CVE-2026-48109: MessagePack MessagePack's LZ4 Decompression May Fail Vulnerability

HIGH
Advisory Identifier: GHSA-hv8m-jj95-wg3x
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CVSS 8.2
CWE-20
Published
Advisory Summary

CVE-2026-48109 is a HIGH (CVSS 8.2) messagepack's lz4 decompression may fail vulnerability (CWE-20: Improper Input Validation) affecting MessagePack libraries in the NuGet ecosystem. The issue involves messagepack's lz4 decompression may fail with accessviolationexception after dereferencing memory from bad input. Remediation is available by upgrading affected packages to version 2.5.301, 3.1.7.

CVE-2026-48109 at a Glance

CVE Identifier CVE-2026-48109
GHSA Identifier GHSA-hv8m-jj95-wg3x
Severity HIGH
CVSS v3.1 Base Score 8.2
CWE Classification CWE-20: Improper Input Validation
Affected Product MessagePack
Package Ecosystem NuGet
Published Date
Last Updated

Security Impact

CVSS: 8.2

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-48109

To remediate this vulnerability, upgrade all affected MessagePack dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

MessagePack fixed: 2.5.301 
Upgrade MessagePack to 2.5.301
MessagePack fixed: 3.1.7 
Upgrade MessagePack to 3.1.7
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for MessagePack

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-48109, inspect your project dependencies for MessagePack:

inspect dependencies for MessagePack

Analyze the terminal output to locate any version of MessagePack in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.5.301), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-20: Improper Input Validation, where the product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. In MessagePack, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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: 1. Disable LZ4 compression for untrusted input paths (`Lz4Block`, `Lz4BlockArray`). 2. Only accept compressed payloads from strongly trusted producers. 3. Isolate deserialization in a separate process/container with restart supervision to limit availability impact. ### Resources - MESSAGEPACKCSHARP-010

CVE-2026-48109 Affected MessagePack Versions

2 component(s)
Fixed Versions:
2.5.3013.1.7
Product / Package Ecosystem Affected Versions Fixed Version
MessagePack
nuget/MessagePack
NuGet
< 2.5.301
2.5.301
MessagePack
nuget/MessagePack
NuGet
≥ 3.0.214-rc.1
< 3.1.7
3.1.7

References

GitHub Advisory / Wiki ADVISORY
https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-hv8m-jj95-wg3x
NVD — CVE-2026-48109 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-48109
MessagePack Source Repository REPOSITORY
https://github.com/MessagePack-CSharp/MessagePack-CSharp

Frequently Asked Questions

What is CVE-2026-48109?
CVE-2026-48109 is a security vulnerability in MessagePack involving MessagePack's LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input under CWE-20: Improper Input Validation.
Which MessagePack versions are affected?
Affected versions include releases prior to 2.5.301, 3.1.7 across affected packages: MessagePack.
What version fixes CVE-2026-48109?
Upgrading to 2.5.301, 3.1.7 (or the latest patched release for your respective branch) resolves the vulnerability.
Is MessagePack affected?
Yes, MessagePack is affected in releases prior to 2.5.301, 3.1.7. Upgrade your dependency to 2.5.301, 3.1.7 or later.
What is the CVSS score of CVE-2026-48109?
CVE-2026-48109 has a CVSS v3.1 base score of 8.2 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H.
What CWE is associated with CVE-2026-48109?
CVE-2026-48109 is classified under CWE-20: Improper Input Validation (The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.).
How can I check NuGet dependencies for CVE-2026-48109?
Run the dependency tree command: 'inspect dependencies for MessagePack'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hv8m-jj95-wg3x",
  "modified" : "2026-06-25T15:01:13Z",
  "published" : "2026-06-11T20:34:10Z",
  "aliases" : [ "CVE-2026-48109" ],
  "summary" : " MessagePack's LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input",
  "details" : "### Impact\n\nA vulnerability exists in the optional LZ4 decompression path used by MessagePack compression modes `Lz4Block` and `Lz4BlockArray`.\n\nThe 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.\n\nThis issue affects applications that deserialize untrusted data while LZ4 compression is enabled.\n\n### Patches\n\nThe v2 versions are patched as of 2.5.301.\nThe v3 versions are patched as of 3.1.7.\n\n### Workarounds\n\nInstead of upgrading, an application may take the following precautions:\n\n1. Disable LZ4 compression for untrusted input paths (`Lz4Block`, `Lz4BlockArray`).\n2. Only accept compressed payloads from strongly trusted producers.\n3. Isolate deserialization in a separate process/container with restart supervision to limit availability impact.\n\n### Resources\n\n- MESSAGEPACKCSHARP-010",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "MessagePack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.5.301"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "MessagePack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.0.214-rc.1"
      }, {
        "fixed" : "3.1.7"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-hv8m-jj95-wg3x"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-48109"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/MessagePack-CSharp/MessagePack-CSharp"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-11T20:34:10Z",
    "nvd_published_at" : "2026-06-22T22:16:46Z"
  }
}