AIKIDO-2026-943815

TL;DR

The PPMd8 decoder in ppmd-rust mishandles model-restoration bookkeeping, turning an inconsistent model state directly into raw heap pointer arithmetic with no bounds or tag check. A crafted PPMd-compressed stream, such as a ZIP entry using compression method 98 whose restoration method does not match the one the encoder used, drives the decoder into the cut_off reduction path with corrupt accounting. This produces out-of-bounds heap reads and writes, including an oversized unit copy and a write through a freed pointer, corrupting memory during routine decompression of untrusted input. The fix corrects the frequency accumulation, flag handling, stats relocation copy size, successor write cursor, and restart-guard accounting so the decoder stays inside the model arena.