Auth0 vs Firebase Auth vs Supabase Auth vs Clerk vs Keycloak: The Complete 2026 Authentication Service Comparison

Almost nobody builds a signup-and-login screen from scratch anymore. Rolling your own social login, passkeys, MFA, and SSO carries too much security risk and maintenance overhead, so most teams hand this whole layer off to a proven authentication service. The problem is that there are too many candidates, and none of them play by quite the same rules. This authentication service comparison isn’t built on marketing copy — it’s built on the numbers actually printed on official pricing pages and release notes as of August 2026.

The last six months in particular have moved this market more than most people realize. Clerk expanded its free tier 5x in February 2026, Supabase raised a $500M round (roughly ₩500 billion, at a $10.5B valuation — about ₩14 trillion) in June 2026 and got noticeably bigger as a company, and Keycloak shipped 26.7.0 with an SCIM preview and experimental multi-cluster HA. Firebase Auth, meanwhile, is still sitting on the same gap it’s had for a while: no first-party passkey support. This is not a space where a blog post from a few months ago can be trusted at face value.

This piece lines up five products side by side: Auth0, the long-standing heavyweight in enterprise SaaS auth; Firebase Authentication, riding on Google’s ecosystem; Supabase Auth, the auth module baked into a Postgres-based open-source backend; Clerk, built specifically around React/Next.js developer experience; and Keycloak, the fully free, self-hosted open-source option. We cover everything you actually need for a purchasing decision — free-tier limits, paid entry pricing, passkey/SSO support, and whether self-hosting is even on the table.

Below, we start with a five-way summary table, then go deep on each product’s features, benchmark free tiers and scalability, do a full pricing comparison, give recommendations by user type, and close with practical tips, common mistakes, and an FAQ. Anywhere a figure couldn’t be officially confirmed, we’ve flagged it plainly as “third-party estimate, not officially confirmed” rather than stating it as fact — keep that in mind when you’re building a budget around these numbers.

1. Executive Summary: The 2026 Authentication Service Comparison at a Glance

For developers short on time, here’s the essential comparison of the five major authentication services as of August 2026, compressed into one table.

📊 Authentication Service Comparison Table

The line to be most careful with in this table is “free tier.” Look only at the numbers and Firebase, Supabase, and Clerk all seem to cluster around 50,000 — but Clerk’s unit isn’t MAU at all, it’s a separate metric called MRU (Monthly Retained User), and treating the two as interchangeable will lead you to the wrong conclusion. We unpack that distinction in detail in Section 2.

2. Deep Dive: Core Features of the Five Major Authentication Services

🔐 1) Auth0 — The Benchmark for Enterprise SSO and Compliance

• The free tier has gotten noticeably more generous. The Free plan now covers up to 25,000 MAU with no credit card required, and includes unlimited logins, passwordless auth, unlimited social connections, 5 organizations, 1 enterprise connection, and even self-service SSO. Some third-party sources describe this as an increase from a previous 7,500-MAU cap, but the exact date of that change couldn’t be independently reverified.
• Paid tiers split into B2C and B2B lines. B2C Essentials starts at $35/month (billed in 500-MAU increments), and B2C Professional starts at $240/month, adding MFA, user roles, SSO integrations, and Actions/Forms extensibility. B2B Essentials starts at $150/month (includes 3 enterprise connections, $100 per additional connection, up to 30), and B2B Professional starts at $800/month (5 connections plus MFA included). Enterprise is custom-quoted and adds a 99.99% SLA and SCIM provisioning.
• Overage pricing past the MAU cap is not officially published. Auth0’s own pricing page only says you’ll be billed at “the next tier’s rate” once you cross a threshold, without stating an exact per-MAU overage price. Third-party sources cite a figure around $0.07/MAU, but that’s an unverified estimate — if you’re a growing startup, it’s worth getting this in writing from sales before you sign anything.
• Passkeys are GA on the web; native apps need re-verification. Web passkeys through Universal Login are confirmed as generally available on every plan, including the free one. Native iOS/Android passkey support, however, only shows up in older documentation with a limited track record, and whether it’s fully GA as of August 2026 needs to be checked against the latest release notes.
• The last six months have been rougher than usual on reliability. There was roughly a 2-hour-35-minute outage in the US-3 region on February 22, 2026, and an officially acknowledged ~10-minute outage on July 24, 2026 that hit four production endpoints simultaneously (US-1/3/4/5). Since there’s no self-hosting option, an outage like that translates directly into login failures on your own product.

