We are going to explore a newly discovered vulnerability in the Android operating system, specifically in the way it handles file paths in the ExternalStorageProvider.java component. This vulnerability, titled CVE-2024-43093, can potentially be exploited to bypass the file path filter, resulting in unauthorized access to sensitive directories. Successful exploitation could lead to local elevation of privilege without needing any additional execution privileges. However, user interaction is required for the exploit to work.

This blog post will provide an in-depth analysis of the vulnerability, including code snippets, links to original references, and details about the exploit. Get ready to dive into the world of Android security!

Vulnerability Description

The vulnerability exists in the "shouldHideDocument" function of the ExternalStorageProvider.java class. This function is responsible for determining if a given file path should be accessible by the user or not. When the function processes file paths, it performs Unicode normalization on the input. However, this normalization is done incorrectly, potentially allowing an attacker to bypass the filter and access sensitive directories.

Here is a code snippet illustrating the issue in the shouldHideDocument function

private boolean shouldHideDocument(String docId) {
    if (ENABLE_HIDE || ENFORCE_HIDE) {
        boolean shouldHide = false;
        try {
            // Get the file path
            String path = mDocs.getFileForDocId(docId).getCanonicalPath();

            // Normalize the path
            String normalizedPath = Normalizer.normalize(path, Normalizer.Form.NFC);

            // Check if the path matches any of the filters
            for (String filter : HIDE_FILTERS) {
                if (normalizedPath.contains(filter)) {
                    shouldHide = true;
                    break;
                }
            }
        } catch (IOException e) {
            Log.e(TAG, "Failed in shouldHideDocument", e);
        }
        return shouldHide;
    } else {
        return false;
    }
}

As seen in the code above, the input path is first obtained and then normalized using the normalize method. However, this normalization is not properly implemented, leaving room for potential bypass.

Exploitation Details

In order to exploit this vulnerability, an attacker would have to craft a malicious application that requests access to external storage. The user must then grant permission for the app to access that storage. Once the app has the required permissions, the attacker can use this vulnerability to access sensitive directories by crafting a Unicode string that bypasses the file path filter.

For instance, the attacker might create a Unicode string containing special characters, such as Unicode combining marks. When the file path filter processes these characters, it might not recognize them as part of a restricted path due to the incorrect normalization. Thus, the attacker could craft a file path that passes through the filter without being blocked.

Mitigations

Although successful exploitation requires user interaction, it is still crucial to rectify the issue to prevent potential attacks. Google has already acknowledged this vulnerability and is working on a patch to address it. In the meantime, users can protect themselves by being cautious about granting apps permission to access external storage.

Conclusion

CVE-2024-43093 demonstrates how a seemingly small misstep in implementing Unicode normalization can lead to a security vulnerability. It highlights the importance of thoroughly understanding and correctly implementing security-critical code in any application. By staying informed and vigilant, developers and users alike can protect themselves from these types of threats.

For further details and updates on this vulnerability, you can refer to the following sources

1. Android Security Bulletin
2. CVE Details
3. NVD - National Vulnerability Database

Keep an eye out for patches from Google and other Android device manufacturers to ensure your devices remain secure. Stay safe in the ever-evolving world of cybersecurity!

Timeline

Published on: 11/13/2024 18:15:21 UTC
Last modified on: 11/14/2024 21:42:34 UTC