Common Weakness Enumeration

A Community-Developed Dictionary of Software Weakness Types

Common Weakness Scoring System
Common Weakness Risk Analysis Framework

Presentation Filter:

CWE-502: Deserialization of Untrusted Data

Deserialization of Untrusted Data
Weakness ID: 502 (Weakness Variant)Status: Draft
+ Description

Description Summary


Extended Description

It is often convenient to serialize objects for communication or to save them for later use. However, deserialized data or code can often be modified without using the provided accessor functions if it does not use cryptography to protect itself. Furthermore, any cryptography would still be client-side security -- which is a dangerous security assumption.

Data that is untrusted can not be trusted to be well-formed.

+ Alternate Terms
Marshaling, Unmarshaling:

Marshaling and unmarshaling are effectively synonyms for serialization and deserialization, respectively.

Pickling, Unpickling:

In Python, the "pickle" functionality is used to perform serialization and deserialization.

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms







+ Common Consequences

Technical Impact: Varies by context

The consequences can vary widely, because it depends on which objects or methods are being deserialized, and how they are used.


Technical Impact: Modify application data; Unexpected state

Attackers can modify unexpected objects or data that was assumed to be safe from modification.


Technical Impact: DoS: resource consumption (CPU)



Technical Impact: Other

Code could potentially make the assumption that information in the deserialized object is valid. Functions that make this dangerous assumption could be exploited.

+ Likelihood of Exploit


+ Demonstrative Examples

Example 1

This code snippet deserializes an object from a file and uses it as a UI button:

(Bad Code)
Example Language: Java 
try {
File file = new File("object.obj");
ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
javax.swing.JButton button = (javax.swing.JButton) in.readObject();

This code does not attempt to verify the source or contents of the file before deserializing it. An attacker may be able to replace the intended file with a file that contains arbitrary malicious code which will be executed when the button is pressed.

+ Observed Examples
Use of PHP unserialize function on untrusted input allows attacker to modify application configuration.
Use of PHP unserialize function on untrusted input in content management system might allow code execution.
Use of PHP unserialize function on untrusted input in content management system allows code execution using a crafted cookie value.
Content management system written in PHP allows unserialize of arbitrary objects, possibly allowing code execution.
Python script allows local users to execute code via pickled data.
Unsafe deserialization using pickle in a Python script.
Web browser allows execution of native methods via a crafted string to a JavaScript function that deserializes the string.
+ Potential Mitigations

Phases: Architecture and Design; Implementation

If available, use the signing/sealing features of the programming language to assure that deserialized data has not been tainted. For example, a hash-based message authentication code (HMAC) could be used to ensure that data has not been modified.

Phase: Implementation

When deserializing data, populate a new object rather than just deserializing. The result is that the data flows through safe input validation and that the functions are safe.

Phase: Implementation

Explicitly define final readObject() to prevent deserialization. An example of this is:

(Good Code)
Example Language: Java 
private final void readObject(ObjectInputStream in) throws {
throw new"Cannot be deserialized"); }

Phases: Architecture and Design; Implementation

Make fields transient to protect them from deserialization.

An attempt to serialize and then deserialize a class containing transient fields will result in NULLs where the transient data should be. This is an excellent way to prevent time, environment-based, or sensitive variables from being carried over and used improperly.

+ Background Details

Serialization and deserialization refer to the process of taking program-internal object-related data, packaging it in a way that allows the data to be externally stored or transferred ("serialization"), then extracting the serialized data to reconstruct the original object ("deserialization").

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory858CERT Java Secure Coding Section 13 - Serialization (SER)
ChildOfWeakness ClassWeakness Class913Improper Control of Dynamically-Managed Code Resources
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory994SFP Secondary Cluster: Tainted Input to Variable
Software Fault Pattern (SFP) Clusters (primary)888
PeerOfWeakness BaseWeakness Base915Improperly Controlled Modification of Dynamically-Determined Object Attributes
Research Concepts1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPDeserialization of untrusted data
CERT Java Secure CodingSER01-JDo not deviate from the proper signatures of serialization methods
CERT Java Secure CodingSER03-JDo not serialize unencrypted, sensitive data
CERT Java Secure CodingSER06-JMake defensive copies of private mutable components during deserialization
CERT Java Secure CodingSER08-JDo not use the default serialized form for implementation defined invariants
Software Fault PatternsSFP25Tainted input to variable
+ References
Heine Deelstra. "Unserializing user-supplied data, a bad idea". 2010-08-25. <>.
Nadia Alramli. "Why Python Pickle is Insecure". 2009-09-09. <>.
+ Maintenance Notes

The relationships between CWE-502 and CWE-915 need further exploration. CWE-915 is more narrowly scoped to object modification, and is not necessarily used for deserialization.

+ Content History
Submission DateSubmitterOrganizationSource
CLASPExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Common_Consequences, Description, Relationships, Other_Notes, Taxonomy_Mappings
2009-10-29CWE Content TeamMITREInternal
updated Description, Other_Notes, Potential_Mitigations
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2012-10-30CWE Content TeamMITREInternal
updated Demonstrative_Examples
2013-02-21CWE Content TeamMITREInternal
updated Alternate_Terms, Applicable_Platforms, Background_Details, Common_Consequences, Maintenance_Notes, Observed_Examples, Potential_Mitigations, References, Relationships
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
Page Last Updated: July 30, 2014