CVE-2026-48109: MessagePack for C# is a MessagePack serializer for C#

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HIGH 8.2REMOTEUPGRADE

CVE-2026-48109 is a high-severity flaw (CVSS 8.2) affecting messagepack. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, A vulnerability exists in the optional LZ4 decompression path used by MessagePack compression modes Lz4Block and Lz4BlockArray. The decoder implementation is based on a deprecated fast-decompression algorithm that does not take a source-length bound. A remote attacker can send a crafted MessagePack payload with manipulated LZ4 token/length fields to force out-of-bo

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	8.2 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 57%)
CISA exploited list	No
Weakness	CWE-20
Published	2026-06-22
Last modified	2026-06-23

Publisher (CNA): GitHub, Inc. · Vendor: messagepack

Last updated 2026-06-23 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-22 22:16 UTCPublished CVE record published
2026-06-23 17:25 UTCUpdated Record last modified

messagepack messagepack UPGRADE

Affected: < 2.5.301 · >= 3.0.3 and < 3.1.7

Fixed in: 2.5.301, 3.1.7

Safe to downgrade to: a release before 3.0.3

1. Check installed version

apt-cache policy messagepack || rpm -q messagepack || messagepack --version

2. UPGRADE to 2.5.301 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade messagepack

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade messagepack

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.0.3 (Debian / Ubuntu)

apt-cache madison messagepack sudo apt install messagepack=<VERSION_BEFORE_3.0.3> sudo apt-mark hold messagepack

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list messagepack sudo dnf downgrade messagepack-<VERSION_BEFORE_3.0.3> sudo dnf versionlock add messagepack

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-48109?

messagepack: upgrade to 2.5.301, 3.1.7. Exact commands for each OS are on this page.

Is CVE-2026-48109 being exploited?

CVE-2026-48109 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-48109 affect?

CVE-2026-48109 affects messagepack. Severity is high (CVSS 8.2). MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, A vulnerability exists in the optional LZ4 decompression path used by MessagePack compression modes Lz4Block and Lz4BlockArray. The decoder implementation is

How urgent is CVE-2026-48109?

Priority 44/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-57585 HIGH 7.5 UPGRADE MessagePack is the serializer implementation for Python msgpack.org
CVE-2026-54522 MEDIUM 5.4 UPGRADE MessagePack for Ruby is an implementation of the MessagePack binary serialization format
CVE-2026-48517 HIGH 7.5 UPGRADE MessagePack for C# is a MessagePack serializer for C#
CVE-2026-48516 HIGH 7.5 UPGRADE MessagePack for C# is a MessagePack serializer for C#
CVE-2026-48515 HIGH 7.5 UPGRADE MessagePack for C# is a MessagePack serializer for C#

References

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