{"id":"CVE-2026-48516","title":"MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings","summary":"MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings","severity":"medium","cwe":["CWE-407"],"vendor":"MessagePack","product":"MessagePack","ecosystem":"nuget","affected":["MessagePack < 2.5.301","MessagePack >= 3.0, < 3.1.7"],"patched":["MessagePack 2.5.301","MessagePack 3.1.7"],"published":"2026-06-25","updated":"2026-06-25","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-q2h6-ghwm-5qm8","references":[{"url":"https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-q2h6-ghwm-5qm8"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-48516"},{"url":"https://github.com/advisories/GHSA-q2h6-ghwm-5qm8"}],"tags":["ghsa","nuget"],"epss":0.00405,"epssPercentile":0.34561,"ingestedAt":"2026-06-26T16:43:14.221Z","slug":"CVE-2026-48516","body":"## Overview\n\n## Summary\n\n`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>()`.\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<TKey,TElement>` 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<TKey,TElement>` 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<TKey,TElement>.Create`\n- Data type: `ILookup<TKey,TElement>`\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<TKey>()`, 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<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.\n\n## Resources\n\n- `MESSAGEPACKCSHARP-041`: `InterfaceLookupFormatter` missing security comparer\n- CWE-407: Inefficient Algorithmic Complexity\n\n## Affected packages\n\n- `MessagePack < 2.5.301`\n- `MessagePack >= 3.0, < 3.1.7`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `MessagePack 2.5.301`\n- `MessagePack 3.1.7`","depth":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}