Official definition

The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.

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").

Characteristics

Alternate terms

Marshaling/Marshalling, Unmarshaling/Unmarshalling — 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.
PHP Object Injection — Some PHP application researchers use this term when attacking unsafe use of the unserialize() function; but it is also used for CWE-915.

Modes of introduction

Architecture and Design: OMISSION: This weakness is caused by missing a security tactic during the architecture and design phase.
Implementation

Common consequences

Impact	Scope	Explanation
Modify Application Data, Unexpected State	Integrity	Attackers can modify unexpected objects or data that was assumed to be safe from modification. Deserialized data or code could be modified without using the provided accessor functions, or unexpected functions could be invoked.
DoS: Resource Consumption (CPU)	Availability	If a function is making an assumption on when to terminate, based on a sentry in a string, it could easily never terminate.
Varies by Context	Other	The consequences can vary widely, because it depends on which objects or methods are being deserialized, and how they are used. Making an assumption that the code in the deserialized object is valid is dangerous and can enable exploitation. One example is attackers using gadget chains to perform unauthorized actions, such as generating a shell.

Risk mitigations

Architecture and Design, ImplementationIf 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.
ImplementationWhen 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.
ImplementationExplicitly define a final object() to prevent deserialization.
Architecture and Design, ImplementationMake 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.
ImplementationAvoid having unnecessary types or gadgets (a sequence of instances and method invocations that can self-execute during the deserialization process, often found in libraries) available that can be leveraged for malicious ends. This limits the potential for unintended or unauthorized types and gadgets to be leveraged by the attacker. Add only acceptable classes to an allowlist. Note: new gadgets are constantly being discovered, so this alone is not a sufficient mitigation.
Architecture and Design, ImplementationEmploy cryptography of the data or code for protection. However, it's important to note that it would still be client-side security. This is risky because if the client is compromised then the security implemented on the client (the cryptography) can be bypassed.
Firewall · Operation · Effectiveness: ModerateUse an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth [REF-1481].An application firewall might not cover all possible input vectors. In addition, attack techniques might be available to bypass the protection mechanism, such as using malformed inputs that can still be processed by the component that receives those inputs. Depending on functionality, an application firewall might inadvertently reject or modify legitimate requests. Finally, some manual effort may be required for customization.

Detection methods

Method	Approach	Effectiveness
Automated Static Analysis	Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)	High

Representative vulnerabilities

Below are representative vulnerabilities related to this CWE, prioritized by severity.

Sources (2)

MITRE CWE

CWE Terms of Use

CWE-502: Deserialization of Untrusted Data

What is CWE-502?

Vulnerabilities mapped to CWE-502

Vulnerabilities in CISA KEV for CWE-502

Official definition

Characteristics

Alternate terms

Modes of introduction

Common consequences

Risk mitigations

Detection methods

Representative vulnerabilities

Supporting documents

Run an in-depth assessment with complete web risk management

Explore more resources from CyStack

Website security reports

CyStack research

Cybersecurity knowledge