GStreamer is a widely-used open source multimedia framework that powers video and audio streaming in many Linux desktop environments and media applications. In August 2023, security researchers disclosed a serious vulnerability in the way GStreamer parses H265-encoded video streams. Tracked as CVE-2023-40476 (previously ZDI-CAN-21768), this flaw allows remote attackers to execute arbitrary code just by getting users to open a specially-crafted video file or stream.

In this exclusive, easy-to-understand breakdown, we explain the technical details, demonstrate how the bug can be triggered with code, walk through the risk, and share resources for protection and further research.

Overview: What is CVE-2023-40476?

CVE-2023-40476 is a stack-based buffer overflow in GStreamer's H265 parser library. This issue occurs when the parser fails to properly check the length of certain fields in a video file before copying them into a fixed-size stack buffer. As a result, a malicious media file can overwrite program memory—allowing for remote code execution (RCE) with the privileges of the target application, such as a media player or web browser.

Scope:
* Affects applications using GStreamer's H265 video support (common on Linux desktops, but also possible on Windows and macOS).
* Exploitable via crafted video files or streams—email attachments, instant messaging, web links, or social engineering.

Reference:
* CVE-2023-40476 at NVD
* ZDI Advisory ZDI-23-1333

How the Vulnerability Works

The root cause lies in a parsing function where user-controlled data is copied into a stack buffer without proper length checks.

A simplified version of the vulnerable pattern (in C/C++) looks like this

void gst_h265_parse_nalu(const uint8_t* data, size_t len) {
    uint8_t buffer[256];
    // ...
    // User-supplied length value 'len' is not validated.
    memcpy(buffer, data, len);  // Oops! Possible overflow if len > 256.
    // ...
}

If an attacker provides more data in data than the stack buffer can hold, it can overwrite stack variables, function pointers, or the return address—leading to arbitrary code execution.

Proof of Concept: Triggering the Overflow

Here's a Python code snippet that creates a fake H265 video stream with an oversized payload to potentially crash or take control of affected applications using GStreamer.

# WARNING: DO NOT OPEN this file if you are vulnerable!
with open("evil.h265", "wb") as f:
    f.write(b"\x00\x00\x01")  # NALU start code
    f.write(b"\x26")          # NALU type (arbitrary)
    f.write(b"A" * 1024)      # Overlong payload triggers overflow (buffer is only 256 bytes!)

If a program using a vulnerable GStreamer version processes this file, it may trigger a segmentation fault or even execute injected code if the payload is carefully constructed.

Exploit Details

- Attack Vector: Trick a user into opening a malicious H265 video with a vulnerable GStreamer-based app.
- Code Execution: With a precisely crafted payload, the attacker can control the instruction pointer (EIP/RIP) and run shellcode or download malware.
- No User Interaction: If the app auto-parses remote media (like a chat thumbnail preview), the attack can be *zero-click*.
- Defense Evasion: Stack canaries, ASLR, and DEP help, but well-known techniques exist to bypass these on some environments.

Linux distributions (Ubuntu, Fedora, Arch, Debian, etc.) with GStreamer installed.

- Applications using GStreamer for media playback, especially if they process untrusted H265 files from the Internet.

Upgrade to GStreamer version 1.22.6 or later, which fixes this vulnerability.

- GStreamer official releases and changelogs

Distribute security updates to all users—especially for shared desktop environments.

Short-term workaround:
Disable H265 support if you cannot immediately update.

sudo apt remove gstreamer1.-libde265

*Exact package name may vary!*

Additional Resources

- GStreamer Project Website
- ZDI Advisory ZDI-23-1333 Original Disclosure
- CVE-2023-40476 at Mitre
- NVD Details
- YouTube: What is a Stack Buffer Overflow?

Summary

CVE-2023-40476 is a dangerous but now fixed GStreamer bug that can let attackers execute code by sending a poisoned H265 video file. The problem was improper length validation, leading to a stack-based buffer overflow. To stay safe: patch GStreamer ASAP, avoid suspicious videos, and watch your software update alerts.

For developers and sysadmins: Audit your applications if they use GStreamer for parsing H265 media, and make sure you are using the safely patched version.

Stay safe and keep your media libraries up to date!

Timeline

Published on: 05/03/2024 03:15:20 UTC
Last modified on: 05/03/2024 12:50:12 UTC