jackson-databind has a PolymorphicTypeValidator bypass via generic type parameters that allows arbitrary class instantiation

`jackson-databind`'s `PolymorphicTypeValidator` (PTV) is the primary safety mechanism guarding polymorphic deserialization. When polymorphic typing is enabled and a type identifier contains generic parameters (i.e. the type ID string contains `<`), `DatabindContext._resolveAndValidateGeneric()` validates **only the raw container class name** (the substring before `<`) against the configured PTV. If the container type is approved, the method parses the full canonical type string via `TypeFactory.constructFromCanonical()` and returns the fully parameterized type **without ever validating the nested type arguments** against the PTV. The nested type arguments are then resolved, instantiated, and populated as beans during deserialization. An attacker who controls the type ID can therefore place a denied class as a generic type parameter of an allowed container — for example `java.util.ArrayList<com.evil.Gadget>` when only `java.util.ArrayList` is allow-listed. The container passes the PTV check; `com.evil.Gadget` is loaded via `Class.forName(name, true, loader)`, instantiated, and its properties are set from attacker-controlled JSON. This completely bypasses an explicitly configured PTV allow-list. This is the same vulnerability class responsible for the historical sequence of jackson-databind deserialization CVEs; here it manifests as a validator bypass rather than a missing deny-list entry. ## Impact - **Bypass of the PTV allow-list**, including the recommended `BasicPolymorphicTypeValidator` configured with name-prefix allow rules. - **Arbitrary class instantiation** of any type assignable to the container's element/parameter position, with attacker-controlled property values (setter/field injection). - **Potential unauthenticated remote code execution** when a class with exploitable side effects (JNDI lookup, JDBC/connection-pool gadgets,`TemplatesImpl`-style loaders, etc.) is present on the classpath. Applications that accept untrusted JSON and rely on a configured PTV — the documented, security-conscious configuration — are affected. ## Proof of Concept Configuration restricting polymorphic deserialization to a single safe container: ```java BasicPolymorphicTypeValidator ptv = BasicPolymorphicTypeValidator.builder() .allowIfSubType("java.util.ArrayList") .build(); ObjectMapper mapper = JsonMapper.builder() .polymorphicTypeValidator(ptv) .build(); ``` Malicious payload (`Wrapper.value` is `Object` with `@JsonTypeInfo(use = Id.CLASS, include = As.WRAPPER_ARRAY)`): ```json {"value":["java.util.ArrayList<com.evil.EvilGadget>",[{"cmd":"calc.exe"}]]} ``` On vulnerable versions, `com.evil.EvilGadget` is instantiated and its `cmd` property is set, despite only `java.util.ArrayList` being allow-listed. On `2.18.8` / `2.21.4` / `3.1.4` the deserialization throws `InvalidTypeIdException` before instantiation. **Variant payloads** (all bypass an `ArrayList`/`HashMap` allow-list): | Type ID | Smuggled type position | |---|---| | `java.util.ArrayList<Evil>` | list element | | `java.util.HashMap<Evil,String>` | map key | | `java.util.HashMap<String,Evil>` | map value | | `java.util.ArrayList<java.util.ArrayList<Evil>>` | nested element | | `java.util.ArrayList<Evil[]>` | array element | --- ## Patches Fixed in **2.18.8**, **2.21.4** and **3.1.4** via the changes for [FasterXML/jackson-databind#5988](https://github.com/FasterXML/jackson-databind/issues/5988), commit `434d6c511`. The fix adds recursive validation of each non-trivial type parameter (and array element types appearing as parameters) through the full PTV chain, with documented exemptions for `Object` (wildcard resolution) and `Enum` types. `PolymorphicTypeValidator` was added in 2.10.0 so vulnerability N/A for versions prior to that.