's component (element) type against the configured allowlist. A PTV built with `allowIfSubTypeIsArray()` plus an explicit concrete-type allowlist therefore still permits `EvilType[]` even though `EvilType` is not allowlisted. When Jackson deserializes the elements and no per-element type IDs are present, it instantiates the component type directly with no further PTV check, bypassing the allowlist.

## Impact Applications using `BasicPolymorphicTypeValidator` with `allowIfSubTypeIsArray()` as a safeguard get no protection for concrete array component types; an attacker controlling JSON can instantiate non-allowlisted types via an array wrapper, re-opening the gadget-instantiation risk PTV is meant to prevent.

## Affected / Patched (verified via `git tag --contains`) - 2.18 line: `>= 2.10.0, < 2.18.8` -> fixed in **2.18.8** - 2.19-2.21 line: `>= 2.19.0, < 2.21.4` -> fixed in **2.21.4** - 3.x line: `>= 3.0.0, < 3.1.4` -> fixed in **3.1.4**

`PolymorphicTypeValidator` was added in 2.10.0 so vulnerability N/A for versions prior to that.

## Severity / CWE Maintainer: significant. Reporter: HIGH. CWE-184 (Incomplete List of Disallowed Inputs); related CWE-502.

## Upstream fix FasterXML/jackson-databind#5981; fix PR #5983 (`24529da`), 2.18 backport PR #5984 (`01d1692`). Released 2026-06-04 in 2.18.8 / 2.21.4 / 3.1.4.

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GHSA-rmj7-2vxq-3g9f — jackson-databind has an array subtype allowlist bypass in BasicPolymorphicTypeValidator (allowIfSubTypeIsArray)