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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 asGHSA-cxmj-83gh-fp49
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Oct 4, 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 ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs32th percentile — riskier than 32% of all scored CVEsHighest risk
0.00%0.30%0.60%0.90%0.4%0.4%0.4%Sep 26Oct 26Oct 26

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

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
EcosystemPackageVulnerable rangeFix
.NETNuGetMessagePackall versions2.5.301dotnet add package MessagePack --version 2.5.301
.NETNuGetMessagePack≥ 3.0&&< 3.1.73.1.7dotnet 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

  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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CVE-2026-48515: MessagePack — Fixed in 2.5.301