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+# 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:
+
+```text
+sb_rows = (64 + 63) >> 6 = 1
+entries allocation = 1 * sizeof(atomic_int) = 4 bytes
+```
+
+For a later `64x8192` frame:
+
+```text
+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:
+
+```c
+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:
+
+```bash
+python poc.py
+```
+
+Generate the sample at a custom path:
+
+```bash
+python poc.py -o sample.ivf
+```
+
+Optionally replay it with a local VLC binary:
+
+```bash
+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:
+
+```text
+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.