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
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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
MessagePack.NETMessagePackReal-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[,,], andT[,,,] - Finding IDs:
MESSAGEPACKCSHARP-040, duplicate/open variantMESSAGEPACKCSHARP-OPEN-003
Patches
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePackto the patched version for your release line. - 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 dimensionsMESSAGEPACKCSHARP-OPEN-003: duplicate/open finding for the multi-dimensional array issue- CWE-770: Allocation of Resources Without Limits or Throttling
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | MessagePack | all versions | 2.5.301dotnet add package MessagePack --version 2.5.301 |
| .NETNuGet | MessagePack | ≥ 3.0&&< 3.1.7 | 3.1.7dotnet add package MessagePack --version 3.1.7 |
Detection & mitigation playbook
Open-source dependencyDetect
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-2026-48515 is resolved across your whole dependency graph.
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-48515 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-48515. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-48515 in your dependencies?
O3 Security finds CVE-2026-48515 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.