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How CVE-2026-54522 Works
MessagePack for Ruby is a popular gem that implements the MessagePack binary serialization format. The `MessagePack::Buffer` class provides a low‑level interface for managing serialization buffers. Prior to version 1.8.2, the `Bufferclear` method – defined in the C extension under `ext/msgpack/buffer.c` – contained a critical memory management flaw.
When `Bufferclear` is invoked, it shifts out every chunk currently held by the buffer and returns each 4 KiB rmem (read‑memory) page to a shared pool for reuse. However, the function fails to reset three internal cursor variables: rmem_last, rmem_end, and rmem_owner. These pointers continue to reference the now‑freed page, effectively leaving stale metadata inside the buffer object.
The next time `Bufferwrite` is called on the same cleared buffer, the method checks for “unused rmem space” by looking at the stale `rmem_last` and `rmem_end` values. Because these values still point into the freed page, the write operation incorrectly believes that part of that page is still available and hands back a slice of memory that has already been returned to the shared pool.
Meanwhile, a second independent `MessagePack::Buffer` object may later acquire the same physical page from the shared pool for its own use. At this point, both buffers alias the same memory region – the first buffer through its stale cursors, and the second buffer through a legitimate allocation. This creates a classic use‑after‑free scenario within the same process.
The consequences are twofold. If an attacker can control the data written into the first buffer after the clear, that data will actually be written into the aliased page, which is now owned by the second buffer. Consequently, reading from the second buffer will disclose the attacker‑supplied data (information disclosure). Conversely, if the second buffer contains sensitive information, a subsequent read from the first buffer (using the stale cursors) will expose that data – a cross‑buffer information leak. In both cases, the attacker can also corrupt the second buffer’s contents by writing arbitrary data through the first buffer’s stale view.
This vulnerability is particularly dangerous in multi‑tenant Ruby applications that use `MessagePack::Buffer` directly (e.g., for caching, session storage, or inter‑service communication) because it allows a local attacker to break isolation between buffers. The issue is fixed in version 1.8.2 by properly resetting the rmem cursors before returning the page to the pool.
DailyCVE Form:
Platform: ……. Ruby (msgpack gem)
Version: …….. < 1.8.2
Vulnerability :.. Use‑After‑Free (CWE‑416)
Severity: ……. LOW (CVSS 2.1)
date: ……….. 2026‑07‑30
Prediction: ….. Patch already released (1.8.2)
What Undercode Say (Analytics)
The following bash commands and code snippets can be used to detect vulnerable versions and reproduce the behaviour:
Check installed msgpack gem version gem list msgpack Audit the gem for known vulnerabilities (using bundler-audit) bundle audit check --update Search for CVE-2026-54522 in your Gemfile.lock grep -A 2 "msgpack" Gemfile.lock | grep -E "1.[0-7].[0-9]|1.8.[bash]"
Ruby snippet to trigger the stale cursor behaviour (proof‑of‑concept):
require 'msgpack'
buf1 = MessagePack::Buffer.new
buf1.write("sensitive")
buf1.clear rmem_last, rmem_end, rmem_owner become stale
Simulate a second buffer re‑acquiring the freed page
buf2 = MessagePack::Buffer.new
buf2.write("attacker_data")
Now buf1's stale cursors still point into the page now owned by buf2
A subsequent read from buf1 would disclose buf2's content
puts buf1.read may output "attacker_data"
Exploit
An attacker who can invoke `Bufferclear` on a MessagePack buffer and later trigger a `write` on the same buffer can cause the buffer to alias a memory page that has been returned to the shared pool. By orchestrating the timing so that a second buffer re‑acquires that page, the attacker can:
– Read sensitive data from the second buffer through the first buffer’s stale cursors.
– Write arbitrary data into the second buffer’s memory, corrupting its contents.
Because the vulnerability requires local process access and the ability to call the affected API, it is not remotely exploitable in typical deployments. However, in shared environments (e.g., Ruby on Rails servers handling multiple requests), an attacker could use this to break buffer isolation and leak or corrupt data from other requests.
Protection
- Upgrade to `msgpack` version 1.8.2 or later, which resets the rmem cursors before returning pages to the pool.
- If upgrading is not immediately possible, avoid using `MessagePack::Bufferclear` in production code, or wrap buffer operations in a mutex to prevent concurrent access.
- Use static analysis tools (e.g.,
brakeman) to detect direct usage of the vulnerable API. - Apply runtime monitoring to alert on unexpected memory access patterns or crashes that could indicate exploitation attempts.
Impact
- Confidentiality: An attacker may read arbitrary data from other buffers within the same process, potentially exposing session tokens, cryptographic keys, or user‑specific information.
- Integrity: By writing into aliased memory, the attacker can corrupt the state of other buffers, leading to data corruption or logical errors in the application.
- Availability: While not directly causing crashes, memory corruption can lead to unpredictable behaviour, segmentation faults, or denial of service in extreme cases.
- Scope: Limited to the local process; remote exploitation is unlikely unless the attacker can already execute code or influence buffer operations via an exposed API. The CVSS score is 2.1 (LOW) , reflecting the local attack vector and the need for specific conditions to be met.
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Sources:
Reported By: nvd.nist.gov
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