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HighVulnerabilityCVE-2026-114613updated 27 Oct 2026, 15:04 UTC

msgpack Python library leaks memory on malformed map keys

When the msgpack library rejects a bad input, it keeps some of the data in memory and does not free it. Sending many bad inputs can slowly use up all the memory and crash the app.

Fixed in msgpack 1.2.3Official, 2 sources
SeverityHighCVSS 7.5 of 10
Being exploited?UnclearNo reports either way yet
Fix available?Yesmsgpack 1.2.3 · since 27 Oct 2026
How sure is this?Official, 2 sourcesBacked by an official source

How the attack works simplified

  1. 1Attacker sends inputMalformed MessagePack with bad map keys
  2. 2App deserializes dataunpackb() or Unpacker parses the input
  3. 3Parser rejects keyDecoded value is never released
  4. 4Memory keeps growingRepeated bad inputs exhaust process memory
  5. ResultService becomes unavailable (denial of service)

What happened

The msgpack C extension (msgpack._cmsgpack) leaks a decoded map value when deserialization rejects a key under strict_map_key=True (the default). The streaming Unpacker also has a stale parser-stack reference issue that can cause a use-after-free.

Why it is dangerous: Repeated malformed MessagePack input can cause unbounded memory growth and denial of service. The Unpacker issue can also cause crashes or unexpected errors in applications using that API.

Are you affected?

You are affected if your application deserializes attacker-controlled MessagePack data with msgpack 1.2.2 or earlier using the C extension (unpackb() or the streaming Unpacker).

Package / productAffected versionsFixed in
msgpack · pip< 1.2.31.2.3

What to do

  1. Update msgpack to 1.2.3 or later (pip install --upgrade msgpack)
  2. If you cannot update yet, consider limiting or validating MessagePack input from untrusted sources
  3. Monitor memory usage of services that parse untrusted MessagePack data

Timeline

  1. 22 Oct 2026, 19:17 UTC
    Publicly disclosed
  2. 22 Oct 2026, 19:17 UTC
    Reported by NVD · NVD
  3. 27 Oct 2026, 15:04 UTC
    GitHub Advisory reported on it · GitHub Advisory
  4. 27 Oct 2026, 15:04 UTC
    Fixed version published: msgpack 1.2.3 · GitHub Advisory

Sources 2 reports, oldest first

Official vendor, government CERT or advisory database

Technical details

ID
CVE-2026-114613 · GHSA-j586-36cw-2gc2
CVSS
7.5 / 10
EPSS
0.5% chance of exploitation within 30 days
Disclosed
22 Oct 2026, 19:17 UTC
On Zero Radar
before the radar started
Last change
27 Oct 2026, 15:04 UTC

The plain-language parts and the attack diagram were written by an AI model (anthropic/claude-haiku-5.5) from the sources above. Facts like versions, exploitation and dates come from the sources themselves. How Zero Radar works

msgpack Python library leaks memory on malformed map keys · Zero Radar