In recent years, Android has seen its fair share of vulnerabilities relating to image handling. One such vulnerability, CVE-2023-21036, affects the BitmapExport.java file in the Android source code. This post will walk you through how the flaw works, why it happens, show a simplified code snippet, and discuss what an attacker might do with it.

The Background

Android uses the BitmapExport.java class to handle exporting bitmap (image) data from applications. Correct image truncation is crucial—especially when dealing with memory-limited devices or user-supplied image files.

The bug described by Google as “a possible failure to truncate images due to a logic error in the code” (see Google’s own Android Security Bulletin) was reported and tracked as A-264261868.

The Vulnerability

In the affected versions, BitmapExport.java did not properly check the image’s size before exporting, due to a logic mistake in truncation. The faulty logic means an image buffer might not be fully cut off when needed, possibly leading to extra data being written, or memory reading errors. This could allow for information leakage, application crashes, or—if chained with other bugs—code execution.

Here’s a simplified mockup to show how the truncation check could fail

// Suppose this is inside BitmapExport.java

public byte[] exportImage(Bitmap bitmap, int maxSize) {
    byte[] raw = bitmapToBytes(bitmap);

    // Faulty truncation logic
    if (raw.length >= maxSize) {
        // Should be '>' not '>=' or better: always enforce a max limit.
        return Arrays.copyOf(raw, maxSize);
    }
    // Missing proper truncation or length validation here.
    return raw; // Can return data larger than maxSize!
}

Logic error: The length check doesn’t properly enforce the maximum size.

- No fallback for oversized images: The function might not truncate at all for certain sizes, and might pass the full (too-large) image buffer onwards.

The exported image buffer is then saved, sent over a network, or further processed.

A malicious user could provide an oversized or carefully crafted image. If not truncated correctly, this could result in:

- Information leakage: memory after the intended buffer could be included, exposing potentially sensitive data.

Denial of Service (DoS): Processing an overly large image could crash the app or device.

- Preparation for further attacks: Combined with other vulnerabilities, it may allow arbitrary code execution or privilege escalation.

Suppose an app calls exportImage with maxSize = 1024

// Malicious oversized bitmap (120 bytes)
Bitmap maliciousBitmap = getMaliciousBitmap(120); // Returns 120-byte image

byte[] exported = exportImage(maliciousBitmap, 1024);
// exported length may be >1024 due to faulty logic,
// or may unintentionally copy adjacent memory or fail unpredictably.

Attackers can use this to trigger a crash, retrieve leftover memory, or sow confusion.

Fixing the Vulnerability

Google’s fix is to correctly enforce image size limits within BitmapExport.java—performing a strict length check and always truncating excess data, regardless of edge cases:

public byte[] exportImage(Bitmap bitmap, int maxSize) {
    byte[] raw = bitmapToBytes(bitmap);
    if (raw.length > maxSize) {
        return Arrays.copyOf(raw, maxSize); // Always truncate
    }
    return raw;
}

References

- Android Security Bulletin
- CVE-2023-21036 entry at CVE.org
- Google Issue Tracker (A-264261868) (may require permissions)

Conclusion

CVE-2023-21036 is a great reminder that even small logic errors in critical code like image processing can lead to security issues. Always validate and truncate user input carefully—especially with files, images, and inter-app communication on Android.

Timeline

Published on: 03/24/2023 20:15:00 UTC
Last modified on: 03/29/2023 12:53:00 UTC