CVE-2024-49738 - Deep Dive into the Android Parcel.cpp writeInplace Out-of-Bounds Write Exploit

A new security vulnerability, CVE-2024-49738, was discovered in Android's core codebase. This bug is particularly concerning because it allows for a local privilege escalation. It resides in the writeInplace function in Parcel.cpp, which under certain circumstances lets attackers write beyond allocated memory—an *out-of-bounds write*. This post breaks down what the vulnerability is, how it can be exploited, and how you can protect affected systems.

What Is CVE-2024-49738?

The vulnerability was found in the Android operating system's low-level Parcel class. The function writeInplace is supposed to reserve a region of memory for the caller to write to and guarantee that writing will not go beyond the memory boundary. Unfortunately, due to insufficient checks, it's possible to make writeInplace allocate less memory than actually needed—leading to an out-of-bounds write.

> Severity: High
>
> Attack vector: Local
>
> Privileges required: None extra
>
> User interaction: Not needed

The vulnerable function looks roughly like this (simplified for illustration)

// Parcel.cpp
status_t Parcel::writeInplace(size_t len, void** ptr) {
    if (len > UINT32_MAX - mDataPos) {
        return BAD_VALUE;
    }

    status_t err = continueWrite(len);
    if (err != NO_ERROR)
        return err;

    if (ptr) {
        *ptr = mData + mDataPos;
    }

    mDataPos += len;
    return NO_ERROR;
}

mData is a pointer to the allocated memory buffer; mDataPos is the current write position.

Root Cause

The key issue is improper bounds checking. The function continueWrite(len) is supposed to make sure enough memory is available but lacks robust checks. If an attacker can control the value of len and the state of mDataPos, it's possible to move mDataPos past the end of mData.

Impact

A local attacker—say, through a malicious app—can trigger this vulnerability *without* needing extra permissions or user action. Successful exploitation can:

Gain a handle to a Parcel object.

2. Craft a call to writeInplace with malicious input, setting len so that writing post-offset goes outside the buffer.

Here's a pseudocode snippet to illustrate the exploit

// Assume 'parcel' is a valid pointer to a Parcel object (in a native extension/hooked Java code)
void* ptr;
size_t maliciousLen = xFFFFFFF; // Large enough to overflow
status_t res = parcel->writeInplace(maliciousLen, &ptr);

// Write controlled data to out-of-bounds memory
if (res == NO_ERROR && ptr != nullptr) {
    memset(ptr, x41, maliciousLen); // Fill with 'A's
}

If exploited, this could overwrite internal data structures—possibly function pointers or object metadata—within the same process, potentially leading to code execution.

Official References

- Android Security Bulletin (June 2024)
- Google Git repo - Parcel.cpp
- CVE Details: CVE-2024-49738

Mitigation and Fix

Google fixed this issue in the June 2024 Android Security Bulletin. The patch strengthens checks in writeInplace and related functions, ensuring mDataPos + len cannot exceed buffer size.

Conclusion

CVE-2024-49738 is a textbook example of why buffer overflow protections and careful memory handling are crucial. It underscores the fact that simple logic errors in low-level system code can open the door to severe privilege escalations. If you manage or develop for Android devices, check your patch status and review your code for similar unchecked memory operations.

Timeline

Published on: 01/21/2025 23:15:14 UTC
Last modified on: 03/19/2025 15:15:51 UTC