CWE-491: Public cloneable() Method Without Final ('Object Hijack')

Definition

What is CWE-491?

This vulnerability occurs when a class implements a public clone() method without declaring it final. This allows attackers to create copies of objects without invoking their constructors, potentially leaving the cloned object in an inconsistent or insecure state.

When a clone() method isn't marked final, subclasses can override it and bypass the original class's intended construction logic. This object hijacking technique lets an attacker create instances that skip critical initialization steps like security checks, resource allocation, or state validation, leading to unexpected behavior or security flaws. To prevent this, always declare the clone() method as final in security-sensitive classes, or consider alternative object creation patterns like copy constructors or factory methods. This ensures that object duplication follows a controlled path and maintains the object's integrity, preventing attackers from manipulating the cloning process to create malicious or unstable instances.

Real-world impact

Real-world CVEs caused by CWE-491

No public CVE references are linked to this CWE in MITRE's catalog yet.

How attackers exploit it

Step-by-step attacker path

1
Identify a code path that handles untrusted input without validation.
2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
3
Deliver the payload through a normal request and observe the application's reaction.
4
Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable Java

In this example, a public class "BankAccount" implements the cloneable() method which declares "Object clone(string accountnumber)":

Vulnerable Java

public class BankAccount implements Cloneable{
  		public Object clone(String accountnumber) throws
  		CloneNotSupportedException
  		{
  				Object returnMe = new BankAccount(account number);
  				...
  		}
  }

Secure code example

Secure pseudo

// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}

What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.

Prevention checklist

How to prevent CWE-491

Implementation Make the cloneable() method final.

Detection signals

How to detect CWE-491

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

CWE-491

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Frequently asked questions

What is CWE-491?

This vulnerability occurs when a class implements a public clone() method without declaring it final. This allows attackers to create copies of objects without invoking their constructors, potentially leaving the cloned object in an inconsistent or insecure state.

How serious is CWE-491?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-491?

MITRE lists the following affected platforms: Java.

How can I prevent CWE-491?

Make the cloneable() method final.

How does Plexicus detect and fix CWE-491?

Plexicus's SAST engine matches the data-flow signature for CWE-491 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-491?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/491.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-491

CWE-668 Parent

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Public cloneable() Method Without Final ('Object Hijack')

What is CWE-491?

Real-world CVEs caused by CWE-491

Step-by-step attacker path

Vulnerable Java

Secure pseudo

How to prevent CWE-491

How to detect CWE-491

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-491

Exposure of Resource to Wrong Sphere

Improper Isolation of Shared Resources on System-on-a-Chip (SoC)

Assumed-Immutable Data is Stored in Writable Memory

Binding to an Unrestricted IP Address

Improper Isolation of Shared Resources in Network On Chip (NoC)

Use of Externally-Controlled Format String

Exposure of Sensitive Information to an Unauthorized Actor

Passing Mutable Objects to an Untrusted Method

Returning a Mutable Object to an Untrusted Caller

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`