Java Deserialization Vulnerabilities

Imagine an avatar in an online game. This avatar can be viewed as an object with specific attributes, such as clothing and weaponry. When the game is paused, how is this avatar saved or transmitted over the internet? The object is transformed into a stream of bytes, a process known as serialization.

Object -> Stream of Bytes

When this stream of bytes is converted back to an object, it is termed deserialization.

Stream of Bytes -> Object

If applications do not handle this process securely, they are at risk of deserialization vulnerabilities. For instance, if an attacker manipulates the byte stream, and the application deserializes and processes it without proper validation, it can lead to privilege escalation or remote code execution.

Java, being an Object-Oriented Programming Language, employs serialization through the Serializable interface. When conducting code audits, special attention should be given to classes implementing Serializable.

Identifying and Closing Common Java Web Application Security Vulnerabilities - YouTube

This video discusses the identification and mitigation of common security vulnerabilities in Java web applications.

Server-Side Template Injection (SSTI)

SSTI, or Server-Side Template Injection, is a vulnerability that arises when user input is parsed directly by the web server. The server may treat this input as native template syntax rather than as simple data values.

For instance, consider a Java-based web application that uses the FreeMaker template engine. If a user inputs "bob," the application correctly displays "bob." However, if an attacker inputs ${7*7}, the output becomes 49 because the template engine evaluates the expression rather than just displaying it.

This type of vulnerability can lead to information disclosure and remote code execution. To safeguard against SSTI, it is essential to sanitize all user inputs, especially when employing template engines like Velocity, FreeMaker, Thymeleaf, and others.

XML External Entity (XXE) Vulnerabilities

XML, or eXtensible Markup Language, is a flexible format for data exchange, albeit often overshadowed by JSON. XXE, or XML eXternal Entity, is a vulnerability that can occur with improper XML handling.

XML allows the definition of entities via Document Type Definitions (DTD). If an attacker intercepts an XML request and alters the DTD, it can lead to severe consequences, including local and remote file inclusion, server-side request forgery (SSRF), and denial-of-service attacks.

Consider a standard XML request:

<user>

<name>Bob</name>

<email>[email protected]</email>

</user>

If a malicious user modifies the request to include a new DTD with an external entity, they can manipulate the output significantly.

To mitigate XXE attacks, limit the functionalities of XML parsers and consider disabling DTD processing altogether.

Code Injection Risks

Code injection occurs when an application inadvertently executes malicious user input due to inadequate input sanitization. This vulnerability can allow attackers to gain complete control of the server and application.

To prevent code injection attacks, it is crucial to sanitize all user-provided inputs before they reach the server. Additionally, avoid using sensitive functions within the application.

Insufficient Session Expiration

User sessions are managed via session IDs, remaining active as long as the session is alive. Critical applications, such as banking platforms, typically require users to log out after a period of inactivity, often around 15 minutes.

Unlike social media platforms, maintaining prolonged sessions in sensitive applications can pose significant risks. To enforce session timeouts effectively, you can configure the server settings in the web.xml file.

<session-config>

<session-timeout>15</session-timeout>

</session-config>

Weak Cryptography Practices

The java.util.Random class is commonly used for generating random numbers in Java, but it is not secure for cryptographic purposes. The algorithm it employs can lead to predictable outputs. Instead, developers should utilize the java.security.SecureRandom class, which offers a stronger random number generator.

Denial of Service via readLine() Method

The readLine() method reads user input until it encounters a newline or return character. If an attacker provides input without these characters, the method may read indefinitely, resulting in denial-of-service attacks.

To counter this, consider using a SafeReadLine() method, which limits input reading to a specified character count or until a newline or return carriage is encountered.