Release v0.27.0: security hardening (audit 0026)

Remediates the rfc-app application + deploy-config findings from the
Session 0026 security audit. Cut as the "v0.25.0-security-hardening"
branch (from v0.24.0); reversioned to 0.27.0 on rebase onto main since
v0.26.0 (#28) shipped while this was in flight.

- C1 (Critical): single sanitizeHtml.js chokepoint (DOMPurify) for every
  marked→innerHTML / dangerouslySetInnerHTML sink (MarkdownPreview,
  ProposalView x2, Editor); rel=noopener hook on target=_blank links.
- H1: per-account OTC-verify lockout (migration 023, auto-applied) +
  per-IP throttle via new ratelimit.py; wired on otc verify/request +
  passcode check/verify.
- M1: device_trust.lookup() single indexed-row read — cookie value is now
  "<row_id>.<raw_token>"; bcrypt-checks one row, not a global table scan.
  (Behavior change: existing device-trust cookies re-prompt once.)
- M2: HTTP security headers (CSP/HSTS/XFO/XCTO/Referrer-Policy) at nginx.
- M4: session cookie Secure-by-default (SESSION_COOKIE_SECURE opt-out).
- M5: bounce webhook fails CLOSED (503) when secret unset, instead of open;
  RFC_APP_INSECURE_BOUNCE_WEBHOOK=1 dev opt-in.
- L2/L3: per-IP cooldown + check-endpoint throttle.
- L4: systemd sandbox knobs. L8/I1: nginx server_tokens off + TLS1.0/1.1 out.

VERSION + frontend/package.json → 0.27.0; CHANGELOG documents the upgrade
steps (incl. the out-of-band nginx + systemd apply, which the flotilla
deploy gesture does not perform).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/otc.py

@@ -85,6 +85,15 @@ def _cooldown_seconds() -> int:
         return 60
 
 
+# v0.25.0 / security audit 0026 (H1): per-email OTC verify lockout,
+# mirroring the passcode path (passcode.py). Five consecutive wrong codes
+# for an email lock its OTC verify for 15 minutes. The per-IP limiter in
+# ratelimit.py is the primary brute-force brake; this is the durable,
+# passcode-parity layer.
+LOCKOUT_AFTER_FAILED_ATTEMPTS = 5
+LOCKOUT_DURATION_MINUTES = 15
+
+
 # ---------------------------------------------------------------------------
 # Code generation + hashing
 # ---------------------------------------------------------------------------
@@ -206,6 +215,61 @@ class VerifyOutcome:
     ok: bool
     user: SessionUser | None
     reason: str
+    # v0.25.0 (H1): ISO-8601 stamp when reason == 'locked'.
+    locked_until: str | None = None
+
+
+def _verify_lockout_until(email: str) -> str | None:
+    """Return the active lockout stamp for `email`, or None if not locked.
+
+    Clears an elapsed lockout (and resets the counter) as a side effect so
+    the next failure starts a fresh budget — mirrors passcode.verify_passcode.
+    """
+    row = db.conn().execute(
+        "SELECT failed_attempts, locked_until FROM otc_verify_state WHERE email = ?",
+        (email,),
+    ).fetchone()
+    if row is None or not row["locked_until"]:
+        return None
+    still_locked = db.conn().execute(
+        "SELECT datetime(?) > datetime('now') AS locked", (row["locked_until"],),
+    ).fetchone()["locked"]
+    if still_locked:
+        return row["locked_until"]
+    db.conn().execute(
+        "UPDATE otc_verify_state SET failed_attempts = 0, locked_until = NULL WHERE email = ?",
+        (email,),
+    )
+    return None
+
+
+def _record_verify_failure(email: str) -> None:
+    """Increment the per-email failure counter; stamp a lockout once it
+    crosses the threshold. Mirrors the passcode lockout shape."""
+    db.conn().execute(
+        """
+        INSERT INTO otc_verify_state (email, failed_attempts)
+        VALUES (?, 1)
+        ON CONFLICT(email) DO UPDATE SET failed_attempts = failed_attempts + 1
+        """,
+        (email,),
+    )
+    count = db.conn().execute(
+        "SELECT failed_attempts FROM otc_verify_state WHERE email = ?", (email,),
+    ).fetchone()["failed_attempts"]
+    if count >= LOCKOUT_AFTER_FAILED_ATTEMPTS:
+        db.conn().execute(
+            f"""
+            UPDATE otc_verify_state
+               SET locked_until = datetime('now', '+{LOCKOUT_DURATION_MINUTES} minutes')
+             WHERE email = ?
+            """,
+            (email,),
+        )
+
+
+def _clear_verify_state(email: str) -> None:
+    db.conn().execute("DELETE FROM otc_verify_state WHERE email = ?", (email,))
 
 
 def verify_code(email: str, code: str) -> VerifyOutcome:
@@ -214,6 +278,13 @@ def verify_code(email: str, code: str) -> VerifyOutcome:
     if not email or not code:
         return VerifyOutcome(ok=False, user=None, reason="invalid")
 
+    # v0.25.0 (H1): refuse before spending any bcrypt if this email is in
+    # its OTC-verify lockout window. The passcode path is unaffected — a
+    # locked-out OTC user can still set/use a passcode, and vice versa.
+    locked_until = _verify_lockout_until(email)
+    if locked_until:
+        return VerifyOutcome(ok=False, user=None, reason="locked", locked_until=locked_until)
+
     rows = db.conn().execute(
         """
         SELECT id, code_hash, expires_at, consumed_at
@@ -237,6 +308,9 @@ def verify_code(email: str, code: str) -> VerifyOutcome:
             break
 
     if matched is None:
+        # A genuine wrong guess against this email — the brute-force
+        # signal. Count it toward the lockout threshold (H1).
+        _record_verify_failure(email)
         return VerifyOutcome(ok=False, user=None, reason="wrong")
 
     if matched["consumed_at"] is not None:
@@ -255,6 +329,8 @@ def verify_code(email: str, code: str) -> VerifyOutcome:
         "UPDATE otc_codes SET consumed_at = datetime('now') WHERE id = ?",
         (matched["id"],),
     )
+    # Success wipes the per-email failure counter (H1).
+    _clear_verify_state(email)
     user = provision_or_link_user(email)
     return VerifyOutcome(ok=True, user=user, reason="ok")