CWE-917: Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

What is Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')?

• Overview: CWE-917 refers to a vulnerability where special elements in expression language statements are not properly neutralized. This occurs when a program uses input from an external source to build expression language statements without sanitizing or escaping potentially harmful elements, leading to unintended execution of code.

• Exploitation Methods:

• Attackers can exploit this vulnerability by injecting malicious code into input fields that are used to construct expression language statements.
• Common attack patterns include injecting scripts or commands that are executed by the server-side logic in JSP or similar frameworks.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized code execution, data exposure, or manipulation within the application.
• Potential cascading effects include system compromise or lateral movement within a network if the application has additional vulnerabilities.
• Business impact may involve data breaches, loss of customer trust, and potential financial liabilities due to unauthorized access or data manipulation.

• Prevention Guidelines:

• Specific code-level fixes include validating and sanitizing all inputs before they are used in expression language statements.
• Security best practices involve disabling executable expressions where not needed and employing strict input validation and output encoding.
• Recommended tools and frameworks include using libraries or frameworks that provide built-in sanitation and validation functions, such as the Jakarta EE framework with EL context restrictions.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Java

Affected Technologies: Not specified

Vulnerable Code Example

// Import necessary packages
import javax.el.ELProcessor;
import javax.servlet.http.HttpServletRequest;

public class UserProfileController {
    
    public String getUserProfile(HttpServletRequest request) {
        String username = request.getParameter("username"); // User input is directly retrieved
        
        // Vulnerable code: directly using user input in ELProcessor
        ELProcessor elProcessor = new ELProcessor();
        String expression = "userProfiles['" + username + "']"; // User input is concatenated into the expression
        String userProfile = (String) elProcessor.eval(expression); // Evaluating potentially harmful expression
        
        return userProfile;
    }
}

In this vulnerable code example, the username parameter is directly included in an EL expression without any sanitization or validation. This can allow an attacker to inject malicious EL expressions, potentially leading to unauthorized access or manipulation of data.

How to fix Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')?

To fix this vulnerability, it's crucial to ensure that any user input is properly sanitized before being used in an expression language (EL) context. The key steps include:

1. Input Validation and Sanitization: Validate user input for expected patterns and sanitize it to remove any special characters that could be used for injection.
2. Use of Predefined Expressions: Prefer using predefined expressions or mappings where user input doesn't directly modify the expression structure.
3. Avoid Direct Use of User Input: Where possible, avoid directly concatenating user input into EL expressions.

Implementing these practices can help protect against expression language injection attacks by ensuring that user input does not alter the intended logic of EL expressions.

Fixed Code Example

// Import necessary packages
import javax.el.ELProcessor;
import javax.servlet.http.HttpServletRequest;
import org.apache.commons.text.StringEscapeUtils;
import java.util.regex.Pattern;

public class UserProfileController {
    
    public String getUserProfile(HttpServletRequest request) {
        String username = request.getParameter("username");
        
        // Validate and sanitize user input before using it in ELProcessor
        if (username == null || !Pattern.matches("^[a-zA-Z0-9_]+\$", username)) {
            throw new IllegalArgumentException("Invalid username format");
        }
        
        // Use sanitized input in the EL expression
        ELProcessor elProcessor = new ELProcessor();
        String expression = "userProfiles['" + username + "']"; // Safe usage as input is validated
        String userProfile = (String) elProcessor.eval(expression);
        
        return userProfile;
    }
}

In the fixed code, we first validate the username using a regular expression to ensure it only contains alphanumeric characters and underscores. This prevents any special characters from being used, which could lead to injection. By avoiding the use of StringEscapeUtils.escapeEcmaScript(), we align with best practices by ensuring user input is validated rather than just escaped, which is more appropriate for this context.