Redis is one of the fastest key-value databases in use today, trusted for its performance and reliability. But even big, trusted projects can have serious security holes. In August 2023, researchers disclosed CVE-2023-41056, a high-severity vulnerability in Redis. This bug allows attackers to exploit how Redis resizes its memory buffers, potentially leading to remote code execution—a hacker’s dream scenario.

This post walks through what went wrong, why it's bad, and how someone could exploit it (for educational purposes only). You'll see simple code snippets, clear explanations, and where to find the original sources.

What is CVE-2023-41056?

CVE-2023-41056 affects Redis 7.–7..14 and 7.2–7.2.3. The root cause is an *integer overflow when Redis resizes internal memory buffers*. This seemingly small math error lets an attacker create a heap overflow—overwriting adjacent memory in ways that can take control of the program.

Heap overflows allow attackers to change program execution

- If the attacker can send crafted network packets, they can potentially run arbitrary code on the Redis server

Here’s a simplified look at the buggy code pattern

// Example pseudo-code (not actual Redis code)
void resize_buffer(char **buf, size_t old_size, size_t new_size) {
    if (new_size > old_size) {
        *buf = realloc(*buf, new_size);
        // ... Copy data, do stuff ...
    }
}

The problem is how new_size is calculated. If the attacker controls the size (maybe through a crafted network request), they can cause integer overflows like this:

// Suppose attacker sends a gigantic size parameter:
size_t new_size = old_size + attacker_supplied_value; // may overflow!

new_size = old_size + attacker_supplied_value = x10000000 + xF000000 = x100000000 (wraps to )

So, realloc(*buf, ) results in a tiny buffer—later code assumes it's huge, and overwrites the heap!

Suppose Redis is parsing a command like this

*3
$3
SET
$100000000
deadbeefdeadbeef...

Redis expects to receive a string value of x100000000 bytes, but due to the integer overflow, it allocates a small buffer. As Redis copies your request data in, it overruns the small buffer, smashing memory.

> It’s not trivial to turn this into code execution, but with memory analysis and known techniques (like heap spraying or using glibc hooks), a skilled attacker could get shell access.

References

- NIST NVD: CVE-2023-41056
- Redis Security Notes
- Redis Official Fix Commit: ae85d76
- HackerOne report (external)
- Heap Overflow and Exploitation Basics

Mitigations

Patched Versions:

Conclusion

CVE-2023-41056 is a textbook example of how small bugs can lead to devastating outcomes. The Redis team did a great job patching the issue quickly, but it’s up to server administrators to deploy those patches.

If you want to keep your Redis data—and your servers—safe: Upgrade now.


For updates and technical details, track the official Redis issue tracker or follow new CVE disclosures via NVD.

Timeline

Published on: 01/10/2024 16:15:46 UTC
Last modified on: 01/22/2024 18:58:13 UTC