.NET 10 LTS migration isn’t just another version bump—it’s a strategic window. Microsoft released .NET 10 on November 11, 2025 with long‑term support through November 14, 2028. That gives you three full years of patches, a fresh C# 14 toolchain, and meaningful ASP.NET Core 10 improvements. Add in October’s 9.9‑severity Kestrel security issue and the calculus gets simple: staying current is now a security and cost posture, not a nice‑to‑have.
What actually shipped in .NET 10 LTS?
Let’s start with the parts you’ll feel on day one. On the runtime side, .NET 10 tightens code generation—better JIT inlining, method devirtualization, and smarter stack allocations. NativeAOT gets steadier, broadening the set of workloads that can go ahead-of-time without fighting toolchain edges. On modern hardware, AVX10.2 support lands, which helps numerics‑heavy code and tight loops.
ASP.NET Core 10 is where most web teams will notice quality‑of‑life gains. Passkeys are first‑class in Identity, so passwordless authentication finally feels supported rather than experimental. Minimal APIs get built‑in validation: annotate your DTOs and let the framework return problem details on bad input. OpenAPI generation is sharper—YAML output, richer descriptions, and better XML comments integration. And there’s a quiet but important change to memory behavior: automatic memory pool eviction reduces long‑running process bloat, especially in services that see uneven traffic.
On the language front, C# 14 delivers the small features teams actually adopt: field‑backed properties to transition beyond auto‑properties without refactors; extension members to organize helpers without static utility clutter; and more ergonomic Span conversions. None of these headline features change your architecture, but they save minutes every day—which compounds.
Why this release matters to the business
Here’s the thing: .NET’s release cadence is predictable, but your risks aren’t. .NET 10 is the current LTS; .NET 8 remains LTS through November 2026; .NET 9 is STS until November 10, 2026. If you’re still on .NET 6/7, you’re already out of runway. LTS means predictable patching, vendor attention, and a stable baseline for auditors. It also means modern platform support: current TLS stacks, up‑to‑date Linux images, and security defaults that won’t embarrass you in a SOC 2 review.
Performance is the quiet ROI. Between runtime codegen wins, memory pool eviction, and incremental ASP.NET improvements, many teams see lower p95 latency and smaller working sets after a clean rebuild. In practice, that translates to fewer containers for the same throughput and lower edge CPU minutes. If your cloud bills have been creeping, a well‑run migration can pay for itself in a quarter—especially if you also revisit noisy neighbors like image optimization or server‑side rendering. For a broader cost lens, our breakdown on model costs and cutting spend is a useful companion when you re‑baseline infra.
Security reality check: the 9.9 Kestrel CVE
In October 2025, Microsoft patched an HTTP request smuggling vulnerability in Kestrel (CVE‑2025‑55315) that earned a rare 9.9 severity. The impact depends on your proxy and app behaviors, but the takeaway is clear: patch, rebuild, and redeploy any .NET apps with public surfaces. If you’re already on supported .NET 8/9 trains and keep pace with monthly updates, you’re covered by servicing releases. Teams that lag on base images or self‑contained deployments need a planned redeploy to pull in the fixed runtime.
Moving to .NET 10 LTS doesn’t just bring features; it forces a tidy‑up of your build pipelines, base images, and hosting bundles. Treat that as a gift. Bake image refreshes into CI, pin secure base images, and add a smoke test for smuggling protections at the edge. If you needed a board‑level reason to fund the work, you have one.
The pragmatic .NET 10 LTS migration checklist
This is the sequence we’ve used to get teams to green without heroics. It assumes you’re on .NET 8 or 9 today; if you’re older, insert a safety stop at .NET 8 first.
1) Inventory and prioritize
Map apps and services with these fields: target framework, hosting model (self‑contained vs framework‑dependent), base image tag, public ingress, auth mechanism, and deployment frequency. Prioritize outward‑facing APIs, auth flows, and money‑touching surfaces. If you have multi‑tenant workloads, they bubble to the top.
2) Upgrade the toolchain in isolation
Install the .NET 10 SDK on your build agents and add a feature branch that bumps TargetFramework to net10.0. Keep your production agents pinned to the old SDK so you can dogfood side‑by‑side. Enable C# 14 in a small subset first to surface analyzer changes before the floodgates open.
3) Fix the obvious breakpoints
Common hotspots: analyzers that became warnings‑as‑errors, custom JSON converters, and packages tied to older compiler semantics. For web apps, exercise new OpenAPI generation and verify response descriptions render as expected in Swagger UI. If you use minimal APIs, turn on built‑in validation—then remove redundant endpoint guards and update tests to assert problem details.
4) Clean your auth: adopt passkeys where they fit
If you use ASP.NET Core Identity, test the passkey flow in a dedicated environment. Decide if passkeys are primary or a strong option alongside passwords. Update your privacy notice and user comms before you flip it on. If you’re pure OAuth/OIDC, your lift should be minimal—still run through sign‑in/out, challenges, and metrics (ASP.NET 10 exposes more counters you can scrape into your dashboards).
5) Rebase and lock base images
If you ship containers, rebase to a patched .NET 10 image and lock a digest. Don’t rely on floating tags like :10 or :latest. Ensure your registry policy rebuilds images weekly even if your app code hasn’t changed, so you catch base‑layer CVEs automatically.
