VLC VP9 Resolution-Change Crash PoC

This repository contains a small Python reproducer for a VLC 3.0.23 Windows VP9 decoder crash condition.

Research status: incomplete and continuing.

Summary

The PoC writes a 405-byte VP9 IVF file with two frames:

frame 1: 64x64
frame 2: 64x8192

The important detail is that the second frame changes the frame height while keeping the VP9 tile-column layout stable. In VLC 3.0.23's bundled FFmpeg VP9 decoder, that shape reaches a stale slice-thread progress allocation.

Why It Happens

The VP9 decoder tracks slice-thread progress in an entries array. That array is allocated using the number of superblock rows for the current frame.

For a 64x64 frame:

sb_rows = (64 + 63) >> 6 = 1
entries allocation = 1 * sizeof(atomic_int) = 4 bytes

For a later 64x8192 frame:

sb_rows = (8192 + 63) >> 6 = 128

The stale allocation remains sized for the first frame when the tile-column count does not change. During decode, the VP9 slice-thread reset loop writes zero to each row entry for the new frame:

for (i = 0; i < s->sb_rows; i++)
    atomic_store(&s->entries[i], 0);

That turns the second frame into a sequence of 4-byte zero writes past the original 4-byte allocation. On Windows VLC 3.0.23, the process behavior depends on heap layout and runtime state; observed outcomes include heap-corruption termination and access violation.

Files

poc.py: stdlib-only Python reproducer
generated output: vp9_reschange_64x64_to_64x8192_tc0.ivf

No external Python dependencies are required.

Usage

Generate the IVF sample:

python poc.py

Generate the sample at a custom path:

python poc.py -o sample.ivf

Optionally replay it with a local VLC binary:

python poc.py --vlc "C:\Path\To\VLC\vlc.exe"

The script prints JSON containing the generated sample path, SHA256 hash, size, and optional VLC process result.

Expected sample hash:

F26BDEFBDFD0B44359E314E0BFDE7AEA979D29F80F598749DCCA68AB34F54649

Tested Target

Tested against:

VLC media player 3.0.23 for Windows x64
decoder module: plugins/codec/libavcodec_plugin.dll
VP9 decoder source lineage: FFmpeg 4.4.x VP9 decoder

The relevant decoder behavior is the stale entries allocation on a resolution change that does not change tile-column count.

Research Notes

Local instrumentation observed the stale reset loop in libavcodec_plugin.dll at RVA 0x698a5c, executing the fixed zero-write pattern against the stale entries allocation.

For the 64x64 -> 64x8192 sample, direct-store tracing observed:

129 total entries stores
127 stores past the requested 4-byte allocation
114 stores past the allocator raw usable block

This repository is a compact crash reproducer. Research on the full exploitability of this primitive is incomplete and continuing.

Responsible Use

Run this only in a local test environment you control. The generated media file is intended for reproducing and studying the decoder fault path.