ASP.NET Core 10 LTS: Security, Speed, Next Steps

ASP.NET Core 10 LTS arrived on November 11, 2025 with a three‑year support window through November 10, 2028. If you run production APIs or web apps, this is the upgrade you plan around. The ASP.NET Core 10 LTS release isn’t just “faster”—it closes a critical Kestrel request‑smuggling class of bugs patched in October, adds passkey support to identity, strengthens diagnostics, and trims memory in long‑running services. Let’s break down the high‑impact changes and how to adopt them without slowing your roadmap.

Why ASP.NET Core 10 LTS matters now

.NET’s even-numbered releases are LTS. That matters for budgets, audits, and risk: you get critical fixes for three years. With the recent policy changes, .NET 9 (STS) gets 24 months total support, while .NET 8 (LTS) ends support on November 10, 2026. Many organizations will jump straight from 8 to 10; others sitting on 9 should plan an accelerated hop. Either way, the scheduling math favors moving now rather than stacking debt into 2026.

Security is the other forcing function. In October 2025, Microsoft patched a Kestrel parsing flaw rated 9.9 CVSS that enabled HTTP request smuggling in certain proxy/app setups. The fix is available on supported trains, but landing on 10 means you’re current on the runtime, libraries, and the evolving diagnostics you’ll use to prove you’re safe. Treat this release as part security response, part platform reset.

What’s actually new for web apps and APIs

The headline is performance plus safety. Under the hood, the runtime tightened JIT and allocation patterns; on the web stack, you’ll notice smoother memory behavior, simpler auth, and better signals when something goes wrong.

Security and identity

Passkeys are now a first‑class option in ASP.NET Core Identity. You can let users register and sign in with FIDO2/WebAuthn—no passwords to breach, phish, or rotate. The Blazor Web App template ships with passkey management scaffolding, which makes pilot rollouts feasible without a greenfield rewrite. If you operate B2C auth, passkeys won’t instantly replace passwords, but they’re the cleanest way to reduce credential stuffing and reset tickets.

Performance and efficiency

Automatic memory‑pool eviction helps long‑running services give unused buffers back to the OS. On busy Kestrel or SignalR servers that spike traffic, this reduces RSS drift after peak loads. The change isn’t magic—your allocations still matter—but you get a friendlier steady state.

NativeAOT keeps maturing. For small API workers and background jobs, AOT brings cold‑start and memory wins that add up in container density. You’ll need to trim reflection and watch dynamic code paths, but for compute‑bound workflows, it’s finally a practical lever.

Developer ergonomics

Server‑Sent Events (SSE) is a one‑liner with typed results now, which is great for dashboards, long‑running jobs, and lightweight real‑time updates without the complexity budget of WebSockets. Error responses are more consistent via IProblemDetailsService so API consumers see uniform payloads when validation or auth fails.

Observability and diagnostics

You get more built‑in metrics across Blazor, Identity, and memory pools. Blazor Server tracing is clearer, and WebAssembly apps gained better CPU and memory tooling. For teams standardizing on OpenTelemetry, .NET 10 makes it simpler to export useful, high‑cardinality signals without littering code with custom counters.

ASP.NET Core 10 LTS vs. your 2026 roadmap

Here’s the strategic lens: 2026 is already crowded. Browser privacy changes will keep shifting ad/attribution flows. Budget cycles tighten when macro noise gets loud. Sliding your platform migration into Q1–Q2 2026 is how projects slip. If you move now, you roll security response and platform uplift into a single change window, and you keep your feature work unblocked next year.

If you’re weighing the jump, we’ve already covered the organizational angle in our .NET 10 series—what changes in your stack, why it’s worth it, and the pitfalls to avoid. Start with a practical overview in what .NET 10 LTS changes for your web stack, then sanity‑check timing with our 2026 upgrade reality check.

Security first: close the request‑smuggling window

The October 2025 Kestrel fix targeted inconsistent parsing paths that could let a malicious client “smuggle” a second request through proxies or middlewares that interpret headers differently. Not every deployment was exploitable, but the class of errors is exactly the sort that slips past unit tests and shows up under load or at the edge.

Do this before anything else:

• Patch your currently deployed runtime to the latest supported patch level on 8/9 or migrate to 10, then redeploy everything that terminates HTTP with Kestrel behind a proxy.
• Turn up logging briefly in staging to capture suspicious Transfer‑Encoding/Content‑Length combinations and malformed chunk extensions, then roll those log rules back to normal to avoid noise.
• Audit reverse proxies and API gateways for header normalization and body size limits; ensure origin and proxy agree on how to handle invalid chunk extensions.
• Run a smuggling test suite against a non‑prod environment. Include cache poisoning and CSRF bypass attempts; verify that auth and anti‑forgery checks still gate all protected routes.

If you need a deeper primer on planning the jump, our field notes in Ship on .NET 10 LTS: What’s New, What to Do walk through sequencing the cut‑over by service, not by repo.

Hands‑on: a 30‑60‑90 plan you can copy

Day 0–30: prove stability

• Create a global.json pin for the .NET 10 SDK in the monorepo. Update CI runners and self‑hosted agents to match.
• Upgrade one low‑risk API first. Target net10.0, enable nullable/trim warnings as errors, and switch the container image to the 10 runtime.
• Enable passkeys in a staging branch of your main sign‑in experience. Test platform authenticators on Windows, macOS, and Android. Define recovery: a passkey‑only flow without recovery codes is an outage waiting to happen.
• Turn on the new ASP.NET Core metrics and export via OpenTelemetry. Capture a 24‑hour baseline in staging with realistic traffic.
• Run load tests to observe memory‑pool eviction behavior; confirm steady‑state RSS is lower after traffic spikes.

