CVE-2026-48509: MessagePack for C# is a MessagePack serializer for C#

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CRITICAL 9.1REMOTEUPGRADE

CVE-2026-48509 is a critical-severity flaw (CVSS 9.1) 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, the parameterless MessagePackInputFormatter() constructor uses default serializer options, which resolve to MessagePackSerializerOptions.Standard with MessagePackSecurity.TrustedData. The formatter is designed for ASP.NET Core MVC request bodies, which commonly cross an HTTP trust boundary. This insecure default can expose applications to denial-of-service attacks

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 64%)
CISA exploited list	No
Weakness	CWE-1188
Published	2026-06-22
Last modified	2026-06-25

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

Last updated 2026-06-25 · 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
2026-06-25 16:16 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-2026-48509?

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

Is CVE-2026-48509 being exploited?

CVE-2026-48509 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-2026-48509 affect?

CVE-2026-48509 affects messagepack. Severity is critical (CVSS 9.1). MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, the parameterless MessagePackInputFormatter() constructor uses default serializer options, which resolve to MessagePackSerializerOptions.Standard with Messag

How urgent is CVE-2026-48509?

Priority 46/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

.NET vulnerabilities

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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