CVE-2026-48514 — MessagePack
CVE-2026-48514 is a CWE-770 vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below.
MessagePack-CSharp: Unity unsafe blit formatter allocates from unbounded byte length
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-2026-48514.
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
UnsafeBlitFormatterBase<T>.Deserialize reads an attacker-controlled byteLength from an extension payload and allocates an array based on that value before validating it against the extension header length or remaining payload bytes.
The outer extension header is bounded by available input, but that bound is not used to constrain the inner byteLength before allocation. A very small payload can therefore request a very large T[] allocation.
Impact
Applications are affected when they deserialize untrusted payloads using Unity blit resolvers such as UnityBlitResolver or UnityBlitWithPrimitiveArrayResolver.
This is especially relevant to Unity multiplayer clients or servers that use MessagePack-CSharp for networked values such as vectors, matrices, or primitive arrays. A hostile peer can send an extension payload with a large declared byte length and cause an out-of-memory exception or process termination on memory-constrained platforms.
The resolver is opt-in, but the vulnerable value is pure wire input and the allocation happens before the formatter verifies that the declared bytes are actually present in the extension body.
Affected components
- Package:
MessagePack.UnityClient - Resolvers:
UnityBlitResolver,UnityBlitWithPrimitiveArrayResolver - API:
UnsafeBlitFormatterBase<T>.Deserialize - Finding IDs:
MESSAGEPACKCSHARP-080, duplicate/open variantMESSAGEPACKCSHARP-OPEN-010
Patches
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePack.UnityClientto 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 byteLength before allocation. It should reject negative lengths, lengths greater than the extension body length after metadata, and lengths that are not a valid multiple of the element size.
Workarounds
Patching is recommended.
Until a patched version is available, do not use Unity blit resolvers on data received from untrusted peers. Use safer resolvers or explicitly validate and size-limit messages before deserialization.
Resources
MESSAGEPACKCSHARP-080: unsafe blit formatter allocation from unbounded byte lengthMESSAGEPACKCSHARP-OPEN-010: duplicate/open finding for the same root cause- 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-2026-48514 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-2026-48514 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-48514. 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-2026-48514 in your dependencies?
O3 Security finds CVE-2026-48514 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.