CVE-2020-5234 is a medium-severity (CVSS 6.5) CWE-121 vulnerability in MessagePack. O3 Security confirms whether CVE-2020-5234 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Untrusted data can lead to DoS attack due to hash collisions and stack overflow in MessagePack
Real-World Exposure
MessagePack.NETMessagePack.NETMessagePack.ImmutableCollection.NETMessagePack.ImmutableCollection.NETMessagePack.ReactiveProperty.NETMessagePack.ReactiveProperty.NETMessagePack.UnityShims.NETMessagePack.UnityShims+2 moreReal-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
MessagePack for C# and Unity before version 1.9.11 and 2.1.90 has a vulnerability where untrusted data can lead to DoS attack due to hash collisions and stack overflow. Review the linked GitHub Security Advisory for more information and remediation steps.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | MessagePack | all versions | 1.9.11 |
| .NETNuGet | MessagePack | ≥ 2.0.0&&< 2.1.90 | 2.1.90 |
| .NETNuGet | MessagePack.ImmutableCollection | all versions | 1.9.11 |
| .NETNuGet | MessagePack.ImmutableCollection | ≥ 2.0.0&&< 2.1.90 | 2.1.90 |
| .NETNuGet | MessagePack.ReactiveProperty | all versions | 1.9.11 |
| .NETNuGet | MessagePack.ReactiveProperty | ≥ 2.0.0&&< 2.1.90 | 2.1.90 |
Detection & mitigation playbook
Open-source dependencyDetect
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.
Fix
Update MessagePack to 1.9.11 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-5234 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 pinpoints whether CVE-2020-5234 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 CVE-2020-5234. 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-2020-5234 in your dependencies?
O3 detects CVE-2020-5234 across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.