Day 31–60: consolidate and ship

• Migrate the “chattiest” service next. If it’s a Blazor Server app, verify tracing in staging sessions and check for missed exceptions in circuit logs.
• Introduce SSE where you currently poll. Start with a read‑only stream (e.g., order status or job progress) behind your existing CDN. Confirm idle timeouts on your proxy don’t sever the stream.
• Trial NativeAOT for a small worker or API without heavy reflection. Measure cold starts, P95 latency, and image size. Keep a non‑AOT variant ready until you’ve tested observability and compatibility.
• Align tooling: Visual Studio 2026 or latest C# Dev Kit in VS Code for everyone touching these services. Discrepant toolchains are sneaky productivity killers.

Day 61–90: scale the pattern

• Roll passkeys to a small customer cohort with clear UX copy. Track help‑desk contact rate and completion times for account recovery.
• Adopt the problem details service across APIs so clients get consistent error payloads. Update API docs once, not per controller.
• Finalize platform policy: what services stay JIT, which go AOT, and how you decide next time.
• Freeze framework drift in December change windows; keep patching but avoid feature‑level switches until January.

People also ask: common upgrade questions

Should we jump from .NET 8 straight to ASP.NET Core 10 LTS?

Yes, if your test suite is healthy. .NET 8 is supported until November 10, 2026, but you’ll spend the same effort later with less runway. Jumping now compresses security remediation and platform uplift into one release plan, and you start the 36‑month clock today.

Will ASP.NET Core 10 break my middleware and filters?

Most middleware continues to work unchanged. Watch for packages that baked in old HTTP parsing assumptions or reflection‑heavy behaviors if you consider AOT. Turn on trimming analysis early to surface risky code paths.

How do passkeys actually land in our stack?

Use the updated Identity templates to scaffold registration and login. Store the WebAuthn public key credential IDs server‑side, ensure origin checks are strict, and ship clear recovery flows (backup codes or a second factor). Plan time to update your risk engine—passkey logins behave differently from passwords.

Is SSE enough for real‑time features?

SSE is perfect for one‑way streams like logs, progress, or notifications. For two‑way, high‑frequency messaging (think collaborative editors), stay with WebSockets. On the edge, make sure your CDN or load balancer allows long‑lived HTTP connections.

Gotchas we’ve seen in real migrations

Reverse proxies and timeouts. Some proxies aggressively buffer or terminate idle HTTP connections. For SSE, set explicit read timeouts and heartbeat messages; otherwise you’ll see mysterious disconnects at the five‑minute mark.

Chunked requests at the edge. Even with the October fixes, keep origin and proxy behavior aligned. Hard‑reject invalid chunk extensions rather than trying to be “helpful” and re‑parsing.

Key management and passkeys. Passkeys reduce your credential risk but increase responsibility for recovery and device changes. Document your rotation and revocation process. Customer support needs scripts for account recovery that don’t explode compliance.

AOT trade‑offs. NativeAOT improves startup and memory, but certain libraries that rely on dynamic code gen will fight you. Keep a layered approach: AOT small workers; JIT larger, reflection‑heavy services; measure both.

Framework and tooling alignment

Standardize your SDK and IDE story before you scale the upgrade. Pin the .NET 10 SDK with global.json, ensure your build agents pull matching container images, and align developer machines on Visual Studio 2026 or the C# Dev Kit. This reduces “works on my box” drift when language features (like newer C# field‑backed properties or extension members) appear in code reviews.

For context on how we think about version strategy and cost of upgrades, browse our deep dive: why .NET 10 LTS is worth the upgrade now. It covers timelines, change budgets, and the total cost of waiting.

A simple upgrade readiness checklist

• Inventory: list all apps exposing HTTP, their target frameworks, and proxy topology.
• Patch: apply the latest supported runtime updates on your current train; verify staging with smuggling tests.
• Pin: add global.json and container base images for .NET 10; update CI/CD runners.
• Pilot: move one API and one UI (e.g., Blazor Server) first; capture performance and memory baselines.
• Auth: enable passkeys for internal users or a small customer cohort; define recovery pathways.
• Observe: wire up the new metrics, export with OpenTelemetry; set SLOs.
• Decide AOT: select 1–2 services for AOT trials; document wins and caveats.
• Communicate: set a freeze window and publish cut‑over dates; give support teams the recovery scripts and runbooks.

What to do next

Pick your entry point and move:

• Read our concise playbook in What .NET 10 LTS Changes for Your Web Stack and share it with your platform lead.
• Schedule a half‑day spike to upgrade a non‑critical service to ASP.NET Core 10 LTS; collect before/after metrics.
• Plan a passkey pilot for a real user group with clear recovery and support workflows.
• If you’d like help sequencing the rollout or building guardrails, our team can assist—see our services and reach out via contacts.

Zooming out

Teams ship faster when platform friction drops. ASP.NET Core 10 LTS brings practical wins—leaner memory behavior, better built‑in telemetry, one‑click passkeys—while addressing a serious security class. You don’t need a heroic rewrite. You need a steady plan, a small pilot, and the discipline to finish. Do that, and your 2026 roadmap gets simpler—even as everything else gets noisier.

If you want more release‑by‑release detail and action items, keep our series handy: the upgrade timing piece in The .NET 10 LTS Reality Check for 2026 Upgrades and the onboarding guide in Ship on .NET 10 LTS: What’s New, What to Do. When you’re ready to move, we’re ready to help.