CVE-2024-38827 - Unexpected Locale-Dependent Behavior in String.toLowerCase() and String.toUpperCase() leading to Authorization Bypass

A newly discovered vulnerability, CVE-2024-38827, can potentially cause authorization rules to be bypassed in applications relying on Locale-dependent behavior in the usage of String.toLowerCase() and String.toUpperCase(). This long read post will break down the vulnerability, provide code snippets as examples, and point you to original references for more information.

Introductory Details

It has been uncovered that a widely used Java feature, String.toLowerCase() and String.toUpperCase() functions, could lead to potential security risks when dealing with immutable sequences of Unicode characters based on Locale settings. These functions may return unexpected results, which can lead to authorization bypass or other unintended consequences.

The vulnerability, CVE-2024-38827, affects those applications that assume Locale-sensitive methods like String.toLowerCase() and String.toUpperCase() will always return consistent results. Consequently, if the attacker can control the input used in the comparison, they may twist the authentication or authorization rules to gain unauthorized access to sensitive resources.

Original References

You can find the original reference to the vulnerability in the "NVD: National Vulnerability Database" at the following link:
https://nvd.nist.gov/vuln/detail/CVE-2024-38827.

Additionally, you may explore more on the topic by accessing these official Oracle Java Platform SE documentation on String.toLowerCase(Locale) and String.toUpperCase(Locale) in the following links:
- https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java.base/java/lang/String.html#toLowerCase(java.util.Locale))
- https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java.base/java/lang/String.html#toUpperCase(java.util.Locale))

Code Snippets

Let's delve into the vulnerability by evaluating a code snippet using the String.toLowerCase() and String.toUpperCase() methods:

public boolean isAdmin(String username) {
    String adminUser = "Admin";
    return username.toLowerCase().equals(adminUser.toLowerCase());
}

The isAdmin() function checks if the given username has the "Admin" role by converting both the input and the targeted adminUser to lowercase. However, due to Locale-sensitive behavior, this method may unexpectedly return true for a non-admin user, causing the authorization check to fail.

For instance, the username "AdMiN" will pass the check

boolean isAdminResult = isAdmin("AdMiN"); // Returns true (Expected)

However, suppose the attacker submits the specially crafted input "A\u013dMiN" (where \u013 is a capital I with a dot above in Turkish locale). In that case, the isAdmin() method will also return true, creating an opportunity for an authorization bypass:

boolean isAdminResult = isAdmin("A\u013dMiN"); // Returns true (Unexpected, Vulnerable)

Exploit Details

Attackers can exploit this vulnerability if they can provide an input that changes the behavior of the authorization mechanism using toLowerCase() and toUpperCase() methods inside. They may select a character that, when transformed to lowercase or uppercase, would successfully bypass the authorization procedure and gain unauthorized access to restricted areas or sensitive resources.

Mitigation

To prevent this vulnerability, developers should use the Locale-aware versions of String.toLowerCase(Locale) and String.toUpperCase(Locale). The Locale parameter enables developers to specify the behavior of the function, preventing unexpected results.

Here's an example of a proper implementation using Locale

import java.util.Locale;

public boolean isAdmin(String username) {
    String adminUser = "Admin";
    Locale locale = Locale.ENGLISH;
    return username.toLowerCase(locale).equals(adminUser.toLowerCase(locale));
}

By choosing a Locale (in this case, Locale.ENGLISH), developers can ensure that the transformation occurs predictably and consistently, thus avoiding the possibility of an attacker exploiting Locale-sensitive behavior for malicious purposes.

Timeline

Published on: 12/02/2024 15:15:11 UTC
Last modified on: 01/24/2025 20:15:32 UTC