Two-Factor Authentication

Scope: the self-service account surface (/rustfs/admin/v3/account/*), the login gate on AssumeRole, and the administrative reset (/rustfs/admin/v3/user/mfa).

This document records what the second factor does and does not protect, and why. The boundaries are deliberate; several of them look like gaps until the alternative is spelled out.

What is protected

TOTP gates session minting: the AssumeRole call that turns a long-term credential into a short-lived STS session. That is the only interactive login RustFS has — the Console holds nothing but an STS session, and obtains it by signing an AssumeRole request with the access key the user typed.

When an identity has an active enrollment:

access key + secret key
        │
        ▼
GET /v3/mfa/challenge          (SigV4-signed; answers "is a factor needed?")
        │  required: true
        ▼
POST /  Action=AssumeRole
        SerialNumber=<challenge>
        TokenCode=<6-digit TOTP | recovery code>
        │
        ▼
STS credentials, with the claim  x-rustfs-mfa-verified: true

Without a TokenCode, AssumeRole fails with AccessDenied and a message carrying the MultiFactorAuthRequired marker. Clients match on that marker to prompt for a code rather than reporting a failed login — the password was accepted.

SerialNumber and TokenCode are AssumeRole's own parameters, so an SDK or a script authenticates the same way the Console does, with no RustFS-specific protocol.

What is deliberately not protected

A request signed directly with a long-term access key is not gated. This is the most important boundary in the design, and it is intentional:

• Gating it would break every script, SDK client and rc invocation the moment a human enabled 2FA on their own account. An operator who turned on a security feature would discover it by way of a production outage.
• It would add no protection. Whoever holds the secret key already has full access to everything that identity can reach; they never need to present a code, because they never need to mint a session.

This is the same division AWS draws: MFA gates AssumeRole and is enforced for API calls through the aws:MultiFactorAuthPresent policy condition, not by refusing signed requests.

Consequence to state plainly: 2FA raises the cost of a stolen password. It does not contain a stolen secret key. Because in RustFS the password is the S3 secret key (see below), those are the same string — so 2FA protects the console login path against credential reuse and phishing, and nothing more, until the policy-condition work lands.

The tracked follow-up is an aws:MultiFactorAuthPresent condition key populated from the x-rustfs-mfa-verified session claim, which would let an operator write a policy that denies administrative actions to a session that presented no second factor. That is the mechanism that makes 2FA meaningful for API access.

OIDC and Keystone sessions are not gated either. Those identities are authenticated by their provider; a RustFS-side TOTP enrollment would not be consulted at login and would give a false impression of protection. MFA for a federated identity belongs to its IdP. CallerIdentity reports such sessions as FederatedIdentity and refuses enrollment.

Service-account credentials cannot manage their parent's factor. A machine credential must not be able to take over the human identity it was minted from.

Password reality: there is no password hash

RustFS is an S3 server. SigV4 requires the server to know the secret key itself in order to recompute a request signature, so secret keys cannot be hashed — not here, and not in any S3-compatible implementation. The "password" a user types into the Console is their S3 secret key.

What protects it instead:

There is deliberately no maximum length. The previous Console capped passwords at 40 characters, which was a client-side invention with no server constraint behind it; capping password length is an anti-pattern.

At-rest protection is mandatory for TOTP secrets

A TOTP secret is credential-equivalent: anyone holding it can mint valid codes forever. So enrollment is refused when RUSTFS_IAM_MASTER_KEY is not configured, rather than writing the secret in plaintext:

POST /v3/account/mfa/enroll  →  501 NotImplemented
"two-factor authentication requires RUSTFS_IAM_MASTER_KEY to be configured
 so the shared secret can be encrypted at rest"

IAM identities tolerate a missing master key for backward compatibility with existing deployments. A new feature has no such history to honour, and a second factor that can be lifted off a disk is worse than none, because the user believes they have one.

GET /v3/account/mfa reports enrollment_available: false with the reason, so the Console and rc explain the remedy instead of offering a control that fails.

Root credentials cannot be changed at runtime

The root identity comes from RUSTFS_ACCESS_KEY / RUSTFS_SECRET_KEY and lands in a process-wide OnceLock (crates/credentials/src/credentials.rs). It cannot be rotated while the server runs, and the account surface reports this as credentials_source: "env" with mutable.password: false.

This is not merely a missing feature. The root secret key feeds three things:

1. STS session token signing (root_credentials::token_signing_key) — every live session in the cluster is HMAC-signed with it.
2. The internode RPC secret (derive_rpc_secret), unless RUSTFS_RPC_SECRET is set explicitly.
3. Legacy IAM at-rest decryption for blobs migrated from MinIO.

Rotating it at runtime would therefore invalidate every session cluster-wide and break node-to-node authentication. Making root mutable is a separate piece of work with those three couplings as prerequisites; it is not a side effect of adding a profile page.

Operational recommendation: treat root as a bootstrap identity. Create a built-in IAM user with the consoleAdmin policy for day-to-day administration. That identity has a working password change and full 2FA support.

