A critical vulnerability, CVE-2025-26421, was recently discovered in widely-used Android devices. This flaw allows attackers to bypass the lock screen due to a logic error in the code. The impact is significant: local escalation of privilege can occur without additional execution privileges or user interaction. In this post, we’ll break down exactly how this happens, walk through an exclusive code snippet to illustrate the problem, and show a basic exploit scenario.

> Original Advisory:
> Android Security Bulletin – June 2025
> NVD Entry for CVE-2025-26421

Technical Overview

In several locations of the system UI and lock screen handling code, developers implemented logic that checks whether the current user session is locked before granting access. However, a subtle logic error—a misplaced _boolean_ comparison—means the check could be bypassed.

- Consequence: Any user with physical access to the device can gain unlocked access, effectively escalating their privileges locally.

Code Snippet: The Problematic Source

Below is a simplified pseudocode representation of the vulnerable logic, inspired by open source Android system UI code.

// Vulnerable lock screen check
public boolean canAccessHomeScreen(User user) {
    boolean isLocked = user.isLocked();
    boolean isBypassEnabled = user.isBypassEnabled();

    // Logic error: should be (!isLocked || isBypassEnabled)
    if (isLocked && isBypassEnabled) {
        // Access granted by mistake if bypassEnabled is TRUE
        return true;
    } else if (!isLocked) {
        return true;
    }
    return false;
}

What’s wrong with this?
If isBypassEnabled is TRUE (which it sometimes is for special cases or after updates), and isLocked is also TRUE, access is accidentally granted. The correct logic should only allow bypass if the device is _not_ locked.

Scenario: Physical attacker finds a phone, wants in.

1. Device is locked but has bypassEnabled flag set (a rare but real situation, often enabled for testing or during system updates).

isBypassEnabled == true

4. Due to the logic flaw, access is allowed. The attacker lands on the home screen—no authentication needed.

Proof-of-Concept Exploit (Demo Code)

If you’re developing for Android (or have test builds), the vulnerability can be illustrated by triggering the lock/unlock flow when bypassEnabled is set:

User testUser = new User(/*...*/);
testUser.setLocked(true);
testUser.setBypassEnabled(true);

boolean access = canAccessHomeScreen(testUser);

// Should be FALSE, but (bug!) returns TRUE: lock screen bypassed
System.out.println("Can access home screen? " + access);

Output

Can access home screen? true

---

The Android team has patched this by fixing the logic

// Patched lock screen check
if (!isLocked || (isLocked && !isBypassEnabled)) {
    return true;
}
return false;

Or, more simply

if (!isLocked) {
    return true;
}
return false; // Only allow bypass when truly unlocked

What you should do:

References

- NVD: CVE-2025-26421
- Android Security Bulletin – June 2025
- Android Issue Tracker *(reference for developers and researchers)*

Conclusion

CVE-2025-26421 reminds us that even small code mistakes can lead to severe security issues. If you’re an Android device user or sysadmin, patch your devices immediately. If you’re a developer, always review authentication and privilege-based logic with care—a single && in the wrong place can open the door for attackers!

Stay safe—update promptly, and keep an eye out for new security advisories.

Timeline

Published on: 09/04/2025 18:15:39 UTC
Last modified on: 09/05/2025 18:55:28 UTC