Android smartphones are everywhere, powering billions of devices across the globe. But with widespread adoption comes a bigger target for hackers. In this long read, we’re examining CVE-2022-20127—a serious vulnerability in the Android operating system’s NFC (Near Field Communication) subsystem, specifically inside the ce_t4t_data_cback function of the ce_t4t.cc file. This bug enables a remote attacker to achieve code execution with alarming consequences. Let’s break down how this works and what you need to watch out for.

What is CVE-2022-20127?

CVE-2022-20127 is a vulnerability reported in AOSP (Android Open Source Project), which could let an attacker gain remote code execution on an Android device—right over NFC. No interaction is needed from the user. This affects Android versions 10, 11, 12, and 12L, as detailed in Android’s Security Bulletin.

Product affected:

Android (various versions, see above)

Component:

ce_t4t.cc (part of NFC stack)

Issue:

Out of bounds write via double free

Impact:

Remote code execution with no extra permissions required

Android Security Issue ID:  
- A-221862119

The Vulnerability: Double Free in ce_t4t_data_cback

A "double free" happens when a program tries to release (free) a region of memory that has already been released. If not checked properly, this can allow an attacker to re-allocate that memory region, inject malicious data or code, and force the vulnerable program to execute it.

Code Snippet (Simplified, Illustrative)

void ce_t4t_data_cback(tNFA_STATUS status, tNFC_DATA_CEVT* p_data) {
    BUFFER* buf = allocate_buffer();

    if (error_condition) {
        free_buffer(buf); // First free
        // ... some other logic ...
        free_buffer(buf); // Second free: DOUBLE FREE VULNERABILITY!
    }
}

In the real source, memory management is more complex, but this shows the gist: if an error occurs, the same pointer is freed twice. On many systems, this sort of bug can be exploited for code execution.

How can an attacker exploit this?

1. Remote vector: NFC can be triggered passively without user action (for example, with a malicious NFC tag).

Heap Tampering: Attacker can potentially overwrite function pointers or critical data.

5. Execute Arbitrary Code: Ultimately leading to remote code execution, possibly with the privileges of the NFC daemon.

Proof-of-Concept Exploit Idea

As this issue is deeply embedded within the NFC stack and relies on heap handling, most public exploits are proof-of-concept at best. Simplified pseudo-exploit:

# Pseudo-code: Craft special NFC tag data
malicious_tag_data = b'C' * 4096  # Oversized payload to trigger the bug

# Approach target Android device with crafted NFC tag
send_nfc_payload(malicious_tag_data)
# Device processes the tag, hitting the double free in ce_t4t_data_cback
# Heap is corrupted, potentially hijacking execution flow

Note: In the real world, heap manipulation and bypassing modern security mitigations like ASLR and heap hardening is much more complex. But the root flaw is the double free.

Real-World Impact

- Remote code execution: Attacker can run malicious code on your phone with almost unlimited power, just via NFC.
- No User Interaction: You don’t have to tap anything; just having NFC enabled and being in range is enough.

Android Security Advisory:

- Android Security Bulletin—May 2022
 - AOSP Commit Fix

Never Double Free Pointers: Ensure all code paths release resources only once.

- Use Safe Memory Management: Modern programming habits and tools can help catch these bugs before shipping.

Conclusion

CVE-2022-20127 is a classic example of how low-level memory handling flaws can still haunt even the most popular and seemingly secure platforms. Double free bugs are rare today, but when they happen in code like Android’s NFC stack, the impact can be global.

Stay safe, keep your devices updated, and learn from these bugs to make systems more secure!

Learn More:  
- Android Open Source Project: system/nfc
- NVD - CVE-2022-20127


*Written exclusively for you, in plain American English, by an expert on software security.*

Timeline

Published on: 06/15/2022 13:15:00 UTC
Last modified on: 06/23/2022 17:55:00 UTC