Summary
MessagePack-CSharp: Multi-dimensional array formatters allocate from unchecked dimensions
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.
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
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
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.
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
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.
Frequently Asked Questions
- What is CVE-2026-48515? CVE-2026-48515 is a medium-severity allocation of resources without limits or throttling vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- Which versions of MessagePack are affected by CVE-2026-48515? MessagePack (nuget) versions < 2.5.301 is affected.
- Is there a fix for CVE-2026-48515? Yes. CVE-2026-48515 is fixed in 2.5.301, 3.1.7. Upgrade to this version or later.
- Is CVE-2026-48515 exploitable, and should I be worried? Whether CVE-2026-48515 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-48515 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-48515?
- Upgrade
MessagePackto 2.5.301 or later - Upgrade
MessagePackto 3.1.7 or later
- Upgrade