How severe is CVE-2026-48511?

No CVSS score has been assigned to CVE-2026-48511 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-48511?

CVE-2026-48511 affects the following packages: MessagePack (NuGet), MessagePack (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2026-48511?

Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48511 is resolved across your whole dependency graph.

How do I detect CVE-2026-48511 in my NuGet dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-48511 if there is no patch (or I can't update yet)?

Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Is CVE-2026-48511 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-48511 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-48511?

CVE-2026-48511 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 33th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-48511?

CVE-2026-48511 is classified as CWE-407 (CWE-407). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-48511 published, and has it been updated?

CVE-2026-48511 was published on June 22, 2026 and was last updated on August 27, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

.NET NuGet

Not in CISA KEV

CVE-2026-48511 — MessagePack

CVE-2026-48511 is a CWE-407 vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below.

MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps

Also known asGHSA-2x83-8g95-xh59

Published

Jun 22, 2026

Updated

Aug 27, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-48511.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs33th percentile — riskier than 33% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

2 pkgs affected

.NETMessagePack.NETMessagePack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

Summary

ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary<string, object>.Add for each map entry. ExpandoObject internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies.

For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because ExpandoObjectResolver.Options is configured with MessagePackSecurity.UntrustedData, but collision-resistant dictionary comparers cannot protect ExpandoObject insertion internals.

Impact

Applications are affected when they deserialize untrusted MessagePack maps into ExpandoObject using ExpandoObjectResolver or related resolver options.

A hostile payload containing many distinct keys can cause CPU exhaustion and allocation churn disproportionate to the input size. This can make a server unresponsive or exhaust memory under concurrent request load.

This is not a hash-collision attack against a configurable dictionary comparer. The super-linear behavior comes from ExpandoObject's insertion model, so MessagePackSecurity.UntrustedData does not eliminate the cost.

Affected components

Package: MessagePack
APIs: ExpandoObjectFormatter.Deserialize, ExpandoObjectResolver
Data type: System.Dynamic.ExpandoObject
Finding ID: MESSAGEPACKCSHARP-102

Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

Upgrade MessagePack to the patched version for your release line.
Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.

Potential fixes include applying a map-entry count limit for ExpandoObject under untrusted-data settings, buffering into a security-aware dictionary before materializing a bounded ExpandoObject, or otherwise rejecting maps large enough to trigger quadratic behavior.

Workarounds

Patching is recommended.

Until a patched version is available, avoid deserializing untrusted payloads into ExpandoObject. Prefer strongly typed DTOs or dictionaries with security-aware comparers and explicit count limits. Enforce request-size and map-entry limits at the transport or application layer.

Resources

MESSAGEPACKCSHARP-102: ExpandoObjectFormatter quadratic insertion behavior
CWE-407: Inefficient Algorithmic Complexity

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`MessagePack`	all versions	2.5.301`dotnet add package MessagePack --version 2.5.301`
.NETNuGet	`MessagePack`	≥ 3.0&&< 3.1.7	3.1.7`dotnet add package MessagePack --version 3.1.7`

Affected Products

1 product · 2 configurations

Application

messagepackmessagepack

≥ 3.0.3 && < 3.1.7

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48511 is resolved across your whole dependency graph.
Workarounds
Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Frequently Asked Questions

## Summary `ExpandoObjectFormatter.Deserialize` populates `System.Dynamic.ExpandoObject` by calling `IDictionary<string, object>.Add` for each map entry. `ExpandoObject` internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies. For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because `ExpandoObjectResolver.Options` is configured with `MessagePackSecurity.UntrustedData`, but collision-resistant dictionary comparers cannot protect `Exp

O3 Security · Impact-Aware SCA

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