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Security Model

This page documents how MFB's security-relevant controls actually work: credential encryption, defenses against server-side request forgery (SSRF), the read-only IMAP boundary, multi-tenant isolation, OAuth CSRF protection, and the reserved restore username prefix. It describes the current model, not a history of fixes.

Credential encryption

IMAP credentials (passwords and OAuth2 refresh tokens) are stored encrypted in PostgreSQL using Fernet symmetric encryption (src/mailfallback/security.py). The Fernet key is not SECRET_KEY itself — it is derived from it with PBKDF2-HMAC-SHA256 (600,000 iterations, a fixed application-specific salt):

_KDF_ITERATIONS = 600_000
_KDF_SALT = b"mailfallback-fernet-v1"

def _derive_fernet_key(secret_key: str) -> bytes:
    dk = hashlib.pbkdf2_hmac("sha256", secret_key.encode(), _KDF_SALT, _KDF_ITERATIONS)
    return base64.urlsafe_b64encode(dk)

Operator duty: set a strong SECRET_KEY

The entire scheme's strength rests on SECRET_KEY. A short or guessable value makes the PBKDF2 derivation brute-forceable offline. Generate a long random value (e.g. openssl rand -base64 32) and treat it like any other root secret — losing it makes stored credentials unrecoverable; leaking it exposes them.

Legacy KDF self-heal

An older, weaker key derivation (unsalted SHA-256 of SECRET_KEY) predates the PBKDF2 scheme. decrypt_credentials still accepts ciphertext produced by it: a modern decrypt is tried first, and only on InvalidToken does it fall back to the legacy derivation. Rather than leaving legacy ciphertext in place, every read path upgrades it in-line. account_service.decrypt_account_credentials is the sanctioned entry point for reading an account's credentials — sync, restore, and UI code call it instead of decrypt_credentials directly:

def decrypt_account_credentials(db: Session, account: Account) -> str | None:
    if not account.credentials:
        return None
    plaintext, upgraded = decrypt_credentials_with_upgrade(account.credentials, settings.secret_key)
    if upgraded is not None:
        account.credentials = upgraded
        db.commit()
    return plaintext

decrypt_credentials_with_upgrade decrypts once with the modern key; if that fails, it decrypts with the legacy key and immediately re-encrypts the plaintext with the modern key, returning the new ciphertext for the caller to persist. The net effect: every account whose credentials are actually read gets migrated off the weak KDF automatically, with no batch migration step required. Rows that are never read stay on the legacy KDF until they are.

SSRF defenses

Account IMAP hosts are user-supplied and are used to open outbound TCP/TLS connections from the server — both for one-off "test connection" calls and for the recurring mbsync sync itself. Without guardrails, an attacker with account-edit access could point imap_host at an internal service (e.g. the PostgreSQL container, the Docker metadata endpoint, a private admin panel) and use MFB as an SSRF proxy. src/mailfallback/services/imap_check.py implements the defense in three layers.

1. Host validation

validate_host_not_internal(host) resolves the hostname and rejects it if any resolved address is:

  • private (RFC 1918), loopback, or link-local
  • CGNAT (100.64.0.0/10 — not covered by Python's is_private, so checked explicitly)
  • reserved, multicast, or unspecified
  • an IPv4-mapped IPv6 address wrapping an internal IPv4 (::ffff:10.0.0.1) — unwrapped before the check so a crafted AAAA record can't smuggle an internal address past it
  • unresolvable at all — a gaierror is treated as a rejection, not silently passed through, because a swallowed resolution failure at validation time followed by a successful resolution at connection time is itself an SSRF bypass window

2. IP pinning for untrusted connects

Passing host validation once is not enough on its own: DNS is looked up again when the connection is actually opened, and an attacker controlling the DNS record (or exploiting the propagation window) could return a public IP for the validation lookup and an internal IP for the connection lookup — DNS rebinding.

For untrusted, user-supplied hosts (test-connection, account create/edit), connect_imap(..., pin_public_ip=True) closes this gap by resolving once via resolve_public_ip, validating that result, and then connecting directly to that pinned IP — _PinnedIMAP4SSL / _PinnedIMAP4 open a raw socket to the IP rather than letting imaplib re-resolve the hostname. The original hostname is still passed to the TLS layer for SNI and certificate verification, so TLS correctness isn't sacrificed for the pin:

class _PinnedIMAP4SSL(imaplib.IMAP4_SSL):
    def __init__(self, host, port, pinned_ip, timeout):
        self._pinned_ip = pinned_ip
        super().__init__(host, port, timeout=timeout)

    def _create_socket(self, timeout=None):
        return socket.create_connection((self._pinned_ip, self.port), timeout)

3. Sync-time re-validation

An account's host can be valid at creation time and later start resolving to an internal address (the operator's DNS changes, or a deliberately delayed rebind). sync_worker re-runs validate_host_not_internal(account.imap_host) immediately before every sync, and fails the job (sync_state = error) rather than invoking mbsync if the host now resolves internally.

Documented residual: mbsync's own resolution

The sync-time check and the moment mbsync itself opens a connection are not atomic — mbsync (an external binary, not imap_check's pinned-IP path) re-resolves the hostname when it connects. A rebind timed precisely between MFB's check and mbsync's own DNS lookup is a narrow, bounded TOCTOU window that isn't closed without pinning mbsync's connection too, which would give up TLS hostname verification against the real target. This is accepted as a residual risk, not treated as fixed.

