CVE-2026-49451 — Microsoft.OpenApi
HIGHCVE-2026-49451 is a high-severity (CVSS 7.5) CWE-674 vulnerability in Microsoft.OpenApi. A fix is available for Microsoft.OpenApi — see the affected versions and patch details below.
Microsoft.OpenAPI: Circular schema references may terminate OpenAPI parsing
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-49451.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-49451 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 380,066 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
Microsoft.OpenApi.NETMicrosoft.OpenApiReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected versions
>= 2.0.0-preview.11, <= 2.7.4>= 3.0.0, <= 3.5.3
Patches
- 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.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | Microsoft.OpenApi | ≥ 2.0.0-preview.11&&< 2.7.5 | 2.7.5dotnet add package Microsoft.OpenApi --version 2.7.5 |
| .NETNuGet | Microsoft.OpenApi | ≥ 3.0.0&&< 3.5.4 | 3.5.4dotnet add package Microsoft.OpenApi --version 3.5.4 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Microsoft.OpenApi, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update Microsoft.OpenApi to 2.7.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-49451 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.
Frequently Asked Questions
Is CVE-2026-49451 in your dependencies?
Find it across NuGet, including transitive dependencies.