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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

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.

Source published

8 Sep 2026 17:12:20 UTC

Source modified

14 Oct 2026 20:08:53 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

Source published

Sep 8, 2026, 5:12:20 PM (timezone not stated)

Source modified

Oct 14, 2026, 8:08:53 PM (timezone not stated)

Observed here

9 Nov 2026 15:08:52 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

Source published

8 Sep 2026 18:20:16 UTC

Source modified

14 Oct 2026 21:17:39 UTC

Observed here

25 Oct 2026 15:19:03 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

### Impact A small YAML OpenAPI document containing nested anchors and aliases can cause uncontrolled resource consumption in Microsoft.OpenApi.YamlReader and Microsoft.OpenApi.Readers. The issue affects OpenAPI document parsing through the public YAML reader APIs and can cause the parsing process to allocate excessive memory and terminate through out-of-memory conditions. ## Affected versions ### Microsoft.OpenApi.YamlReader * `>= 2.0.0-preview.11, < 2.12.0` * `>= 3.0.0, < 3.10.0` ### Microsoft.OpenApi.Readers * `< 1.6.30` ### Patches * For the 1.X major version, versions 1.6.30 and above are patched. * For the 2.X major version, versions 2.12.0 and above are patched. * For the 3.X major version, versions 3.10.0 and above are patched. ## Impact Applications, CLIs, developer tools, or services that parse untrusted YAML OpenAPI documents in-process may be terminated by a crafted YAML document containing nested anchors and aliases. The impact is availability/process termination only. This report does not claim remote code execution, authentication bypass, credential exposure, privilege escalation, data exposure, or Microsoft hosted service impact. ## Details Microsoft.OpenApi.YamlReader and Microsoft.OpenApi.Readers parses YAML through SharpYaml. SharpYaml represents aliases as shared `YamlNode` instances in a directed acyclic graph, so the initial parsed YAML graph remains small. During conversion to `System.Text.Json.Nodes.JsonNode`, aliases must be materialized as independent nodes because `JsonNode` instances cannot be attached to multiple parents. Without a bound on that conversion work, a small YAML document using nested anchors and aliases can expand exponentially during conversion. With `N` nested anchors each referenced `k` times, conversion can require `k^N` materialized JSON nodes, leading to excessive memory allocation and possible process termination. This is an uncontrolled resource consumption vulnerability, also known as a "billion laughs" style denial of service. ### Example payload ```yaml openapi: 3.0.0 info: title: Test version: 0.0.1 paths: {} x-a: &a ["lol", "lol", "lol", "lol", "lol", "lol", "lol", "lol", "lol"] x-b: &b [*a, *a, *a, *a, *a, *a, *a, *a, *a] x-c: &c [*b, *b, *b, *b, *b, *b, *b, *b, *b] x-d: &d [*c, *c, *c, *c, *c, *c, *c, *c, *c] x-e: &e [*d, *d, *d, *d, *d, *d, *d, *d, *d] x-f: &f [*e, *e, *e, *e, *e, *e, *e, *e, *e] ``` ## Remediation Users should upgrade to Microsoft.OpenApi.YamlReader `2.12.0` or `3.10.0`, or Microsoft.OpenApi.Readers `1.6.30`, depending on the major version line they consume. The patched versions bound YAML-to-JSON conversion work and fail fast instead of allowing unbounded expansion. The default limits are: * Maximum materialized node count: `5,000,000` * Maximum nesting depth: `64` When the limits are exceeded, `OpenApiYamlReader.Read` reports an `OpenApiDiagnostic` error and does not return a parsed document. The limits are exposed as process-wide static properties on `YamlConverter` for consumers that need to tune them for trusted workloads: * `YamlConverter.MaxNodeCount` * `YamlConverter.MaxDepth` Applications that parse OpenAPI documents from untrusted sources should avoid parsing those documents in the primary application process when possible. Running parsing in an isolated process can reduce the blast radius of parser failures.

Source published

14 Oct 2026 20:11:29 UTC

Source modified

14 Oct 2026 20:15:04 UTC

Observed here

23 Oct 2026 16:01:25 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH