diff --git a/backend/tests/test_auth_session.py b/backend/tests/test_auth_session.py
new file mode 100644
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+++ b/backend/tests/test_auth_session.py
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+"""Auth / session security core (TEST-1), DB-lane.
+
+The wallet impersonation gate, epoch-based session revocation, single-use tokens and the
+enumeration-safe uniform 401 are the security mag, and had zero tests (only `_oauth_redirect_uri` was
+covered). Starlette's SessionMiddleware round-trips the signed cookie under TestClient, so the whole
+browser-less flow is exercisable. Uses the real auth stack via the `client` fixture (NO current_user
+override) so login, the wallet gate and epoch revoke are the code under test.
+
+Note: `/api/me` is the bootstrap probe — it returns 200 `{authenticated: False}` when logged out, not a
+401. A revoked session is therefore checked on a PROTECTED endpoint (`/api/feed`), which does 401.
+"""
+from datetime import timedelta
+
+import pytest
+from fastapi.testclient import TestClient
+
+import app.state as state
+from app.auth import ACTIVE_ACCOUNT_HEADER
+from app.main import app
+
+_PW = "correct horse battery"
+
+
+@pytest.fixture(autouse=True)
+def _configured(monkeypatch):
+    # The setup gate 503s /auth and /api until the wizard has run; every test here is post-setup.
+    monkeypatch.setattr(state, "_configured_cache", True)
+
+
+def _login(client, email, password):
+    return client.post("/auth/password-login", json={"email": email, "password": password})
+
+
+def test_password_login_then_me_is_authenticated(client, make_user):
+    make_user(email="alice@example.com", password=_PW)
+    assert _login(client, "alice@example.com", _PW).status_code == 200
+    me = client.get("/api/me")
+    assert me.status_code == 200
+    assert me.json()["authenticated"] is True
+    assert me.json()["email"] == "alice@example.com"
+
+
+def test_wrong_password_and_unknown_email_give_a_byte_identical_401(client, make_user):
+    make_user(email="bob@example.com", password=_PW)
+    wrong_pw = _login(client, "bob@example.com", "not the password")
+    unknown = _login(client, "nobody@example.com", _PW)
+    assert wrong_pw.status_code == 401 and unknown.status_code == 401
+    # Identical body: a wrong password and a non-existent account must be indistinguishable (no
+    # account-enumeration oracle).
+    assert wrong_pw.json() == unknown.json()
+
+
+def test_a_non_wallet_account_header_falls_back_to_the_default(client, make_user):
+    # The X-Siftlode-Account header may only re-target an account already signed into THIS browser
+    # (in the session wallet). A header naming someone else must be ignored, not honored — otherwise
+    # it's an impersonation gate hole.
+    make_user(email="alice2@example.com", password=_PW)
+    mallory = make_user(email="mallory@example.com", password=_PW)
+    assert _login(client, "alice2@example.com", _PW).status_code == 200
+    me = client.get("/api/me", headers={ACTIVE_ACCOUNT_HEADER: str(mallory.id)})
+    assert me.status_code == 200
+    assert me.json()["email"] == "alice2@example.com"  # still Alice, not Mallory
+
+
+def test_logout_others_revokes_the_other_session_but_keeps_the_caller(client, make_user):
+    # Two independent cookie jars (browsers) signed into the same account. `logout-others` from one
+    # bumps the account epoch so the OTHER's cookie is rejected on its next request, while the caller
+    # is re-stamped and stays in. Checked on /api/feed (a protected endpoint that 401s on revocation).
+    make_user(email="carol@example.com", password=_PW)
+    # A second, independent cookie jar; the `client` fixture already routed get_db to the test DB on
+    # the shared app, so this instance uses it too — it just has its own session cookie.
+    other = TestClient(app)
+    assert _login(client, "carol@example.com", _PW).status_code == 200
+    assert _login(other, "carol@example.com", _PW).status_code == 200
+    assert other.get("/api/feed").status_code == 200  # both valid to start
+
+    assert client.post("/auth/logout-others").status_code == 200
+    assert other.get("/api/feed").status_code == 401  # its epoch is now stale
+    assert client.get("/api/feed").status_code == 200  # the caller stays signed in
+
+
+def test_reset_confirm_is_single_use_and_kills_existing_sessions(client, make_user, db):
+    from app.routes.auth import _issue_token
+
+    user = make_user(email="dave@example.com", password=_PW)
+    assert _login(client, "dave@example.com", _PW).status_code == 200
+    assert client.get("/api/feed").status_code == 200  # session valid at the pre-reset epoch
+
+    raw = _issue_token(db, user, "reset", timedelta(hours=1))
+    db.commit()
+    ok = client.post("/auth/password-reset/confirm", json={"token": raw, "password": "brand new pw!"})
+    assert ok.status_code == 200
+
+    # A reset bumps the epoch (compromise response) -> the pre-reset cookie is now rejected.
+    assert client.get("/api/feed").status_code == 401
+    # The token is single-use: a replay is a clean 400, not a second reset.
+    replay = client.post("/auth/password-reset/confirm", json={"token": raw, "password": "another pw!!"})
+    assert replay.status_code == 400