CVE-2024-38827 - Locale Pitfalls in Java String.toLowerCase()/toUpperCase() Can Break Authorization

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With the growing complexity of globalized Java applications, it’s easy to overlook the subtle but dangerous ways locale can influence code—especially when dealing with user input, authentication, and authorization. CVE-2024-38827 exposes a class of vulnerabilities where using Java’s String.toLowerCase() and String.toUpperCase() without specifying the locale can cause authorization logic to quietly fail, opening security holes.

This post breaks down the issue, shows you code samples, and explains how attackers could possibly bypass security gates because of this.

What’s the Problem?

Let's say you build an authorization mechanism based on username normalization. To be case insensitive, you lowercase user input:

if (userInput.toLowerCase().equals("admin")) {
    // grant admin rights
}

Super simple, right? Except, this code can silently break in certain languages (i.e. locales).

The classic example is Turkish (tr) and Azerbaijani (az). In those alphabets, the standard uppercase "I" and the lowercase "i" don't directly correspond as in English. Java's case conversion can behave quite differently under those locales, leading to edge cases and authorization bugs.

Take the following Java code

public boolean isAdmin(String username) {
    return username.toLowerCase().equals("admin");
}

Imagine a user sets their system locale to Turkish and attempts to log in with the username "ADMİN" (notice the dotted capital ‘İ’ used in Turkish).

Let's see what happens

import java.util.Locale;

public class LocaleTest {
    public static void main(String[] args) {
        String input = "ADMİN";
        // US Locale
        System.out.println("US locale:   " + input.toLowerCase(Locale.US));
        // Turkish Locale
        System.out.println("TR locale:   " + input.toLowerCase(new Locale("tr", "TR")));
        // Default (system) Locale
        System.out.println("Default:     " + input.toLowerCase());
    }
}

Output

US locale:   admin
TR locale:   admın
Default:     ??? (depends on system locale)

Note: In Turkish, "İ" (U+013, capital dotted I) lowercases to "i", but regular "I" (no dot) lowercases to "ı" (dotless i). This means "ADMİN".toLowerCase(new Locale("tr", "TR")) becomes "admın"—which does not match "admin".

Result: Any authorization or authentication relying on naive lowercasing fails. A user might be denied, or (worse) someone could deliberately use this mismatch to bypass rules if the system expects "admin", but the locale-converted string differs just slightly and is treated differently elsewhere in the code.

if (username.toLowerCase().equals("admin")) {

// Admin access
}

"admın".equals("admin") is false.

But other code (like password reset logic) does standard US lowercasing, so emails or logs might handle "admin" and "admın" as identical. This can lead to confusion and privilege escalation, especially if user directory lookups are inconsistent.

Key Guidelines (How to Fix)

Never use String.toLowerCase() or String.toUpperCase() without a specified, explicit locale for security-critical code.

Best Practice: Always use Locale.ROOT for normalization in security code.

input.toLowerCase(Locale.ROOT);

Code Fix Example

public boolean isAdmin(String username) {
    return username.toLowerCase(Locale.ROOT).equals("admin");
}

Now, no matter what the system or user locale is, "ADMİN".toLowerCase(Locale.ROOT) will always return "admin" if the letters match (ignoring case), making your checks robust and secure.

Official References

- Original CVE Description
- Java String Case Conversion Documentation)
- OWASP On Localization Issues
- Blog: Be Careful with toLowerCase and toUpperCase

Conclusion

CVE-2024-38827 is a powerful reminder that tiny globalization details can have huge security implications. If you’re normalizing, comparing, or storing strings for authentication or authorization, always use Locale.ROOT—never leave it to the system default. Review your code today; attackers are hunting for these subtle bugs.

Timeline

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