Microsoft.OpenApi.YamlReader vulnerable to denial of service via YAML alias expansion

Summary

The OpenAPI.NET SDK contains a useful object model for OpenAPI documents in .NET along with common serializers to extract raw OpenAPI JSON and YAML documents from the model. In Microsoft.OpenApi.Readers versions before 1.6.30 and Microsoft.OpenApi.YamlReader from 2.0.0-preview.11 through 2.11.0 and 3.0.0 through 3.9.0, parsing a YAML OpenAPI document with nested anchors and aliases can consume uncontrolled memory. SharpYaml represents aliases as shared nodes in a small directed acyclic graph, but conversion to independent OpenAPI or System.Text.Json.Nodes.JsonNode trees duplicates every aliased path; without a node-count or nesting-depth limit, N nested anchors referenced k times can require k^N materialized nodes and terminate the process through an out-of-memory condition. The fixed readers bound materialization to 5,000,000 nodes and a nesting depth of 64 by default and report an OpenApiDiagnostic error when a limit is exceeded. This vulnerability is fixed in Microsoft.OpenApi.Readers 1.6.30 and Microsoft.OpenApi.YamlReader 2.12.0 and 3.10.0.

Timeline

1. Sep 8, 2026, 5:12 PM UTCCVE published
2. Sep 8, 2026, 5:36 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Oct 14, 2026, 8:08 PM UTCRecord last updated

Affected versions

• microsoft OpenAPI.NET

  From 3.0.0 · fixed in 3.10.0; From 2.0.0-preview.11 · fixed in 2.12.0; fixed in 1.6.30

• Microsoft.OpenApi.YamlReaderNuGet

  From 3.0.0 · fixed in 3.10.0

• Microsoft.OpenApi.YamlReaderNuGet

  From 2.0.0-preview.11 · fixed in 2.12.0

• Microsoft.OpenApi.ReadersNuGet

  fixed in 1.6.30

References 4