{"id":"CVE-2026-48511","aliases":["GHSA-2x83-8g95-xh59"],"url":"https://o3.security/vulnerability/CVE-2026-48511","summary":"MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps","details":"## Summary\n\n`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.\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`'s 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","published":"2026-06-22T21:14:54.127Z","modified":"2026-08-27T03:30:36.877432441Z","cvss":null,"epss":{"score":0.00405,"percentile":0.34308,"asOf":"2026-09-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"NuGet","name":"MessagePack","fixedVersion":"2.5.301"},{"ecosystem":"NuGet","name":"MessagePack","fixedVersion":"3.1.7"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48511.json"},{"type":"ADVISORY","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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-27T03:30:36.877432441Z"}}