Android Call Forwarding Exploit: What Indian SMBs Need to Know

What Happened

In early 2023, security researchers identified a critical vulnerability in Android's Core system (CVE-2023-21313) that allows attackers to silently forward incoming calls without the user's knowledge or permission. The flaw stems from a missing permission check in the Android call handling framework—essentially, the system forgot to verify whether an application had the right to redirect calls before allowing it to do so.

This isn't a theoretical threat. The vulnerability requires no additional execution privileges and zero user interaction to exploit. An attacker with basic app installation access can weaponize this to intercept calls, capture sensitive information during conversations, or redirect calls to attacker-controlled numbers. For businesses relying on phone authentication (OTP verification, two-factor authentication, banking calls), this is a direct pipeline to account compromise.

Originally reported by NIST NVD, this vulnerability affects millions of Android devices worldwide, with particular impact on older and mid-range devices common in India where security patches are deployed slowly.

Why This Matters for Indian Businesses

Let me be direct: this vulnerability hits Indian SMBs harder than most other markets. Here's why.

First, India's regulatory environment now demands accountability. The Digital Personal Data Protection (DPDP) Act, which came into force in 2024, makes businesses liable for breaches involving customer phone data and call records. If an attacker exploits CVE-2023-21313 to intercept calls containing personal data, your business faces:

1. Mandatory breach notification within 72 hours (per DPDP guidelines)
2. Potential penalties up to [pricing available at bachao.ai] crore for negligence
3. Reputational damage and customer trust erosion

Third, device diversity and patch lag in India means millions of devices remain vulnerable. Unlike enterprise environments where I once architected security for Fortune 500 companies—where patch management was centralized and mandatory—Indian SMBs often work with employees using personal devices with sporadic updates. A vulnerability published in 2023 can remain exploitable in your organization until 2025 or later.

Technical Breakdown

How the Exploit Works

Android's call forwarding mechanism is supposed to be gated by the MODIFY_PHONE_STATE permission. This permission requires explicit user grant during app installation. However, CVE-2023-21313 reveals a permission check bypass in the Core framework's call handling routine.

Here's the attack flow:

graph TD
    classDef default fill:#1e3a5f,stroke:#3B82F6,color:#e2e8f0
    classDef danger fill:#5f1e1e,stroke:#EF4444,color:#e2e8f0
    classDef success fill:#1e3d2f,stroke:#10B981,color:#e2e8f0
    A[Attacker develops malicious app] -->|Step 1| B[App requests basic permissions
no call-forwarding permission]
    B -->|Step 2| C[User installs app
sees no red flags]
    C -->|Step 3| D[App exploits missing permission
check in Android Core]
    D -->|Step 4| E[Attacker silently enables
call forwarding]
    E -->|Step 5| F[Incoming calls redirected
to attacker number]
    F -->|Step 6| G[OTPs, sensitive calls
intercepted]
    G -->|Step 7| H[Account compromise
fraud, data theft]

The Technical Root Cause

In Android's TelephonyManager and CallForwarding classes, the permission validation was incomplete. The vulnerable code path looked something like this (simplified):

// VULNERABLE CODE (CVE-2023-21313)
public void setCallForwarding(String number, int reason) {
    // Missing: checkPermission("android.permission.MODIFY_PHONE_STATE")
    
    // Directly forwards call without validation
    mPhone.setCallForwardingOption(reason, number);
}

The fix (applied in patched versions) adds explicit permission checks:

// PATCHED CODE
public void setCallForwarding(String number, int reason) {
    // NOW: Explicitly verify permission
    if (mContext.checkCallingPermission(
        "android.permission.MODIFY_PHONE_STATE") 
        != PackageManager.PERMISSION_GRANTED) {
        throw new SecurityException(
            "Caller does not have MODIFY_PHONE_STATE permission"
        );
    }
    
    mPhone.setCallForwardingOption(reason, number);
}

Attack Variants

Attackers can weaponize this in several ways:

1. Silent OTP interception: Forward calls to a number controlled by attacker, capture SMS-based OTPs
2. Social engineering amplification: Forward calls to attacker's number, impersonate the victim
3. Targeted espionage: Forward specific incoming calls (e.g., from banks, government) to monitor sensitive conversations
4. Credential harvesting: Redirect banking calls to fake IVR systems that collect account details

How to Protect Your Business

Immediate Actions

Quick Fix: Check Your Android Version

Run this on any Android device to verify patch status:

# Via ADB (Android Debug Bridge) on connected device
adb shell getprop ro.build.version.security_patch

# Output example: 2025-02-05
# If date is before March 2023, device is vulnerable

If you see a security patch date before March 2023, your device is vulnerable to CVE-2023-21313.

