CVE-2026-49451
HIGHCVE-2026-49451 is a high-severity (CVSS 7.5) CWE-674 vulnerability in Microsoft.OpenApi. O3 Security confirms whether CVE-2026-49451 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-49451 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 359,229 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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
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. From 2.0.0-preview11 until 2.7.5 and 3.5.4, 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. This vulnerability is fixed in 2.7.5 and 3.5.4.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | Microsoft.OpenApi | ≥ 2.0.0-preview.11&&< 2.7.5 | 2.7.5 |
| .NETNuGet | Microsoft.OpenApi | ≥ 3.0.0&&< 3.5.4 | 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-49451 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-49451. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-49451 in your dependencies?
O3 detects CVE-2026-49451 across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.