Your RSA-2048 keys break in 2030. Find every one of them before attackers do.
.NET NuGet

GHSA-cxmj-83gh-fp49

GHSA-cxmj-83gh-fp49 is a CWE-770 vulnerability in MessagePack. O3 Security confirms whether GHSA-cxmj-83gh-fp49 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

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

Also known asCVE-2026-48515
Published
Jun 25, 2026
Updated
Jun 25, 2026
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed

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 versions2.5.301
.NETNuGetMessagePack≥ 3.0&&< 3.1.73.1.7

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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

  2. Fix

    Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-cxmj-83gh-fp49 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 pinpoints whether GHSA-cxmj-83gh-fp49 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-cxmj-83gh-fp49. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

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

Is GHSA-cxmj-83gh-fp49 in your dependencies?

O3 detects GHSA-cxmj-83gh-fp49 across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works