MessagePack-CSharp CVE-2026-48109
HIGH
GHSA-hv8m-jj95-wg3x
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
Network-reachable and unauthenticated, but AC:H because the target must have opted into LZ4 compression on an untrusted-input path; low confidentiality from possible over-read, no integrity impact, high availability from process crash.
Primary rating from Vendor (https://github.com/MessagePack-CSharp/MessagePack-CSharp).
CVSS VectorVendor: https://github.com/MessagePack-CSharp/MessagePack-CSharp
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
Lifecycle Timeline
2DescriptionCVE.org
Impact
A vulnerability exists in the optional LZ4 decompression path used by MessagePack compression modes Lz4Block and Lz4BlockArray.
The decoder implementation is based on a deprecated fast-decompression algorithm that does not take a source-length bound. A remote attacker can send a crafted MessagePack payload with manipulated LZ4 token/length fields to force out-of-bounds reads from the compressed input buffer. In affected environments, this can trigger an AccessViolationException during decompression, causing process termination (denial of service). Under some conditions, limited unintended memory disclosure from over-read data may also be possible before failure.
This issue affects applications that deserialize untrusted data while LZ4 compression is enabled.
Patches
The v2 versions are patched as of 2.5.301. The v3 versions are patched as of 3.1.7.
Workarounds
Instead of upgrading, an application may take the following precautions:
- Disable LZ4 compression for untrusted input paths (
Lz4Block,Lz4BlockArray). - Only accept compressed payloads from strongly trusted producers.
- Isolate deserialization in a separate process/container with restart supervision to limit availability impact.
Resources
- MESSAGEPACKCSHARP-010
AnalysisAI
Denial-of-service in MessagePack-CSharp's optional LZ4 decompression path (Lz4Block and Lz4BlockArray modes) allows remote unauthenticated attackers to crash .NET applications that deserialize untrusted MessagePack payloads. A crafted payload with manipulated LZ4 token/length fields triggers an out-of-bounds read raising an AccessViolationException, and may also leak limited adjacent memory before the process dies. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires that the target .NET application deserialize attacker-supplied MessagePack data using the Lz4Block or Lz4BlockArray compression modes - applications using MessagePack without LZ4 compression enabled are not affected. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The provided CVSS:3.1 vector AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H (base 8.2) reflects a network-reachable, unauthenticated DoS with low confidentiality impact from possible over-read disclosure - realistic for any service that deserializes MessagePack from clients with LZ4 enabled. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker submits a single crafted MessagePack payload - for example to a public WebSocket, gRPC-over-MessagePack, or HTTP API endpoint that uses MessagePack with LZ4 compression - in which the LZ4 token's literal-length or match-length field encodes a value larger than the remaining compressed bytes. During decompression the decoder dereferences past the source buffer, raising an AccessViolationException that tears down the worker process and denies service to all in-flight users. … |
| Remediation | Vendor-released patch: upgrade MessagePack to 2.5.301 on the v2 branch or 3.1.7 on the v3 branch, per advisory GHSA-hv8m-jj95-wg3x (https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-hv8m-jj95-wg3x). … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours: Inventory .NET applications using MessagePack-CSharp with LZ4 decompression (Lz4Block or Lz4BlockArray modes) that process untrusted input. …
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External POC / Exploit Code
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