Vulnerability-Lookup

CVE-2026-48511 (GCVE-0-2026-48511)

Vulnerability from cvelistv5 – Published: 2026-06-22 21:14 – Updated: 2026-06-25 15:08

Title

MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps

Summary

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary<string, object>.Add for each map entry. ExpandoObject internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies. For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because ExpandoObjectResolver.Options is configured with MessagePackSecurity.UntrustedData, but collision-resistant dictionary comparers cannot protect ExpandoObject insertion internals. This vulnerability is fixed in 2.5.301 and 3.1.7.

Severity

6.3 (Medium)


                        
                          CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-06-23 14:59 UTC

CWE

CWE-407 - Inefficient Algorithmic Complexity

Assigner

GitHub_M mitre root

References

1 reference

URL	Tags
https://github.com/MessagePack-CSharp/MessagePack…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version	CPE status
MessagePack-CSharp	MessagePack-CSharp	Affected: >= 3.0, < 3.1.7 Affected: < 2.5.301	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-48511

Vulnerability from fkie_nvd - Published: 2026-06-22 22:16 - Updated: 2026-06-25 16:16

Severity

7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-2x83-8g95-xh59	Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	messagepack	messagepack	*
	messagepack	messagepack	*

JSON

To clipboard

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          "vendor": "MessagePack-CSharp",
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GHSA-2X83-8G95-XH59

Vulnerability from github – Published: 2026-06-25 19:36 – Updated: 2026-06-25 19:36

Summary

MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps

Details

Summary

ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary<string, object>.Add for each map entry. ExpandoObject internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies.

For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because ExpandoObjectResolver.Options is configured with MessagePackSecurity.UntrustedData, but collision-resistant dictionary comparers cannot protect ExpandoObject insertion internals.

Impact

Applications are affected when they deserialize untrusted MessagePack maps into ExpandoObject using ExpandoObjectResolver or related resolver options.

A hostile payload containing many distinct keys can cause CPU exhaustion and allocation churn disproportionate to the input size. This can make a server unresponsive or exhaust memory under concurrent request load.

This is not a hash-collision attack against a configurable dictionary comparer. The super-linear behavior comes from ExpandoObject's insertion model, so MessagePackSecurity.UntrustedData does not eliminate the cost.

Affected components

Package: MessagePack
APIs: ExpandoObjectFormatter.Deserialize, ExpandoObjectResolver
Data type: System.Dynamic.ExpandoObject
Finding ID: MESSAGEPACKCSHARP-102

Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

Upgrade MessagePack to the patched version for your release line.
Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.

Potential fixes include applying a map-entry count limit for ExpandoObject under untrusted-data settings, buffering into a security-aware dictionary before materializing a bounded ExpandoObject, or otherwise rejecting maps large enough to trigger quadratic behavior.

Workarounds

Patching is recommended.

Until a patched version is available, avoid deserializing untrusted payloads into ExpandoObject. Prefer strongly typed DTOs or dictionaries with security-aware comparers and explicit count limits. Enforce request-size and map-entry limits at the transport or application layer.

Resources

MESSAGEPACKCSHARP-102: ExpandoObjectFormatter quadratic insertion behavior
CWE-407: Inefficient Algorithmic Complexity

Severity

7.5 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

6.3 (Medium)


                  
                    CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Show details on source website

JSON

To clipboard

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      "package": {
        "ecosystem": "NuGet",
        "name": "MessagePack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0"
            },
            {
              "fixed": "3.1.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-48511"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-25T19:36:23Z",
    "nvd_published_at": "2026-06-22T22:16:47Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\n`ExpandoObjectFormatter.Deserialize` populates `System.Dynamic.ExpandoObject` by calling `IDictionary\u003cstring, object\u003e.Add` for each map entry. `ExpandoObject` internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies.\n\nFor large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because `ExpandoObjectResolver.Options` is configured with `MessagePackSecurity.UntrustedData`, but collision-resistant dictionary comparers cannot protect `ExpandoObject` insertion internals.\n\n## Impact\n\nApplications are affected when they deserialize untrusted MessagePack maps into `ExpandoObject` using `ExpandoObjectResolver` or related resolver options.\n\nA hostile payload containing many distinct keys can cause CPU exhaustion and allocation churn disproportionate to the input size. This can make a server unresponsive or exhaust memory under concurrent request load.\n\nThis is not a hash-collision attack against a configurable dictionary comparer. The super-linear behavior comes from `ExpandoObject`\u0027s insertion model, so `MessagePackSecurity.UntrustedData` does not eliminate the cost.\n\n## Affected components\n\n- Package: `MessagePack`\n- APIs: `ExpandoObjectFormatter.Deserialize`, `ExpandoObjectResolver`\n- Data type: `System.Dynamic.ExpandoObject`\n- Finding ID: `MESSAGEPACKCSHARP-102`\n\n## Patches\n\nFixes are prepared and will be released in coordinated patch versions.\n\nUpgrade guidance:\n\n1. Upgrade `MessagePack` to the patched version for your release line.\n2. Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.\n\nPotential fixes include applying a map-entry count limit for `ExpandoObject` under untrusted-data settings, buffering into a security-aware dictionary before materializing a bounded `ExpandoObject`, or otherwise rejecting maps large enough to trigger quadratic behavior.\n\n## Workarounds\n\nPatching is recommended.\n\nUntil a patched version is available, avoid deserializing untrusted payloads into `ExpandoObject`. Prefer strongly typed DTOs or dictionaries with security-aware comparers and explicit count limits. Enforce request-size and map-entry limits at the transport or application layer.\n\n## Resources\n\n- `MESSAGEPACKCSHARP-102`: `ExpandoObjectFormatter` quadratic insertion behavior\n- CWE-407: Inefficient Algorithmic Complexity",
  "id": "GHSA-2x83-8g95-xh59",
  "modified": "2026-06-25T19:36:23Z",
  "published": "2026-06-25T19:36:23Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-2x83-8g95-xh59"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48511"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/MessagePack-CSharp/MessagePack-CSharp"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps"
}

Sightings

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Action not permitted

CVE-2026-48511 (GCVE-0-2026-48511)

FKIE_CVE-2026-48511

GHSA-2X83-8G95-XH59

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