CodeQL documentation

Deserialization of user-controlled data

ID: java/unsafe-deserialization
Kind: path-problem
Security severity: 9.8
Severity: error
Precision: high
   - security
   - external/cwe/cwe-502
Query suites:
   - java-code-scanning.qls
   - java-security-extended.qls
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

Deserializing untrusted data using any deserialization framework that allows the construction of arbitrary serializable objects is easily exploitable and in many cases allows an attacker to execute arbitrary code. Even before a deserialized object is returned to the caller of a deserialization method a lot of code may have been executed, including static initializers, constructors, and finalizers. Automatic deserialization of fields means that an attacker may craft a nested combination of objects on which the executed initialization code may have unforeseen effects, such as the execution of arbitrary code.

There are many different serialization frameworks. This query currently supports Kryo, XmlDecoder, XStream, SnakeYaml, JYaml, JsonIO, YAMLBeans, HessianBurlap, Castor, Burlap, Jackson, Jabsorb, Jodd JSON, Flexjson, Gson, JMS, and Java IO serialization through ObjectInputStream/ObjectOutputStream.


Avoid deserialization of untrusted data if at all possible. If the architecture permits it then use other formats instead of serialized objects, for example JSON or XML. However, these formats should not be deserialized into complex objects because this provides further opportunities for attack. For example, XML-based deserialization attacks are possible through libraries such as XStream and XmlDecoder.

Alternatively, a tightly controlled whitelist can limit the vulnerability of code, but be aware of the existence of so-called Bypass Gadgets, which can circumvent such protection measures.

Recommendations specific to particular frameworks supported by this query:

FastJson -

  • Secure by Default: Partially

  • Recommendation: Call with the argument true before deserializing untrusted data.

FasterXML - com.fasterxml.jackson.core:jackson-databind

  • Secure by Default: Yes

  • Recommendation: Don’t call com.fasterxml.jackson.databind.ObjectMapper#enableDefaultTyping and don’t annotate any object fields with com.fasterxml.jackson.annotation.JsonTypeInfo passing either the CLASS or MINIMAL_CLASS values to the annotation. Read this guide.

Kryo - com.esotericsoftware:kryo and com.esotericsoftware:kryo5

  • Secure by Default: Yes for com.esotericsoftware:kryo5 and for com.esotericsoftware:kryo >= v5.0.0

  • Recommendation: Don’t call com.esotericsoftware.kryo(5).Kryo#setRegistrationRequired with the argument false on any Kryo instance that may deserialize untrusted data.

ObjectInputStream - Java Standard Library

  • Secure by Default: No

  • Recommendation: Use a validating input stream, such as

SnakeYAML - org.yaml:snakeyaml

  • Secure by Default: No

  • Recommendation: Pass an instance of org.yaml.snakeyaml.constructor.SafeConstructor to org.yaml.snakeyaml.Yaml’s constructor before using it to deserialize untrusted data.

XML Decoder - Standard Java Library

  • Secure by Default: No

  • Recommendation: Do not use with untrusted user input.

ObjectMesssage - Java EE/Jakarta EE

  • Secure by Default: Depends on the JMS implementation.

  • Recommendation: Do not use with untrusted user input.


The following example calls readObject directly on an ObjectInputStream that is constructed from untrusted data, and is therefore inherently unsafe.

public MyObject {
  public int field;
  MyObject(int field) {
    this.field = field;

public MyObject deserialize(Socket sock) {
  try(ObjectInputStream in = new ObjectInputStream(sock.getInputStream())) {
    return (MyObject)in.readObject(); // unsafe

Rewriting the communication protocol to only rely on reading primitive types from the input stream removes the vulnerability.

public MyObject deserialize(Socket sock) {
  try(DataInputStream in = new DataInputStream(sock.getInputStream())) {
    return new MyObject(in.readInt());


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