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Expression Language Injection

Definition of Expression Language Injection

Expression Language Injection (EL Injection) refers to a type of security vulnerability that occurs when untrusted data is dynamically inserted into an expression language interpreter. Expression languages are commonly used in web applications to dynamically access and manipulate data, often for the purpose of displaying content or executing server-side logic.

Origin of Expression Language Injection

EL Injection stems from the utilization of expression languages in web development. These languages, such as JavaServer Pages (JSP) Expression Language (EL), are designed to provide a convenient way to access data stored in objects, such as JavaBeans. However, if input from users is not properly sanitized or validated, malicious code can be injected into these expressions, leading to potential security breaches.

Practical Application of Expression Language Injection

One practical application of Expression Language Injection is in the context of web form submissions. Consider a web application that allows users to search for products by entering keywords into a search box. If the application fails to properly validate and sanitize user input, an attacker could inject malicious EL expressions into the search query. These expressions might manipulate the application's behavior, granting unauthorized access to sensitive data or executing arbitrary code on the server.

Benefits of Expression Language Injection

Despite its risks, EL Injection detection and prevention techniques offer several benefits for web developers and security professionals. By understanding and mitigating this vulnerability, developers can enhance the security posture of their applications and protect sensitive data from unauthorized access or manipulation. Additionally, addressing EL Injection vulnerabilities contributes to overall code quality and helps maintain the trust of users by ensuring the confidentiality and integrity of their information.


Common signs of EL Injection vulnerabilities include unexpected behavior in the application, such as displaying error messages or executing unintended commands. Additionally, if the application allows users to input data that is directly incorporated into expression language constructs without proper validation or encoding, it may be susceptible to EL Injection.

Developers can prevent EL Injection by implementing secure coding practices, such as input validation and proper sanitization of user input. Additionally, using parameterized queries instead of dynamically constructing expressions with user input can mitigate the risk of injection attacks. Regular security audits and testing, including code reviews and penetration testing, can also help identify and address potential vulnerabilities.

Expression Language Injection can occur in any web application that utilizes expression languages, regardless of the programming language or framework used. Common targets include applications built with JavaServer Pages (JSP), JavaServer Faces (JSF), and other technologies that incorporate expression language functionality. It is essential for developers to understand the mechanisms of expression languages and the potential security implications to effectively mitigate the risk of injection attacks.


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