Rate limiting, replay and expiry

A wrong code, a replayed code and a malformed code are indistinguishable on the wire: all three return AccessDenied with the same message. The distinction survives only in the audit trail, so an operator can tell a guessing attempt from a replay without an attacker learning that a captured code was genuine.

The lockout is stored in the record and updated under an optimistic compare-and-set, so it holds across the cluster rather than per node.

Storage

.rustfs.sys/config/mfa/<access-key>/totp.json     (encrypted with the IAM master key)

A sibling of config/iam/, not a child: the IAM cache loader walks the whole config/iam/ tree on startup and buckets what it finds by first path segment, so a new prefix under there would be swept into that walk for no benefit.

Records are not cached. Every verification reads from the store, because a cache would need cluster-wide invalidation to keep the replay mark and the lockout counter honest, and getting that wrong reopens exactly the holes this design closes. Verifications are rare enough that the read is not worth optimising.

Writes are read-modify-write under an If-Match precondition with bounded retries — the same optimistic scheme the IAM lazy-rewrite path uses. It degrades to a retry rather than to a distributed lock a crashed node would have to time out.

Login challenges are stateless

A challenge is HMAC-SHA256(root_secret, "rustfs-mfa-challenge:v1" ‖ access_key ‖ issued_at), base64url-encoded with its payload.

The obvious alternative is a TTL cache, the way the OIDC flow stores its PKCE verifiers. That store is node-local, which is fine for OIDC because the whole authorization round trip returns to the node that started it. A second factor does not: a cluster behind a load balancer without session affinity would issue the challenge on one node and receive the code on another, and a node-local challenge would fail there for reasons no operator could debug.

Statelessness costs nothing, because the challenge is not what makes the exchange single-use — the consumed TOTP time step is.

Authorization model

The self-service routes carry no admin action. Giving them one would be wrong in both directions: it would stop an ordinary user from changing their own password, and it would let any holder of that action change somebody else's. They are registered as CredentialOnly in the route-policy matrix.

POST /v3/account/password and POST /v3/account/mfa/disable require a proof-of-knowledge step because a signature only proves a credential was used. The Console signs with a short-lived session, so without it a hijacked browser tab could rewrite the account's credentials or strip its second factor.

Why turning the factor off needs the password too

Requiring only a code would mean a single shoulder-surfed number, in a session someone walked away from, is enough to remove the protection. Requiring the password makes disabling the factor as hard as the thing the factor protects.

Break-glass

DELETE /v3/user/mfa clears another identity's factor, for a user who lost both their authenticator and their recovery codes. It is gated on admin:EnableUser rather than a bespoke action, because that is the same capability that can already re-enable a disabled account — anyone who can do that can already take the identity over, so a separate action would be a distinction without a security difference.

The record is deleted outright rather than disabled, so no stale lockout counter survives to block the user's next enrollment. The acting administrator is recorded in the audit entry.

Recovery codes

Ten codes, XXXX-XXXX-XXXX-XXXX-XXXX, 100 bits of uniform randomness each, in a Crockford base32 alphabet with I, L, O and U removed so a handwritten code cannot be ambiguous.

Stored as domain-separated SHA-256 digests, not a password KDF. With 100 bits of uniform randomness there is no dictionary to try and no human-chosen pattern to exploit, so the attacks a slow KDF defends against do not apply — while a memory-hard KDF would have to run once per stored code on every verification attempt, turning each guess into an attacker-controlled multiple of that cost. This is the standard treatment for high-entropy bearer tokens, and the same reasoning is why there is no per-code salt.

Codes are returned in plaintext exactly once. Activation always replaces the set: reusing a previous one would leave codes valid for a secret they were never issued against. Disabling clears them, so no live bypass survives a factor the user believes is gone.

Audit

Two EventName variants carry the whole surface:

• iam:Identity:CredentialChanged — password rotation, enrollment, activation, disable, recovery-code regeneration, administrative reset.
• iam:Identity:AuthChallenge — challenge issuance and second-factor verification.

The per-operation detail lives in api.name and the iamOperation tag, which is what a SIEM filters on. The enum is coarse because EventName::mask() gives every variant its own bit in a u64 and the budget is nearly spent — 63 of 64 used after these two. Splitting these per-operation needs mask() widened first.

Redaction: no secret key, TOTP secret, provisioning URI, submitted code or recovery code enters an audit entry — not even hashed, and not on the failure paths where the submitted value would be the most tempting thing to record. Failures are described by a closed set of static strings (AccountAuditFailure), so no caller-supplied bytes can reach a log target through this module.

Known limitations

1. 2FA does not gate direct SigV4 access. By design; see above. The fix is the aws:MultiFactorAuthPresent policy condition.
2. Root cannot rotate its own credentials at runtime. By design; see above.
3. GHSA-m77q-r63m-pj89 is unaffected. STS session tokens are signed with the root secret key, so anyone holding it can still forge a session token — including one carrying x-rustfs-mfa-verified. 2FA does not close this; a dedicated STS signing key does, and that advisory is tracked separately.
4. Username changes are not supported for anyone. The access key is the primary key for policy mappings, group membership, service-account parents and bucket-policy principals. A rename is a migration that orphans service accounts and silently breaks bucket-policy ARNs, not an edit; mutable.username is false for every identity.