GHSA-Q2H6-GHWM-5QM8

Vulnerability from github – Published: 2026-06-25 21:29 – Updated: 2026-06-25 21:29
VLAI
Summary
MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings
Details

Summary

InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>().

Other hash-based collection formatters use the security-aware comparer when MessagePackSecurity.UntrustedData is configured. This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture.

Impact

Applications are affected when they deserialize untrusted payloads into schemas containing ILookup<TKey,TElement> with a key type for which attacker-controlled hash collisions are feasible.

Under the default comparer, many colliding keys can degrade dictionary insertion from amortized constant time to quadratic behavior. A payload of colliding keys can consume CPU for a disproportionate amount of time. This bypasses the mitigation that developers intentionally enabled by using MessagePackSecurity.UntrustedData.

Affected components

  • Package: MessagePack
  • API: InterfaceLookupFormatter<TKey,TElement>.Create
  • Data type: ILookup<TKey,TElement>
  • Finding ID: MESSAGEPACKCSHARP-041

Patches

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

Upgrade guidance:

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

The fix should create the internal dictionary with options.Security.GetEqualityComparer<TKey>(), matching the sibling dictionary and lookup formatter behavior.

Workarounds

Patching is recommended.

Until a patched version is available, avoid exposing ILookup<TKey,TElement> in DTOs that deserialize untrusted data. Use collection shapes that are already protected by the security-aware comparer path, or validate and cap collection sizes at the transport boundary.

Resources

  • MESSAGEPACKCSHARP-041: InterfaceLookupFormatter missing security comparer
  • CWE-407: Inefficient Algorithmic Complexity
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
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{
  "affected": [
    {
      "package": {
        "ecosystem": "NuGet",
        "name": "MessagePack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.5.301"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "NuGet",
        "name": "MessagePack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0"
            },
            {
              "fixed": "3.1.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-48516"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-25T21:29:39Z",
    "nvd_published_at": "2026-06-22T22:16:48Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\n`InterfaceLookupFormatter\u003cTKey,TElement\u003e` constructs an internal `Dictionary\u003cTKey, IGrouping\u003cTKey,TElement\u003e\u003e` with the default equality comparer instead of the security-aware comparer supplied by `options.Security.GetEqualityComparer\u003cTKey\u003e()`.\n\nOther hash-based collection formatters use the security-aware comparer when `MessagePackSecurity.UntrustedData` is configured. This formatter omission allows hash-collision CPU denial of service against `ILookup\u003cTKey,TElement\u003e` even when the application has opted into the untrusted-data security posture.\n\n## Impact\n\nApplications are affected when they deserialize untrusted payloads into schemas containing `ILookup\u003cTKey,TElement\u003e` with a key type for which attacker-controlled hash collisions are feasible.\n\nUnder the default comparer, many colliding keys can degrade dictionary insertion from amortized constant time to quadratic behavior. A payload of colliding keys can consume CPU for a disproportionate amount of time. This bypasses the mitigation that developers intentionally enabled by using `MessagePackSecurity.UntrustedData`.\n\n## Affected components\n\n- Package: `MessagePack`\n- API: `InterfaceLookupFormatter\u003cTKey,TElement\u003e.Create`\n- Data type: `ILookup\u003cTKey,TElement\u003e`\n- Finding ID: `MESSAGEPACKCSHARP-041`\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\nThe fix should create the internal dictionary with `options.Security.GetEqualityComparer\u003cTKey\u003e()`, matching the sibling dictionary and lookup formatter behavior.\n\n## Workarounds\n\nPatching is recommended.\n\nUntil a patched version is available, avoid exposing `ILookup\u003cTKey,TElement\u003e` in DTOs that deserialize untrusted data. Use collection shapes that are already protected by the security-aware comparer path, or validate and cap collection sizes at the transport boundary.\n\n## Resources\n\n- `MESSAGEPACKCSHARP-041`: `InterfaceLookupFormatter` missing security comparer\n- CWE-407: Inefficient Algorithmic Complexity",
  "id": "GHSA-q2h6-ghwm-5qm8",
  "modified": "2026-06-25T21:29:40Z",
  "published": "2026-06-25T21:29:39Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-q2h6-ghwm-5qm8"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48516"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/MessagePack-CSharp/MessagePack-CSharp"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "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: InterfaceLookupFormatter bypasses collision-resistant comparer settings"
}


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