Android’s security model is designed to make apps run in isolated sandboxes, keeping them from interfering with the system or one another. But sometimes, a low-level bug can punch a hole through this wall. CVE-2023-20938 is one such flaw—a use-after-free bug in the kernel’s binder driver. In this post, we’ll break down what went wrong, how it could be exploited, and show you the risky code that led to this serious Android vulnerability.

The Basics: What’s Binder and Why is it Vulnerable?

Binder is the key component in Android that lets apps and system services talk to each other efficiently and safely. It’s implemented in the Linux kernel and exposed to user apps via device files like /dev/binder, /dev/vndbinder, etc.

CVE-2023-20938 is a use-after-free vulnerability in the binder_transaction_buffer_release() function inside the kernel’s binder.c file. In simple terms, this means the system may access memory after it’s been returned (freed), letting attackers manipulate or hijack sensitive data structures. Crucially, it can be triggered locally by any app on the device, *without* extra privileges or user interaction.

Technical Breakdown: How Does the Vulnerability Happen?

When an app makes a binder transaction, it passes data through the kernel. The kernel allocates memory buffers for this data and eventually releases them. Here’s where things went wrong.

Code Snippet (Vulnerable Version, Simplified)

void binder_transaction_buffer_release(struct binder_proc *proc, struct binder_buffer *buffer)
{
    struct binder_buffer_object *buf_obj = ...; // details omitted

    if (/* some validation fails */) {
        kfree(buf_obj); // Frees the buffer
    }
    // ... some other code
    if (/* reuse buf_obj again */) {
        /* Use-after-free: buf_obj may point to freed memory! */
        process_buffer(buf_obj);
    }
}

The problem: input validation was incomplete. That means, if an attacker crafts their transaction a certain way, they can cause the buffer to get freed early, then trick the kernel into still using it. This creates a window where malicious code can hijack the freed-and-reused memory.

Exploit Details: How Could an Attacker Use This?

Exploiting this bug requires no special privileges and no user interaction. The steps might look like this (high-level):

1. Open /dev/binder from a regular app process.

Trigger the input validation failure so the buffer is freed but still referenced.

4. Race the kernel: Allocate new data in place of the freed buffer, hoping the kernel will act on attacker-controlled contents.
5. Hijack control: If successful, the attacker can execute code in kernel mode or gain elevated privileges.

Note: Crafting a working exploit requires deep knowledge of the kernel memory allocator and timing, but proof-of-concept codes have emerged for similar bugs.

Proof of Concept: Example Binder Exploit Sketch

*This is a safe, simplified sketch. Real exploits are too dangerous to share and should never be run on production devices!*

// Example: Open binder, send malicious transaction
int fd = open("/dev/binder", O_RDWR);
struct binder_write_read bwr;
struct binder_transaction_data tx = {};

// Craft tx to cause use-after-free (details depend on kernel version)
tx.target.handle = ...; // Target some binder object
tx.data.ptr.buffer = ...; // Fake buffer

bwr.write_size = sizeof(tx);
bwr.write_consumed = ;
bwr.write_buffer = (uintptr_t)&tx;

ioctl(fd, BINDER_WRITE_READ, &bwr);

// Monitor system for privilege escalation or crash

> For real-world exploitation, attackers would need to use advanced heap spraying and race techniques to guarantee control over the freed buffer.

Did Google Patch It? Where?

Yes. Google assigned it Android ID: A-257685302 and released patches in February 2023. All devices with the February 2023 security update or later are protected.

You can track upstream patches here

- Upstream kernel patch
- Google’s Security Bulletin: 2023-02-01

Device makers are responsible for delivering updates. If you haven’t gotten the Feb 2023 update or later, your device could be vulnerable.

Update your device: Check for Android system updates and apply them as soon as available.

- Avoid installing apps from unknown sources: Although this bug could be exploited by any non-privileged app, sticking to trusted stores (like Google Play) reduces risk.
- Monitor Google’s security bulletins: Android Security Bulletins

Conclusion

CVE-2023-20938 is a prime example of how a tiny validation mistake in the kernel can lead to massive security risks. Use-after-free bugs are especially dangerous in low-level drivers like binder, since they can break Android’s isolation between apps and the operating system itself. Thankfully, Google and kernel maintainers acted fast—but as always, updating your phone is the best defense.

References

- Android Security Bulletin—2023-02-01
- Upstream kernel commit
- CVE-2023-20938 NVD entry

Timeline

Published on: 02/28/2023 17:15:00 UTC
Last modified on: 03/06/2023 19:32:00 UTC