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

{
  "id": "f5a9916f53e1dc3a2e31552559619a48f5d914b14249d1bc44735ae85db2e027",
  "url": "https://osv.dev/vulnerability/CVE-2026-48511",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "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.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.",
    "evidence": "osv:CVE-2026-48511",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-48511",
      "aliases": [
        "CVE-2026-48511",
        "GHSA-2x83-8g95-xh59"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "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"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/messagepack-csharp/messagepack-csharp",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "5657b1e34ea2869a2ca6bc0aa6617158620201ba"
                },
                {
                  "introduced": "ff3af77f546138d9da71707baddf2c30f55dfc15"
                },
                {
                  "fixed": "1b53ae8b7681340199faf337bcbb0eb5aed702de"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:messagepack:messagepack:*:*:*:*:*:c\\#:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "2.5.301"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:messagepack:messagepack:*:*:*:*:*:c\\#:*:*",
                    "range": [
                      {
                        "introduced": "3.0.3"
                      },
                      {
                        "fixed": "3.1.7"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v3.1.6",
            "v3.1.5",
            "v2.5.198",
            "v3.1.4",
            "v3.1.3",
            "v3.1.2",
            "v3.1.1",
            "v3.1.0",
            "v3.0.301",
            "v3.0.300",
            "v3.0.3",
            "v2.5.192",
            "v2.5.187",
            "v2.5.172",
            "v2.5.171",
            "v2.5.168",
            "v2.5.140",
            "v2.5.129",
            "v2.5.108",
            "v2.5.124",
            "v2.5.103",
            "v2.4.59",
            "v2.5.94",
            "v2.4.35",
            "v2.4.23-alpha",
            "v2.3.85",
            "v2.1.115",
            "v2.3.75",
            "v2.2.113",
            "v2.2.85",
            "v2.2.60",
            "v2.1.194",
            "v2.1.165",
            "v2.1.152",
            "v2.1.143",
            "v2.1.90",
            "v2.1.80",
            "v2.0.270-rc",
            "v2.0.204-beta",
            "v2.0.123-beta",
            "v2.0.119-beta",
            "v2.0.110-alpha",
            "v2.0.94-alpha",
            "v1.7.3.4",
            "v1.7.3.3",
            "v1.7.3.2",
            "v1.7.3.1",
            "v1.7.3",
            "v1.7.2",
            "v1.7.0",
            "v1.6.2",
            "v1.6.1.2",
            "v1.6.1",
            "v1.6.0",
            "v1.5.1",
            "v1.5.0",
            "v1.4.4",
            "v1.4.2",
            "v1.4.1",
            "v1.4.0",
            "v1.3.3",
            "v1.3.2",
            "v1.3.1",
            "v1.3.0",
            "v1.2.3",
            "v1.2.1",
            "v1.2.0",
            "v1.1.2",
            "v1.1.1",
            "v1.0.3",
            "v1.0.2",
            "1.0.1",
            "0.2.0-beta",
            "0.1.0-beta"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-48511.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:28.391978676Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48511.json",
        "https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-2x83-8g95-xh59",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-48511"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-407"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48511.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-48511",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 495347,
  "published_at": "2026-06-22T21:14:54.127+00:00"
}