# CVE-2026-48515: MessagePack — Fixed in 2.5.301

.NET NuGet

Not in CISA KEV

# CVE-2026-48515 — MessagePack

CVE-2026-48515 is a CWE-770 vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below.

MessagePack-CSharp: Multi-dimensional array formatters allocate from unchecked dimensions

Also known as[GHSA-cxmj-83gh-fp49](/vulnerability/GHSA-cxmj-83gh-fp49)

Published

Jun 22, 2026

Updated

Oct 28, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 8, 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-2026-48515.

## EPSS Exploitation Probability

[via FIRST.org ↗](https://www.first.org/epss/)

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs35th percentile — riskier than 35% of all scored CVEsHighest risk

0.00%0.30%0.60%0.90%0.2%0.4%Jul 26Oct 26Nov 26

Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/).

## Real-World Exposure

2 pkgs affected

.NET`MessagePack`.NET`MessagePack`

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

MessagePack-CSharp's multi-dimensional array formatters read dimension lengths directly from the payload and allocate `T[,]`, `T[,,]`, or `T[,,,]` before validating that the dimension product matches the encoded element count.

The formatter reads a guarded element array header, but allocation of the target multi-dimensional array happens before the dimensions are checked against that element count. A small payload can therefore declare large dimensions, provide an empty or tiny inner array, and cause a large heap allocation before element data is validated.

### Impact

Applications are affected when they deserialize untrusted MessagePack payloads into models containing multi-dimensional arrays such as `T[,]`, `T[,,]`, or `T[,,,]`.

An attacker can encode large dimension integers and a small guarded element array. The formatter allocates the target array from the dimensions before confirming that the product of dimensions is consistent with the element count.

The result can be out-of-memory exceptions, container termination on memory-constrained hosts, large object heap pressure, or severe CPU cost from zero-initializing oversized arrays. `MessagePackSecurity.UntrustedData` does not provide a general allocation cap for this path.

### Affected components

-   Package: `MessagePack`
-   APIs: `TwoDimensionalArrayFormatter<T>.Deserialize`, `ThreeDimensionalArrayFormatter<T>.Deserialize`, `FourDimensionalArrayFormatter<T>.Deserialize`
-   Data shapes: `T[,]`, `T[,,]`, and `T[,,,]`
-   Finding IDs: `MESSAGEPACKCSHARP-040`, duplicate/open variant `MESSAGEPACKCSHARP-OPEN-003`

### Patches

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

Upgrade guidance:

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

The fix should validate dimensions before allocation. Dimension values should be non-negative, their checked product should match the encoded element count, and the product should be bounded by the available payload and any configured security limits before `new T[...]` is executed.

### Workarounds

Patching is recommended.

Until a patched version is available, avoid deserializing untrusted payloads into schemas containing multi-dimensional arrays. Prefer schema shapes that can be validated before allocation, such as bounded lists, dictionaries with application-level count limits, or jagged arrays with application-level limits.

Message-size limits reduce the blast radius but do not fully address allocation amplification where a small payload can encode disproportionate array dimensions.

### Resources

-   `MESSAGEPACKCSHARP-040`: unchecked multi-dimensional array dimensions
-   `MESSAGEPACKCSHARP-OPEN-003`: duplicate/open finding for the multi-dimensional array issue
-   CWE-770: Allocation of Resources Without Limits or Throttling

## 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

`messagepack`messagepack

≥ 3.0.3 && < 3.1.7

range

## Detection & mitigation playbook

Open-source dependency

1.  ### 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.
    
2.  ### Fix
    
    Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48515 is resolved across your whole dependency graph.
    
3.  ### 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 MessagePack-CSharp's multi-dimensional array formatters read dimension lengths directly from the payload and allocate \`T\[,\]\`, \`T\[,,\]\`, or \`T\[,,,\]\` before validating that the dimension product matches the encoded element count. The formatter reads a guarded element array header, but allocation of the target multi-dimensional array happens before the dimensions are checked against that element count. A small payload can therefore declare large dimensions, provide an empty or tiny inner array, and cause a large heap allocation before element data is validated. ## Impact Application

O3 Security · Impact-Aware SCA

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