CVE-2021-28429 - Understanding the Integer Overflow Vulnerability in FFmpeg 4.3.2 and Protecting Your System From A Denial of Service Attack

In this extensive post, we will be discussing a recently discovered vulnerability, CVE-2021-28429, affecting FFmpeg version 4.3.2. We'll dive into the details of the vulnerability, including a code snippet demonstrating the issue, links to the original references, and an overview of how this exploit works. We'll also cover possible mitigation strategies to protect your system from potential denial of service (DoS) attacks.

If you're unfamiliar with FFmpeg, it's an open-source software suite that allows users to manipulate video and audio files by converting them, recording live streams, and more. The vulnerability lies in the Timecode feature within the libavutil library, specifically in the av_timecode_make_string() function located in libavutil/timecode.c.

The Vulnerability

CVE-2021-28429 involves an integer overflow vulnerability in the av_timecode_make_string function. This issue allows an attacker to cause a DoS attack by providing a specially crafted MOV file that, when processed, leads to an integer overflow and eventually a segmentation fault. The vulnerability has the potential to impact systems that use FFmpeg to process video files.

The affected function in the FFmpeg libavutil/timecode.c file looks like this

void av_timecode_make_string(const AVTimecode *tc, char *buf, int64_t framenum)
{
    int flag = tc->flags & AV_TIMECODE_FLAG_DROPFRAME;
    int fps = tc->fps.num + ((!flag) ? () : (-1));
    int64_t ff = (flag * framenum) / fps;
    int minutes = framenum / (60 * fps);
    int64_t frame = framenum - (60 * fps * minutes);
    int hours = minutes / 60;
    int sec = (frame + ff) / fps;

    snprintf(buf, 16, "%s%02d:%02d:%02d%c%02d",
             (tc->flags & AV_TIMECODE_FLAG_FIELD) ? "1" : "",
             hours, minutes % 60, sec, flag ? ';' : ':',
             (int) (frame + ff - sec * fps));
}

The overflow occurs due to the results of the "fps" and "ff" calculations in this function. The calculations use the input values provided in a crafted MOV file, which can cause the overflow. In turn, the subsequent snprintf() function call then leads to memory corruption and a segmentation fault, halting the affected service.

Exploit Details

Given the nature of this vulnerability, an attacker would need to create a malicious MOV file with carefully chosen metadata that triggers the integer overflow when the FFmpeg processes it. Once the file is processed, it would result in service disruption and a potential DoS condition for the service handling the video files or streaming service.

Original References

1. NVD Database: https://nvd.nist.gov/vuln/detail/CVE-2021-28429
2. FFmpeg Bug Tracker: https://trac.ffmpeg.org/ticket/9194
3. Exploit Database: https://www.exploit-db.com/exploits/50277

Mitigation Strategies

To protect your system from the impacts of CVE-2021-28429, consider implementing the following measures:

1. Update FFmpeg: If you're using FFmpeg version 4.3.2, it's recommended to update to the latest available version as soon as possible. The FFmpeg maintainers often release patches and updates addressing various security issues. Keep track of any new releases via their official website or GitHub repository.

2. Scrutinize External Inputs: Be cautious while processing video files from untrusted sources. Validate and sanitize any inputs coming from external sources, including video files, metadata, and links to external resources.

3. Monitor & Alert: Regularly monitor your system for any abnormal behavior and signs of potential attacks, such as unusually high resource utilization or suspicious traffic patterns. Implement alerting mechanisms to notify administrators of possible breaches or vulnerabilities.

Conclusion

It's crucial to understand the nature of vulnerabilities like CVE-2021-28429 and mitigate them promptly. By staying informed and taking necessary precautions, you can minimize the risk of DoS attacks, keeping your system and data secure.

Timeline

Published on: 08/11/2023 14:15:00 UTC
Last modified on: 08/18/2023 14:55:00 UTC