Summary
Microsoft.OpenAPI: Circular schema references may terminate OpenAPI parsing
Affected versions
>= 2.0.0-preview.11, <= 2.7.4>= 3.0.0, <= 3.5.3
Impact
Applications, CLIs, developer tools, or services that parse untrusted OpenAPI documents in-process may be terminated by a crafted OpenAPI document containing circular schema references.
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
A standalone isolated-process harness confirmed repeatable process termination through public OpenAPI.NET reader APIs. The issue reproduces in the affected released NuGet packages and affects both JSON and YAML reader paths.
A separate Microsoft-owned local consumer, microsoft/kiota, also reproduces the termination through the kiota show --openapi <file> workflow. That workflow parses OpenAPI files in-process using Microsoft.OpenApi and Microsoft.OpenApi.YamlReader.
Example payload
{
"openapi": "3.0.0",
"info": {
"title": "Test",
"version": "0.0.1"
},
"paths": {},
"components": {
"schemas": {
"A": {
"$ref": "#/components/schemas/B"
},
"B": {
"$ref": "#/components/schemas/A"
}
}
}
}
Remediation
Users should upgrade to Microsoft.OpenApi 2.7.5 or 3.5.4, depending on the major version line they consume.
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.
Impact
A small OpenAPI document containing a circular schema reference can cause process termination through stack overflow in Microsoft.OpenApi. The issue affects OpenAPI document parsing through public OpenAPI.NET reader APIs and has been confirmed across both JSON and YAML reader paths.
CVE-2026-49451 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.5.4, 2.7.5); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
- For the 2.X major version, versions 2.7.5 and above are patched.
- For the 3.X major version, versions 3.5.4 and above are patched.
- For the 1.X major version, the issue does not apply since that version of the library could not resolve references that pointed to another reference.
Frequently Asked Questions
- What is CVE-2026-49451? CVE-2026-49451 is a high-severity security vulnerability in Microsoft.OpenAPI (nuget), affecting versions >= 3.0.0, <= 3.5.3. It is fixed in 3.5.4, 2.7.5.
- How severe is CVE-2026-49451? CVE-2026-49451 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2026-49451?
Microsoft.OpenAPI(nuget) (versions >= 3.0.0, <= 3.5.3)Microsoft.OpenApi(nuget) (versions >= 2.0.0-preview.11, <= 2.7.4)
- Is there a fix for CVE-2026-49451? Yes. CVE-2026-49451 is fixed in 3.5.4, 2.7.5. Upgrade to this version or later.
- Is CVE-2026-49451 exploitable, and should I be worried? Whether CVE-2026-49451 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-49451 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-49451?
- Upgrade
Microsoft.OpenAPIto 3.5.4 or later - Upgrade
Microsoft.OpenApito 2.7.5 or later
- Upgrade