Read-only IMAP via Dovecot ACL

MFB's Dovecot configuration is not hand-maintained — it's generated at container boot by src/mailfallback/services/config_generator.py::generate_dovecot_config and written into the dovecot_confd volume that the Dovecot container mounts conf.d from. This makes the ACL policy part of MFB's own deployed state rather than something an operator could drift out of sync with the data model.

The generator writes a global ACL (acl_globals_only = yes, acl_global_path = /etc/dovecot/conf.d/dovecot-acl) whose default rule grants the mailbox owner only:

  • l — lookup (see the mailbox exists)
  • r — read (fetch messages)
  • s — write-seen (mark read/unread)

No insert, expunge, flag-write, or mailbox create/delete rights are granted by default — a logged-in IMAP user (Roundcube or any other client against Dovecot) cannot delete, modify, or add to a synced mailbox's replica.

Staging is the one deliberate exception

The generated ACL file also grants the per-user Staging namespace broader rights (lrwstie: lookup/read/write-flags/write-seen/write-deleted/insert/expunge). Staging is the restore workflow's curation surface — where a user reviews and deletes-before-push a set of recovered messages prior to pushing them to a live account — and is never populated by mbsync or included in the always-on local backup semantics. Every other namespace stays lrs-only.

mbsync is unaffected by any of this: it writes to the Maildir filesystem directly on the host, bypassing Dovecot and its ACL layer entirely. The ACL only constrains IMAP clients that connect through Dovecot.

Multi-tenant isolation

MFB is multi-user: one deployment can host many accounts owned by different users, and users must not be able to see or act on accounts they don't own. Ownership is modeled two ways, both checked by account_service:

  • Direct ownership — the account_owners join table (many-to-many: an account can have several owners, a user can own several accounts).
  • Group membership — account_groups + group_members; a group's accounts are visible to all of its members without being individually owned by them.

MFB enforces multi-tenant isolation through two access-control gates in account_service:

  • get_account (read/shared-access gate): returns the account for admins, direct owners, or group members. Used for read-only and operational routes (account detail, sync status, snapshots, backup history).
  • get_account_for_modify (write gate): returns the account for admins and direct owners only — not group members. Used for account settings changes (name, IMAP host, credentials, sync schedule).

This distinction means:

  • Account settings (edit IMAP host, change sync schedule, etc.) require direct ownership or admin role. Group members have no write access to account configuration.
  • Backup/restore/recovery operations (configure off-site policy, back up now, restore a snapshot or attachment, delete a recovery) are intentionally available to group members via get_account — shared group access grants operational access to an account's off-site backup surface. This is by design.
  • Account ownership itself is admin-only (require_admin on /accounts/{id}/owners endpoints), so users cannot self-grant or self-revoke ownership.

All these endpoints are account-scoped: routes that call get_account or get_account_for_modify return 404 rather than leaking whether an account ID exists to an unauthorized user. Before touching any account's data, the endpoint resolves it for the requesting user. One tenant therefore cannot reach another's mailbox, snapshots, or recoveries by guessing an account ID (an IDOR guard).

OAuth CSRF protection

Google and Microsoft account linking uses the OAuth2 authorization-code flow (routers/auth.py). Before redirecting to the provider, MFB generates a random state nonce and stores it server-side in the session (request.session["oauth_state"] = nonce). On the callback, the state returned by the provider is compared against the stored value before any destructive action is taken:

expected_state = request.session.pop("oauth_state", None)
# Validate state BEFORE any destructive action (see google callback).
if not state or not expected_state or state != expected_state:
    ...(db, request, account_id, reason="invalid_state", delete_stub=False)

Both the Google and Microsoft callbacks follow this order deliberately: on a state mismatch, the pending account stub is not deleted (delete_stub=False). Without this check, a forged cross-site callback (an attacker driving a victim's browser to MFB's callback URL with an attacker-chosen code) could otherwise trigger cleanup of the victim's in-progress account-linking state. Only a genuine, state-validated failure proceeds to clean up the stub account.

Reserved _restore_ username prefix

Restoring a snapshot back into a live mailbox (IMAP→IMAP RestoreJob) works by creating a short-lived Dovecot/IMAP user scoped to the accounts being restored, so restic/Tika-adjacent tooling can push through the normal IMAP surface rather than needing direct filesystem access. src/mailfallback/services/dovecot_auth.py reserves the prefix:

TEMP_USER_PREFIX = "_restore_"

create_temp_imap_user names every temporary user _restore_{random hex}, and delete_temp_imap_user / cleanup_temp_imap_users — the latter run periodically to sweep temp users older than one hour — only ever operate on usernames matching that prefix:

def delete_temp_imap_user(db: Session, username: str) -> None:
    if not username.startswith(TEMP_USER_PREFIX):
        return
    ...

Because deletion is gated on the prefix rather than on "the user this restore job created," the prefix has to stay unreachable from every other user-creation path, or cleanup could delete a real user by mistaking it for an expired temp user. Both paths that create users from untrusted input enforce this explicitly:

  • user_service.create_user (admin-created users) raises ValueError if the requested username starts with _restore_, refusing creation outright.
  • The OIDC login path (routers/auth.py) derives a username from the identity provider's claims and rewrites it — prefixing with u — if it happens to start with _restore_, rather than rejecting the login:
# Never let an IdP-supplied name collide with the reserved restore-user
# prefix -- cleanup_temp_imap_users would later delete it as an orphan.
if username.startswith(TEMP_USER_PREFIX):
    username = f"u{username}"