CVE-2026-48502 — MessagePack
CVE-2026-48502 is a Out-of-bounds Read vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below.
MessagePack-CSharp: Denial of service vulnerabilities can swamp the CPU or crash the process with stack and heap overflows
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-2026-48502.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
Real-World Exposure
MessagePackReal-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
MessagePackReader.ReadDateTime() can allocate stack memory based on an attacker-controlled MessagePack extension length. In the slow path for timestamp extension parsing, the computed tokenSize includes the extension body length from the wire and is used in a stackalloc operation before the extension length is validated as one of the valid timestamp sizes.
A very small payload can claim a large timestamp extension body and cause a stack allocation large enough to trigger an uncatchable StackOverflowException, terminating the host process.
Impact
Applications are affected when they deserialize untrusted payloads into types containing DateTime values. This path is available through the standard formatter set and does not require opting into typeless serialization, LZ4 compression, Unity-specific resolvers, or other specialized features.
MessagePackSecurity.UntrustedData and MaximumObjectGraphDepth do not mitigate this issue because the crash is caused by a single-frame stack allocation, not by object graph recursion.
An attacker can send a MessagePack timestamp extension header with an oversized body length and insufficient body bytes. The reader enters the slow path, attempts to stack-allocate a buffer sized from that declared length, and can terminate the process before a catchable serialization exception is thrown.
Affected components
- Package:
MessagePack - API:
MessagePackReader.ReadDateTime - Data types:
DateTimeand formatter paths that callReadDateTime - Finding IDs:
MESSAGEPACKCSHARP-020, related stack allocation findingMESSAGEPACKCSHARP-CROW-MEM-001
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 timestamp extension lengths before any stack allocation. Valid MessagePack timestamp payload lengths are limited to the supported timestamp encodings, so oversized extension lengths should fail with a catchable MessagePack serialization exception before the slow path allocates a buffer.
Workarounds
Patching is recommended.
Until a patched version is available, avoid deserializing untrusted MessagePack payloads into schemas that contain DateTime or DateTimeOffset values. Where possible, enforce strict maximum message sizes and reject malformed extension payloads before they reach MessagePack-CSharp.
There is no complete workaround for applications that must deserialize attacker-controlled MessagePack data containing date/time fields with affected versions.
Resources
MESSAGEPACKCSHARP-020:ReadDateTimestack allocation from attacker-controlled extension lengthMESSAGEPACKCSHARP-CROW-MEM-001: related attacker-controlled stack allocation finding inMessagePackReader- CWE-770: Allocation of Resources Without Limits or Throttling
CVE split rationale
This vulnerability is independently fixable in the DateTime extension parsing path by validating extension lengths before stack allocation. It is separate from recursive stack overflows, LZ4 issues, and collection allocation bugs.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | MessagePack | ≥ 3.0&&< 3.1.7 | 3.1.7dotnet add package MessagePack --version 3.1.7 |
Affected Products
messagepackmessagepackDetection & 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 3.1.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48502 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.
Frequently Asked Questions
Is CVE-2026-48502 in your dependencies?
Find it across NuGet, including transitive dependencies.