GHSA-4mrv-5p47-p938

Описание

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):

Expected output:

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)