21 Bytes Can Crash FFmpeg: Inside the Vibecoded Fuzzer That Found What Years of Audits Missed

Twenty-one bytes. That is the entire attack. A file smaller than a URL, with four zero bytes sitting at exactly the right offset, crashes any FFmpeg-based application that opens it and reads a packet. Not memory corruption, not some exotic heap trick. A division by zero, in code that has been shipping for years, in one of the most fuzzed codebases on the planet. The person who found it, Darío Clavijo, did not write the fuzzer by hand. He built it with AI assistance, the way a growing number of security researchers now work, and posted the result on Hacker News this week under a title that got my attention immediately: "We found a division by zero bug in FFmpeg with a vibecoded fuzzer." The thread climbed past 250 points with hundreds of comments, and the debate underneath it is the real story: AI has been writing application code for two years, but AI writing the tester changes the economics of finding bugs in ways most teams have not priced in yet. Full disclosure before I go further. I am not a C security researcher. I run my own AI agent infrastructure and I write Java for a living. What I did for this article is what I would want you to do: I cloned the fuzzer's public repo, read its findings documents, tried to reproduce the crash on my own Ubuntu box, and studied the harness code line by line. Everything below is sourced from the public FFmpeg issue, the repo, and my own experiment, with the one place my results diverged clearly marked. What the fuzzer actually found The bug lives in libavformat/vpk.c , the demuxer for Sony PS2 VPK audio files, a container format almost nobody has heard of. That obscurity is exactly the point. In issue #24290 on the FFmpeg tracker , the crash chain reads like this: The probe matches. FFmpeg's format detection sees the VPK magic bytes and assigns the VPK demuxer. The header parses. vpk_read_header reads a 24-byte header. The crafted input sets the channel count, nb_channels , to zero at bytes 14 through 17. The header code does