GHSA-2X83-8G95-XH59
Vulnerability from github – Published: 2026-06-25 19:36 – Updated: 2026-06-25 19:36Summary
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
MessagePackto 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:ExpandoObjectFormatterquadratic insertion behavior- CWE-407: Inefficient Algorithmic Complexity
{
"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-2026-48511"
],
"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
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Published Proof of Concept: A public proof of concept is available for this vulnerability.
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- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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