🔥 2) Firebase Authentication — Riding the Google Ecosystem, With a Passkey Gap

• Free limits swing wildly depending on the login method. Email/social/anonymous login is free up to 50,000 MAU on both the Spark (free) and Blaze (pay-as-you-go) plans. But SAML/OIDC login drops that free allowance all the way down to 50 MAU — the moment you wire up enterprise login, you’re effectively forced onto a paid plan.
• Blaze overage pricing and SMS verification costs aren’t spelled out on the official page. The pricing page just says it “follows Google Cloud’s pricing structure” without publishing an exact per-MAU rate. Phone (SMS) verification is also billed separately and is never free. Third-party sources quote specific per-verification prices, but those aren’t officially reconfirmed, so when budgeting, treat “it’s not free” as the only certain fact and check the console directly for exact rates.
• It’s the only one of the five with no first-party passkey support. Passkey/WebAuthn support was still absent from the core managed SDK as of early 2026, leaving only workaround implementations through third-party extensions like firebase-web-authn. That’s a clear gap given that Auth0, Supabase, Clerk, and Keycloak all support passkeys in some form.
• MFA defaults to SMS. To use app-based authenticators like TOTP, you need to upgrade to the higher-tier Identity Platform layer, which makes it hard to build a tight multi-factor policy on the free plan alone.
• The last six months have been quiet. No notable pricing changes or major outages surfaced in the last three months (May–July 2026). But the lack of passkey support continues to be a recurring complaint across comparison blogs well into 2026.

🐘 3) Supabase Auth — Open-Source Auth Built Into a Postgres Backend

• The free tier is generous, but with strings attached. The Free plan covers up to 50,000 MAU permanently (no expiration), and bundles in 500MB of database storage, 1GB of file storage, 5GB of egress, and 500,000 Edge Function invocations. However, free projects auto-pause after a week of inactivity, and you can only keep 2 active projects at a time — a real constraint if you’re juggling multiple side projects.
• Paid tiers split into Pro and Team. Pro is $25/month and includes 100,000 MAU (overage billed at $0.00325/MAU), 8GB of disk, 250GB of egress, and 7-day backups. Team is $599/month and adds SOC2/ISO 27001 certification, 14-day backups, and dashboard SSO. SAML 2.0 for end users of your app is priced the same across Pro/Team, includes 50 connections, and bills $0.015/MAU past that — though this overlaps conceptually with dashboard-login SSO in a way that’s worth double-checking before you sign.
• It’s the only one of the five that offers genuine self-hosting. There’s an official stack that bundles the open-source engine GoTrue (Go, JWT-based, Apache 2.0) together with PostgREST, Realtime, Storage, Kong, and Studio via Docker Compose, so you can run it on your own infrastructure. That said, self-hosting disables the dashboard’s Auth settings panel, so you have to configure things like OAuth providers directly through environment variables in docker-compose.yml — a real UX tradeoff. In October 2025, the company said in an official GitHub discussion that it had hired a maintainer dedicated to improving self-hosting, which suggests this is a company priority going forward.
• Passkeys are the newest addition among all five products. They launched in beta in May 2026, with WebAuthn-based support for Face ID, Touch ID, Windows Hello, and security keys. MFA covers TOTP, SMS, and email-based methods together.
• A funding round has significantly scaled up the company. Supabase closed a $500M Series F on June 4, 2026, reaching a $10.5B valuation. GIC led the round, with existing investors Accel, Y Combinator, Craft, Felicis, Peak XV, and Coatue participating alongside a secondary investment from Stripe and new participation from Salesforce Ventures. The valuation jumped from $2B (April 2025) to $5B (October 2025) to $10.5B (June 2026) in just eight months, and the company itself attributes this growth partly to a surge in new databases created by AI coding tools. If you’re specifically interested in Postgres, our PostgreSQL vs MySQL vs MongoDB database comparison is worth a look too.

⚡ 4) Clerk — The Rising Star Built Around React/Next.js Developer Experience

• It made a huge free-tier expansion in early 2026. The Hobby (free) plan was raised to 50,000 MRU free effective February 5, 2026. That said, a lot of older content online still references the previous, lower cap, which can cause confusion.
• MRU is a different metric from MAU, and comparing them at face value is risky. MRU only counts users who return at least one full day after signing up, under a “First Day Free” policy that excludes signup-day activity from billing entirely. That’s a narrower metric than the MAU definitions used by Auth0, Supabase, and Firebase, so you shouldn’t treat “50K free” here the same way you’d treat “50K MAU free” on another service.
• Paid tiers are Pro at $25/month ($20/month billed annually) and Business at $300/month ($250/month billed annually), with Enterprise custom-quoted and adding a 99.99% SLA plus an optional HIPAA BAA. MRU overage pricing is officially tiered: $0.02/MRU between 50K–100K, $0.018/MRU between 100K–1M, $0.015/MRU between 1M–10M, and $0.012/MRU above 10M.
• The free plan has the broadest feature set of the bunch. Passkeys, social login, MFA, and React UI components all ship by default on the Hobby plan. Conditional UI (browser-native autofill passkey prompts) is also enabled by default, which Clerk itself credits for a high passkey adoption rate. There’s no self-hosting path, though — it’s purely managed SaaS.
• The funding and growth trajectory stand out too. Clerk closed a $50M Series C on October 15, 2025, led by Menlo Ventures and Anthropic’s Anthology Fund, bringing total funding to $134M across 7 rounds and 15 investors. The exact post-money valuation hasn’t been disclosed.

🛡️ 5) Keycloak — Fully Free, Self-Hosted, Open-Source IAM

• As of August 2026, the latest version is 26.7.0 (released July 9, 2026), adding a native SCIM API preview, experimental multi-cluster high availability (HA), and SAML step-up authentication. Over the last six months it’s shipped a steady minor-release cadence roughly every one to two months: 26.5.6 (March) → 26.6.2 (May) → 26.6.3 (June) → 26.7.0 (July).
• There’s no LTS (long-term support) version, and that matters a lot. Keycloak’s official policy is that only the latest release ever gets active development and security patches. Put off an upgrade for too long and you’re left running without security patches — teams adopting Keycloak need to plan for the operational load of chasing new versions every few months.
• It’s fully free, but infrastructure costs are separate. There’s no charge tied to user count or features at all — the only cost is whatever server and database infrastructure you run it on yourself. It’s generally known that Red Hat sells a commercial-support edition called “Red Hat build of Keycloak,” but we couldn’t reverify exact commercial support pricing for this comparison, so we’re not stating a figure.
• It’s the strongest of the five for legacy on-premises integration. It supports OIDC, OAuth 2.0, and SAML 2.0 across the board, and offers user federation that lets you plug in existing LDAP/Active Directory users as-is. Identity brokering — delegating login to upstream providers like Google, Microsoft Entra ID, or GitHub — is supported too.
• It’s the only one of the five where self-hosting is the default assumption, not an option. For organizations with data-sovereignty or on-premises requirements, it’s effectively the default open-source choice. Since you’ll need to provision your own deployment infrastructure, if you need a server to run it on in containers, our web hosting & PaaS comparison covers Docker-friendly platforms like Render and Fly.io as candidates.

3. Free Tier & Scalability Benchmark Showdown

In this authentication service comparison, the axis you actually need to check before signing anything isn’t speed — it’s “how generous is the free tier” and “how predictable is the bill once you exceed it.” Ranking purely on figures confirmed from official pages gives us this:

[Official free limits & billing transparency, as of August 2026]
Keycloak       : ⭐⭐⭐⭐⭐ Unlimited (self-hosted), no billing to speak of
Supabase Auth  : ⭐⭐⭐⭐☆ 50K MAU free + overage rate officially published ($0.00325/MAU)
Clerk          : ⭐⭐⭐⭐☆ 50K MRU free + overage rate officially published for every tier
Firebase Auth  : ⭐⭐⭐☆☆ 50K MAU free (SAML capped at 50), overage rate undisclosed
Auth0          : ⭐⭐⭐☆☆ 25,000 MAU free, overage rate undisclosed (third-party estimates only)

As the table shows, Clerk and Supabase publish exact overage pricing, which makes it possible to actually model a budget — while Auth0 and Firebase Auth only offer vague language like “next tier’s rate” or “see Google Cloud pricing.” Specific figures floating around from third-party sources (roughly $0.07/MAU for Auth0, roughly $0.0055/MAU for Firebase in the 50K–100K range, etc.) aren’t officially confirmed estimates, so if you’re expecting large-scale traffic, getting a direct quote from sales before you sign is really the only safe approach.

Looking at the free tier number alone, Firebase, Supabase, and Clerk all cluster around 50,000 — but as covered above, Clerk’s MRU is a narrower metric than MAU. If your product has a low return-visit rate relative to new signups, Clerk’s “50K free” can actually feel more generous in practice than another service’s “50K MAU free.” That’s exactly why, in this authentication service comparison, you shouldn’t judge purely by the raw numbers — you need to weigh them against your own product’s return-visit pattern.

4. Complete Pricing Comparison

[Barrier to paid entry — sorted by minimum monthly cost, per official pricing pages]
Keycloak                 : $0/month    (infrastructure cost only, no billing at all)
Clerk (Pro)              : $25/month   ($20/month billed annually, usage-based past 50K MRU)
Supabase Auth (Pro)      : $25/month   (includes 100K MAU, overage $0.00325/MAU)
Auth0 (B2C Essentials)   : $35/month   (billed in 500-MAU blocks)
Auth0 (B2B Essentials)   : $150/month  (includes 3 enterprise connections)
Clerk (Business)         : $300/month  ($250/month billed annually)
Auth0 (B2B Professional) : $800/month  (5 enterprise connections + MFA included)
Supabase Auth (Team)     : $599/month  (SOC2/ISO 27001, 14-day backups)

💰 Detailed Pricing Comparison Table (per official pages, verified August 2026)

Three hidden-cost issues are worth calling out. First, Auth0’s overage billing past the MAU cap is close to unpredictable. Since the official pricing page never publishes an exact rate, if you run a seasonal product with traffic spikes, you need to get the overage rate written into your contract at signing time. There are individual community reports of people migrating away after a pricing overhaul led to a much larger-than-expected bill — that’s not an official company statement, just user-reported anecdotes, so it shouldn’t be generalized, but it’s worth keeping in mind. Second, don’t treat Clerk’s MRU metric like a bandwidth number. Since activity on signup day itself doesn’t count toward billing, a product with high onboarding drop-off could end up with far fewer billable users than its raw signup count suggests. Third, Keycloak’s “fully free” doesn’t include labor costs. With no LTS, you have to chase new versions every few months, and once you convert that operational effort into engineering hours, it can end up costing a small team more than a managed SaaS would.

5. Final Recommendations by User Type

🎯 1) A solo developer just starting a side project

• Best pick: Clerk
• Why: The free (Hobby) plan gives you passkeys, social login, MFA, and React UI components right out of the box, letting you ship a polished login screen faster than anywhere else without paying a cent. Within the 50K MRU cap, you’re essentially free of cost concerns.

🎯 2) A startup shipping an MVP quickly on React/Next.js

• Best pick: Clerk
• Why: With framework-native components built in, you can implement login, signup, and profile management UI in just a few lines of code. If the MRU metric feels unfamiliar, it’s worth checking your actual billable user count directly in the dashboard early on.

🎯 3) A team running a Postgres-based full-stack SaaS that also wants a self-hosting option open

• Best pick: Supabase Auth
• Why: Database, storage, and auth are unified on one platform, and it’s the only option that lets you move the open-source GoTrue stack onto your own infrastructure if you ever need to. If you want to dig deeper into Postgres itself, our PostgreSQL vs MySQL vs MongoDB database comparison is a useful companion read.

🎯 4) A B2B SaaS where enterprise SSO, SAML, and compliance are non-negotiable

• Best pick: Auth0
• Why: Self-service SSO is available starting from the free plan, and the B2B Professional tier bundles in 5 enterprise connections and MFA by default. Enterprise-tier SCIM provisioning and advanced compliance certifications are the most mature among all five products.

🎯 5) A mobile app already built on the Firebase/Google Cloud ecosystem

• Best pick: Firebase Authentication
• Why: Integration with existing Google infrastructure like Firestore and Cloud Functions is seamless. Just go in aware that passkeys aren’t in the core SDK — if passwordless login is a must-have, evaluate adopting a third-party extension first.

🎯 6) A public-sector or financial institution that requires data sovereignty, or an organization stuck with legacy LDAP/AD

• Best pick: Keycloak
• Why: It’s completely free and open source, with SAML 2.0 and LDAP/Active Directory federation built in by default, making it the strongest choice for integrating with legacy on-premises systems. If you still need to stand up your own self-hosting infrastructure, our web hosting & PaaS comparison covers container-friendly platforms worth considering alongside it.

6. Practical Tips & Common Mistakes

✅ Tip 1) Never compare Clerk’s MRU to another service’s MAU at face value

Don’t treat Clerk and Supabase as equivalent just because both say “50,000 free.” MRU only counts users who return at least one full day after signing up, excluding signup-day activity entirely — a narrower metric. If your product has high onboarding drop-off, your actual billable user count could be far below your total signup count, so check your own D1 return-visit rate before signing anything.

✅ Tip 2) Get Auth0’s overage rate in writing before you commit

Since the official page only says “next tier’s rate” without disclosing an exact per-MAU price, if there’s any chance of a traffic spike, getting that overage rate written into your contract during sales negotiations is the only real safeguard. There are community reports of people getting bills far larger than expected after neglecting to do exactly this.

✅ Tip 3) If you’re using Firebase Auth, plan around the passkey gap from day one

Finding out about the lack of first-party passkeys later means expensive refactoring. If passwordless login is on your roadmap, either build a third-party extension into your architecture from the start, or, if passkeys are non-negotiable for your project, just evaluate Clerk, Auth0, Supabase, or Keycloak instead from the outset.

✅ Tip 4) Supabase self-hosting is configured through environment variables, not the dashboard

Once you run GoTrue on your own infrastructure, the dashboard’s Auth settings panel is disabled, unlike the managed version. OAuth provider keys and the like have to go directly into environment variables in docker-compose.yml, so if you go in expecting the managed dashboard’s UX, you’ll likely burn some time re-reading the docs early on.

✅ Tip 5) Factor “no LTS” into your operational plan before adopting Keycloak

Keycloak only ever patches the latest version for security. With minor versions shipping roughly every one to two months (like 26.7.0), delaying an upgrade for several months means running exposed to known vulnerabilities. It’s worth putting a quarterly upgrade schedule on the calendar in advance.

✅ Tip 6) If you plan to leave a free project idle for a long time, remember Supabase’s auto-pause

Supabase’s Free plan auto-pauses a project after a week of inactivity. If you want to keep a demo or portfolio project running long-term, either send periodic minimal health-check requests, or plan to move to Pro once real usage kicks in.

7. Frequently Asked Questions (FAQ)

Q. What’s the first thing to check in an authentication service comparison?

Two things: “do I need self-hosting” and “do I need enterprise SSO/SAML right now.” If you have data-sovereignty or on-premises requirements, that narrows the field to Keycloak; if you need SSO starting from the free tier, Auth0 has the edge. If neither of those applies to you, move on to comparing free-tier limits and developer experience across Clerk, Supabase, and Firebase.

Q. Which of the five is the best place to start for free?

Purely on feature breadth, Clerk’s Hobby plan is the most generous — passkeys, social login, MFA, and UI components, all free. But if you want something completely free and don’t mind running your own server, Keycloak is a solid alternative, and if you’re planning to use a Postgres backend anyway, Supabase’s free tier (50K MAU) is also a strong contender.

Q. Which is cheaper, Auth0 or Clerk?

Based on official pricing pages, Clerk is cheaper to get into — Clerk Pro at $25/month versus Auth0 B2C Essentials at $35/month. Some third-party sources claim “Auth0 costs several times more than Clerk” at large usage volumes, but since the underlying methodology behind those multiples isn’t disclosed, the exact figures aren’t reliable. Getting a direct quote from each company’s sales team based on your projected MAU/MRU is the most accurate approach.

Q. Can Supabase Auth be self-hosted?

Yes. There’s an official stack that bundles the open-source engine GoTrue (Apache 2.0) with PostgREST, Realtime, Storage, Kong, and Studio via Docker Compose, letting you run it on your own infrastructure. Note that self-hosting disables the dashboard’s Auth settings panel, so things like OAuth providers need to be configured directly through environment variables.

Q. Does Firebase Auth support passkeys?

The core managed SDK itself had no first-party passkey support as of early 2026. Only workaround implementations via third-party extensions like firebase-web-authn are possible, which is widely cited as a clear feature gap compared to the other four products.

Q. Keycloak is completely free — is there no commercial support option?

Keycloak itself is completely free, with no charges based on user count or features. It’s generally known that Red Hat sells a commercial-support edition called “Red Hat build of Keycloak,” but we weren’t able to officially confirm exact commercial support pricing for this piece — if you need it, contacting Red Hat’s sales team directly is the way to go.

Q. How exactly does Clerk’s MRU differ from MAU?

MRU (Monthly Retained User) only counts users who return at least one full day after signing up, and signup-day activity is excluded from billing entirely under the “First Day Free” policy. MAU, by contrast, typically counts every unique user active in a given month. Since MRU will always be equal to or lower than MAU, you shouldn’t compare a “50,000 free” figure against other services on the same footing.

Q. Is it easy to migrate away from Auth0 to another service?

Supabase publishes an official Auth0-to-Supabase migration guide, and there’s a documented technical blog case study of someone migrating around 125,000 users (though it’s an individual’s account, not an official company case study). Given that other vendors like WorkOS also publish separate Auth0 migration guides, there does seem to be steady demand in this market from people looking to leave Auth0. That said, migrations involve plenty of edge cases around things like session and password-hash handling, so it’s worth starting with a small pilot.

8. Conclusion: The Practical Takeaway on the 2026 Authentication Service Comparison

As of August 2026, the conclusion of this authentication service comparison isn’t “one right answer” — it’s closer to “pick your priority axis first.” If you’re one of the many startups that doesn’t need self-hosting, choose between Clerk’s broad free feature set and Supabase’s Postgres integration based on your framework preference. If enterprise SSO is a day-one requirement, Auth0 is still the most mature choice. And if data sovereignty or legacy LDAP integration is a hard requirement, there really isn’t much of an alternative to Keycloak from the start.

Trusting any single number blindly when building a budget is especially risky in this market. Neither Auth0 nor Firebase Auth officially publishes overage pricing, and Clerk’s free-tier limit is measured in MRU, a separate metric from MAU. Before signing anything, always re-verify the official pricing page, and get any ambiguous point in writing from sales. If you’re evaluating a move to passkeys, our password manager comparison is also worth a read for a broader picture of the passwordless experience from the user’s side.

[Summary: the right pick for your situation]

Just-starting side project / React & Next.js MVP
  → Clerk (Hobby free tier, passkeys/social/MFA all free up to 50K MRU)

Postgres full-stack SaaS, want a self-hosting option too
  → Supabase Auth (Free tier 50K MAU, GoTrue open-source self-hosting available)

B2B SaaS where enterprise SSO/SAML/compliance is non-negotiable
  → Auth0 (self-service SSO from free, B2B Professional at $800/month)

Mobile app already on the Firebase/GCP ecosystem
  → Firebase Auth (50K MAU free, just account for the passkey gap)

Data-sovereignty-bound institution, legacy LDAP/AD integration
  → Keycloak (completely free, self-hosting is the default, watch out for no LTS)

Decide first which axis (self-hosting, SSO, framework DX, free-tier limits) matters most to your product right now, then lay this authentication service comparison on top of that decision — that combination is what actually cuts down on surprise bills and migration risk down the line.