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.
Credits Omkhar Arasaratnam (@omkhar) - finder.
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.WRAPPERARRAY)):
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 |
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Patches
Fixed in 2.18.8, 2.21.4 and 3.1.4 via the changes for FasterXML/jackson-databind#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.
A flaw was found in FasterXML Jackson Databind which did not have entity expansion secured properly making it vulnerable to XML external entity (XXE). This vulnerability is similar to CVE-2019-10172. The primary threat from this flaw is data integrity.
Impact
Potential Denial-of-Service when attacker sends deeply nested JSON if (and only if) service:
1. Reads deeply nested (1000s of levels) JSON as JsonNode (ObjectMapper.readTree()) 2. Writes out same (or modifided) node using JsonNode.toString()
which can consume significant amount of resources with concurrent relatively small requests (1000 nested arrays is 2kB).
Patches
Fixed in 2.14.0 via https://github.com/FasterXML/jackson-databind/issues/3447.
Workarounds
Avoid serializing JsonNode using toString(): use ObjectMapper.writeValueAsString(node)
A flaw was found in Jackson Databind. This issue may allow a malicious user to cause a denial of service (2 GB transient heap usage per read) in uncommon situations involving JsonNode JDK serialization.