How Java’s Constant String Pool has a Security implication

The constant string pool in Java, often referred to as the “String Pool,” doesn’t directly provide security features in the sense of access control or data encryption. Instead, it offers a mechanism that indirectly contributes to certain security-related aspects of Java applications:

1. String Immutability for Security: The string pool enforces the immutability of strings in Java. Once a string is created and placed in the pool, it cannot be modified. Immutability is crucial in security because it ensures that sensitive information stored in strings, such as passwords or cryptographic keys, cannot be accidentally or maliciously altered by other parts of the program.
2. Reduced Risk of Data Tampering: Immutable strings in the pool reduce the risk of data tampering. Since the string’s value cannot be changed, it helps maintain data integrity, especially in scenarios where strings represent critical data elements.
3. Security-Critical String Comparison: In security-critical code, string comparison is often used to verify user credentials or validate digital signatures. The use of equals() for comparing strings, combined with the immutability property, ensures that string comparisons are reliable and secure, preventing timing attacks or unexpected behavior.
4. Preventing Inadvertent Data Leaks: Strings containing sensitive information are often logged or printed for debugging purposes. Immutability helps prevent accidental logging or printing of sensitive data because once a string is created, its value cannot be altered. Developers can control when and where such strings are logged or printed.
5. Hashing and Security Tokens: Security mechanisms often involve hashing or generating security tokens from strings. Immutability ensures that the original string’s value remains consistent during these operations, preventing errors that could compromise security.
6. Garbage Collection and Memory Safety: Immutable strings in the pool are automatically managed by the Java garbage collector. This helps prevent memory leaks and ensures that sensitive data does not linger in memory longer than necessary, reducing the risk of unauthorized access to sensitive information.

While the string pool itself is not a security feature in the traditional sense, its properties, such as immutability, contribute to secure coding practices by reducing the risk of common security vulnerabilities related to data integrity and data leakage. However, it’s important to note that security in Java applications involves many other aspects, such as secure network communication, access controls, encryption, and secure coding practices beyond just string immutability.

Below are few applications of String immutability and it’s contribution:

Class loading

A typical approach for loading a class in memory relies on calling the Class.forName(String className) method. Notice the String argument representing the class name. Thanks to string immutability, the class name cannot be changed during the loading process. However, if String is mutable, then imagine loading class A (for example, Class.forName(“A”)), and, during the loading process, its name will get changed to BadA. Now, the BadA objects can do bad things!

Hash code caching

Hash codes should be calculated every time they are involved in hashing specific activities (for example, searching an element in a collection). Since String is immutable, every string has an immutable hash code that can be cached and reused as it cannot be changed after string creation. This means that hash codes of strings can be used from the cache instead of recalculating them at each usage. For example, HashMap hashes its keys for different operations (for example, put(), get()), and if these keys are of the String type, then hash codes will be reused from the cache instead of recalculating them.

Thread safety

Imagine an application using thousands of mutable String objects and dealing with thread-safety code. Fortunately, in this case, our imagined scenario will not become a reality, thanks to immutability. Any immutable object is thread-safe by its nature. This means that strings can be shared and manipulated by multiple threads, with no risk of corruption and inconsistency.

Downside of String immutability:

String cannot be extended

An immutable class should be declared final to avoid extensibility. However, developers need to extend the String class in order to add more features, and this limitation can be considered a drawback of immutability.

Nevertheless, developers can write utility classes (for example, Apache Commons Lang, StringUtils, Spring Framework, StringUtils, Guava, and strings) to provide extra features and simply pass strings as arguments to the methods of these classes.

Sensitive data in memory for a long time

Sensitive data in strings (for example, passwords) may reside in memory (in SCP) for a long time. Being a cache, the SCP takes advantage of special treatment from the garbage collector. More precisely, the SCP is not visited by the garbage collector with the same frequency (cycles) as other memory zones. As a consequence of this special treatment, sensitive data is kept in the SCP for a long time, and can be prone to unwanted usages.

In order to avoid this potential drawback, it is advisable to store sensitive data (for example, passwords) in char[] instead of String.

OutOfMemoryError

The SCP is a small memory zone in comparison with others and can be filled pretty quickly. Storing too many string literals in the SCP will lead to OutOfMemoryError.