Summary
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.
Proof of Concept
Configuration restricting polymorphic deserialization to a single safe container:
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)):
{"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 |
Impact
- Bypass of the PTV allow-list, including the recommended
BasicPolymorphicTypeValidatorconfigured 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.
Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.
CVE-2026-54512 has a CVSS score of 8.1 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.18.8, 3.1.4, 2.21.4); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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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.
Frequently Asked Questions
- What is CVE-2026-54512? CVE-2026-54512 is a high-severity insecure deserialization vulnerability in com.fasterxml.jackson.core:jackson-databind (maven), affecting versions >= 2.10.0, <= 2.18.7. It is fixed in 2.18.8, 3.1.4, 2.21.4. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
- How severe is CVE-2026-54512? CVE-2026-54512 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2026-54512?
com.fasterxml.jackson.core:jackson-databind(maven) (versions >= 2.10.0, <= 2.18.7)tools.jackson.core:jackson-databind(maven) (versions >= 3.0.0, <= 3.1.3)
- Is there a fix for CVE-2026-54512? Yes. CVE-2026-54512 is fixed in 2.18.8, 3.1.4, 2.21.4. Upgrade to this version or later.
- Is CVE-2026-54512 exploitable, and should I be worried? Whether CVE-2026-54512 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-54512 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-54512?
- Upgrade
com.fasterxml.jackson.core:jackson-databindto 2.18.8 or later - Upgrade
com.fasterxml.jackson.core:jackson-databindto 3.1.4 or later - Upgrade
com.fasterxml.jackson.core:jackson-databindto 2.21.4 or later - Upgrade
tools.jackson.core:jackson-databindto 3.1.4 or later
- Upgrade