Disable Call Forwarding Entirely (Temporary Mitigation)

On Android, disable call forwarding to prevent exploitation:

# Via phone dialer, dial:
##002#

# This disables ALL call forwarding (US/India standard)
# Verify with:
*#002#

# To re-enable later:
*21*[FORWARDING_NUMBER]#

Shift to Passwordless Authentication

Instead of relying on voice calls and SMS OTPs, implement push-based authentication:

# Example: Google Authenticator setup via TOTP
# Generate time-based OTP (no SMS/call needed)

# For developers integrating TOTP:
# Use libraries like pyotp (Python) or speakeasy (Node.js)

# Python example:
pip install pyotp

import pyotp
totp = pyotp.TOTP('JBSWY3DPEBLW64TMMQ======')
print(totp.now())  # Generates 6-digit code

How Bachao.AI by Dhisattva AI Pvt Ltd Detects This

When I was architecting security for large enterprises, we built multi-layered detection systems. I built Bachao.AI to bring that same rigor to Indian SMBs at an accessible price.

Here's how our products protect you from CVE-2023-21313 and similar Android vulnerabilities:

Ready to protect your business? Visit Bachao.AI for a comprehensive security assessment of your applications and infrastructure.

Real-World Example: How This Plays Out

Imagine an Indian fintech startup with 50 employees. One employee installs a "productivity app" that exploits CVE-2023-21313. Within days:

1. Attacker silently enables call forwarding on that device
2. Customer calls meant for the employee are redirected to attacker
3. Attacker impersonates the employee, convinces customer to "verify" bank details
4. Customer's account is compromised
5. Customer files complaint with CERT-In
6. Fintech faces DPDP investigation and potential [pricing available at bachao.ai] crore penalty

1. VAPT Scan would have flagged the device as vulnerable before the app was installed
2. Dark Web Monitoring would alert if that app started circulating in exploit forums
3. Security Training would teach the employee to reject permission requests
4. Incident Response would detect the call forwarding within hours and contain it

What You Should Do Right Now

1. Audit your Android fleet: Which devices are running Android versions before March 2023?
2. Check for vulnerable apps: Use ADB to list apps with MODIFY_PHONE_STATE permission
3. Disable call forwarding: Dial ##002# on all business devices
4. Migrate authentication: Replace SMS/voice OTP with push-based 2FA
5. Deploy MDM: Implement Mobile Device Management to enforce security policies
6. Book a free VAPT scan: Get a baseline assessment of your Android device security posture

Key Takeaways

1. CVE-2023-21313 allows silent call forwarding without user knowledge or permission—a direct threat to OTP-based authentication
2. Indian businesses face DPDP liability if call interception leads to personal data breaches
3. Device patch lag means millions of Indian Android devices remain vulnerable years after the fix
4. Call-based authentication is outdated—shift to push-based 2FA immediately
5. Visibility is your first defense—audit your Android fleet, identify vulnerable devices, and remediate systematically

Book Your Free VAPT Scan → Assess your Android device security posture in 15 minutes. No credit card required.

Originally reported by NIST NVD

Written by Shouvik Mukherjee, Founder of Bachao.AI. Follow me on LinkedIn for daily cybersecurity insights for Indian businesses.

Frequently Asked Questions

Q: What is CVE-2023-21313 and how does it affect my Android device? A: CVE-2023-21313 is a critical Android vulnerability that allows attackers to silently forward calls without any user interaction. It exploits a missing permission check in Android's call handling framework, making it dangerous for businesses relying on phone-based OTP or two-factor authentication.

Q: Does an attacker need to target me specifically for this to work? A: No. The vulnerability requires no user interaction and no special app permissions. Any app installed on the device can potentially exploit this, enabling mass opportunistic attacks alongside targeted ones.

Q: How do I verify my Android device is patched? A: Go to Settings → About Phone → Security patch level. A date of March 2023 or later means your device is protected against CVE-2023-21313.

Q: What should Indian businesses do immediately? A: Audit all employee Android devices for patch level, enforce MDM policies requiring current security patches, and disable call forwarding permissions for untrusted apps. Book a VAPT scan at Bachao.AI to identify all vulnerable devices across your organisation.

Q: Does this vulnerability trigger DPDP Act obligations? A: Yes. If a breach occurs via CVE-2023-21313 and customer data or call records are exposed, businesses face DPDP Act notification requirements and potential penalties. Proactive patching is your strongest compliance defence.

Written by Shouvik Mukherjee, Founder of Bachao.AI. Follow me on LinkedIn for daily cybersecurity insights for Indian businesses.