6) Performance gates before production
Recreate your production traffic profile with a realistic data set. Measure p50/p95 latency, memory working set, and CPU per request under both the old and new builds. You’re looking for regressions beyond 5–10% or meaningful improvements you can convert into fewer replicas. Validate that automatic memory pool eviction actually drops footprint after bursts.
7) Roll out safely
Use blue/green or ring deployments: 5% traffic for 30–60 minutes, then 25%, then 100%. Keep rollback one click away. Add extra logging around authentication and serialization for the first week; most surprises hide there. And yes, include a synthetic canary that exercises passkey login if you enabled it.
Benchmarks and dates you can plan around
Key timelines matter for budgeting and risk:
- .NET 10 LTS released November 11, 2025; support through November 14, 2028.
- .NET 9 STS supported through November 10, 2026.
- .NET 8 LTS supported through November 10, 2026.
- Kestrel CVE‑2025‑55315 patched in October 2025—requires rebuilt apps or patched hosting bundles/base images.
If your fiscal year flips January 1, you have December to prove out migration on one or two services and lock a 2026 roadmap with confidence.
People also ask: quick answers for your team
Is .NET 8 still a safe place to be?
Yes, for now. It’s LTS through November 2026, and it received the Kestrel fix via servicing. But if you’ll still be maintaining apps in 2027–2028, moving to .NET 10 LTS avoids another migration cycle mid‑contract.
Do we need to retest everything?
Not everything—but everything public and everything handling money or auth should get full functional, perf, and security passes. Internal batch jobs can ride a lighter path: compile, unit tests, a smoke run on prod‑like data, and go.
Will C# 14 break our analyzers?
Possibly. Preview analyzers or custom rules sometimes assume older syntax. Upgrade analyzer packages and run a dotnet format plus CI checks. Enable C# 14 project by project.
What about NativeAOT—should we flip it on?
Only where it wins: short‑lived CLIs, latency‑sensitive edge workers, and functions with small dependency graphs. For large ASP.NET apps, measure before you move; startup wins can be offset by complexity and bundle size.
Tradeoffs and gotchas you should budget for
OpenAPI upgrades can expose stale doc comments or mismatched attributes; fix them now while you have attention. Minimal API validation is fantastic, but it may change error payloads your clients expect—version your APIs if necessary. Passkeys reduce phishing risk, but they carry UX edge cases around device migration; plan your recovery story and customer support scripts.
Finally, watch memory after traffic spikes. Automatic pool eviction helps, but if you’re running tight container limits, a surge followed by GC can still trigger restarts. Keep headroom until you’ve seen a few real cycles.
Let’s get practical: a 10‑day pilot for .NET 10 LTS migration
Use this to prove feasibility before you commit company‑wide.
- Day 1: Pick one public API and one internal job. Capture baseline perf and error budgets. Confirm current CVE status.
- Day 2–3: Bump to
net10.0on a branch, fix build, update analyzers, enable C# 14. Turn on minimal API validation if used. - Day 4: Switch OpenAPI to include YAML output. Clean descriptions. Regenerate client SDKs if you maintain them.
- Day 5: Test Identity flows. If you plan passkeys, enable in a staging tenant only.
- Day 6: Rebase containers to .NET 10 images; lock digests. Add weekly image rebuild in CI.
- Day 7: Load test with real data shapes; compare p50/p95 latency and memory.
- Day 8: Security pass—DAST against staging, edge proxy rules for ambiguous headers, request‑smuggling canary.
- Day 9: Blue/green into 5% traffic for one service. Watch auth metrics and serialization logs.
- Day 10: Roll to 100% if clean. Document diffs and write your internal “how we migrated” note.
Cost control while you migrate
Keep spend predictable as you move. Cap preview environments at limited autoscale. Run perf tests off‑peak to avoid stealing headroom from prod. After rollout, recalculate replicas with observed memory savings. And if your stack mixes managed platforms, sanity‑check your usage‑based bills; we’ve seen “quiet” upgrades expose pricing quirks. Our recent note on pricing shifts and cost guardrails has a checklist you can adapt.
Resilience isn’t optional—fold it into the upgrade
If you’re touching infra, add the things you meant to do last quarter: health probes that look at downstreams, idempotent retries, circuit breakers, and a real incident rehearsal. When big providers hiccup, the teams that prepared keep revenue. If you need a nudge, our resilience playbook pairs nicely with this migration work.
Where to bring in help
Some teams want a partner to run the pilot or to harden auth and observability patterns. If that’s you, skim what we do and get in touch—we’ve led no‑surprises upgrades before, including our no‑surprises upgrade playbook for frontend stacks. The recipe is the same: tight scoping, measurable gates, boring rollouts.
What to do next (this week)
- Decide: pick your first two services for .NET 10 LTS migration and book the 10‑day pilot.
- Patch posture: verify you’re on patched Kestrel builds; rebuild any self‑contained apps and refresh base images.
- DX wins: enable minimal API validation and improve OpenAPI descriptions while you’re here.
- Auth plan: run a passkey spike in a staging tenant; write the recovery UX before enabling.
- Cost loop: capture before/after perf and adjust replica counts to bank savings.
- Resilience: add an incident drill tied to your rollout and update runbooks.
If you make one decision today, make it this: schedule the pilot. .NET 10 LTS gives you a supported runway through 2028, better developer ergonomics, and a cleaner security story. Get a small win on the board now, write down what worked, and scale it with confidence.
