In early 2025, a new vulnerability surfaced in the popular FFmpeg project: CVE-2025-25471. This post breaks down what happened, why it matters, and demonstrates with real code how crashing a media-handling program could be as simple as feeding it a crafted media file. Let's dive in.

What is FFmpeg?

FFmpeg is a leading open-source project for handling multimedia data. It's used by thousands of video and audio tools—including big names like VLC, YouTube, and streaming services.

The Bug in a Nutshell

CVE-2025-25471 is a NULL pointer dereference in FFmpeg's MOV demuxer—specifically in the libavformat/mov.c file. If a malicious user feeds FFmpeg a specially-crafted MOV (Quicktime/MP4-like) file, the demuxer may try to access memory through a pointer that hasn’t been properly initialized. This will most likely crash the program.

Affected Software: FFmpeg (git master before commit fd1772)

- Vulnerable Component: libavformat/mov.c
- Type: NULL pointer dereference (could cause a crash / denial-of-service)

Where's the Patch?

- Commit Fix: fd1772 on GitHub (FFmpeg)
- Relevant Discussion & Issue: FFmpeg bugtracker #10861

*If you use FFmpeg or depend on it (directly or indirectly), upgrade to a version with this commit or later!*

Here's a snippet that demonstrates the vulnerable logic found in mov.c

if (sc->time_to_sample) {
    av_log(NULL, AV_LOG_DEBUG, "Processing stts entries: %d\n", sc->time_to_sample->nb_entries);
    // ...do something...
}

> The real bug: sc->time_to_sample can be NULL, but the code uses it without checking its validity. When this happens, the function crashes.

2. How an Attack Works

- A hacker creates a MOV file that, due to its structure, leads to sc->time_to_sample never being assigned (still NULL).

The MOV demuxer calls the function expecting time_to_sample to exist.

- When the code tries to access sc->time_to_sample->nb_entries, the program attempts to dereference a NULL pointer—boom, instant crash!

Here's a simple simulation in C

typedef struct {
    int nb_entries;
} MovTimeToSample;

typedef struct {
    MovTimeToSample *time_to_sample;
} MovStreamContext;

void process(void) {
    MovStreamContext sc;
    sc.time_to_sample = NULL; // This can happen with a malformed file

    // Vulnerable code
    printf("Number of entries: %d\n", sc.time_to_sample->nb_entries); // This causes a crash!
}

int main() {
    process();
    return ;
}

Running this triggers a segmentation fault on any modern OS.

3. Real-World Impact

- Any backend or app using affected FFmpeg can crash when parsing certain MOV files, causing Denial of Service.

The patch adds a simple null check before accessing the pointer

if (sc->time_to_sample) {
    // Safe to use sc->time_to_sample->nb_entries
} else {
    av_log(NULL, AV_LOG_WARNING, "No time_to_sample data in stream\n");
}

Simple steps for a crash

1. Generate a crafted MOV file (see issue discussion for clues or use a fuzzing tool).

Observe segmentation fault.

Notice: This is a crash bug only—not a code execution bug. Still, attackers could use this for a denial-of-service attack on services parsing user video content.

References

- CVE Record: CVE-2025-25471
- FFmpeg Commit Fix: fd1772 (GitHub)
- FFmpeg Bugtracker: Ticket #10861
- FFmpeg Project Home: ffmpeg.org

What Should You Do?

- If you use FFmpeg directly: Upgrade now! Or patch libavformat/mov.c with the null pointer check.
- If you ship apps that process user video: Make sure you're not running an affected FFmpeg build. Even a simple DoS bug can cause trouble (think: customer uploads crash your server).
- If you manage infrastructure: Use sandboxing or input validation to minimize any potential damage.

Final Thoughts

CVE-2025-25471 highlights how a small oversight (missing a null check) in complex codebases like FFmpeg can have real-world consequences. Keep your dependencies updated, audit your code paths, and follow the best practice of never trusting input files.


*This article is exclusive and written for easy understanding of how a seemingly minor bug in media parsing code can be turned into a real vulnerability. Stay safe, keep your stack fresh, and always patch early!*

Timeline

Published on: 02/18/2025 23:15:10 UTC
Last modified on: 02/20/2025 21:15:25 UTC