diff --git a/workspace-server/internal/middleware/session_auth.go b/workspace-server/internal/middleware/session_auth.go
index 78b9d2ff..41748332 100644
--- a/workspace-server/internal/middleware/session_auth.go
+++ b/workspace-server/internal/middleware/session_auth.go
@@ -1,106 +1,232 @@
 package middleware
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"log"
+	"math/rand/v2"
 	"net/http"
+	"net/url"
 	"os"
 	"strings"
 	"sync"
 	"time"
 )
 
-// sessionCache holds short-lived positive results for upstream-verified
-// session cookies. Keyed by the raw Cookie header value so ANY change
-// (logout, fresh session) invalidates by just being different bytes.
+// sessionCache holds short-lived verification results for upstream
+// session-cookie checks. Entries are scoped BY TENANT SLUG so one
+// tenant's cache can't satisfy another tenant's check even when the
+// same cookie is presented.
 //
-// TTL is deliberately short — 30s — because the SaaS session lives on
-// the CP; if ops revokes a token, we want that reflected quickly. A
-// longer TTL would let revoked sessions drift into the tenant. 30s is
-// the sweet spot: fast enough for security, slow enough to avoid CP
-// hammering on every canvas render.
-var sessionCache sync.Map
+// Keyed by a sha256 of (slug + cookie) rather than raw cookie bytes:
+//   - Avoids storing raw session tokens in memory for longer than
+//     needed to look them up.
+//   - Makes the cache lookup deterministic regardless of cookie
+//     ordering / whitespace that browsers sometimes introduce.
+//
+// Bounded: we evict random entries when size breaches sessionCacheMax.
+// Periodic sweeper GCs expired entries even when they aren't re-hit.
+var sessionCache = struct {
+	sync.Mutex
+	entries map[string]sessionCacheEntry
+}{entries: make(map[string]sessionCacheEntry)}
 
-const sessionCacheTTL = 30 * time.Second
+const (
+	// Positive TTL: on the higher end because a valid session is
+	// stable until logout. 30s means logout or role change takes at
+	// most 30s to propagate.
+	sessionCacheTTLOK = 30 * time.Second
+
+	// Negative TTL: shorter, because a transient CP 502 (see
+	// controlplane issue #157 — terms-status flake) must heal
+	// quickly. 5s still absorbs a burst of retries from a single
+	// page render without fanning out to CP.
+	sessionCacheTTLFail = 5 * time.Second
+
+	// Cap on cached entries. 10k × ~100 bytes = ~1 MB — enough
+	// headroom for realistic tenant traffic without a slow leak.
+	sessionCacheMax = 10_000
+
+	// Sweeper runs opportunistically; cost is O(N) per sweep.
+	sessionCacheSweepEvery = 2 * time.Minute
+)
 
 type sessionCacheEntry struct {
-	verifiedAt time.Time
-	ok         bool
+	expiresAt time.Time
+	ok        bool
 }
 
-// cpSessionEndpointURL is where we verify. Reads the same env the
-// router uses for the /cp/* reverse-proxy. Empty string → feature
-// disabled (self-hosted / dev). Computed at first call so tests can
-// override via env.
-func cpSessionEndpointURL() string {
+// cacheKey derives the lookup key. Using sha256 here isn't about
+// cryptographic secrecy — it's about keying by (tenant, cookie) in a
+// fixed-size string and not sprinkling raw tokens around the map.
+func cacheKey(slug, cookie string) string {
+	h := sha256.New()
+	h.Write([]byte(slug))
+	h.Write([]byte{0}) // separator so ("a","bc") ≠ ("ab","c")
+	h.Write([]byte(cookie))
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// sessionCacheGet returns (ok, hit). hit=false means expired or absent.
+func sessionCacheGet(key string) (ok bool, hit bool) {
+	sessionCache.Lock()
+	defer sessionCache.Unlock()
+	e, present := sessionCache.entries[key]
+	if !present {
+		return false, false
+	}
+	if time.Now().After(e.expiresAt) {
+		delete(sessionCache.entries, key)
+		return false, false
+	}
+	return e.ok, true
+}
+
+// sessionCachePut stores the result with the appropriate TTL. On
+// overflow it evicts a pseudo-random entry so the cache stays
+// bounded. This isn't LRU — we don't need precise recency, just
+// ceiling behaviour. Random eviction is O(1) expected and avoids
+// the bookkeeping of a doubly-linked list.
+func sessionCachePut(key string, ok bool) {
+	ttl := sessionCacheTTLFail
+	if ok {
+		ttl = sessionCacheTTLOK
+	}
+	sessionCache.Lock()
+	defer sessionCache.Unlock()
+	if len(sessionCache.entries) >= sessionCacheMax {
+		// Evict N random entries to amortize the sweep cost. Pick
+		// the first N in map-iteration order (Go randomizes this).
+		const evictBatch = 128
+		i := 0
+		for k := range sessionCache.entries {
+			delete(sessionCache.entries, k)
+			i++
+			if i >= evictBatch {
+				break
+			}
+		}
+	}
+	sessionCache.entries[key] = sessionCacheEntry{
+		expiresAt: time.Now().Add(ttl),
+		ok:        ok,
+	}
+}
+
+func init() {
+	go func() {
+		// Jitter startup so restarts don't align sweeps.
+		time.Sleep(time.Duration(rand.Int64N(int64(sessionCacheSweepEvery))))
+		t := time.NewTicker(sessionCacheSweepEvery)
+		defer t.Stop()
+		for range t.C {
+			sweepExpired()
+		}
+	}()
+}
+
+// sweepExpired removes expired entries so a low-hit-rate cache still
+// releases memory. Cheap — we hold the lock briefly per entry.
+func sweepExpired() {
+	now := time.Now()
+	sessionCache.Lock()
+	defer sessionCache.Unlock()
+	for k, e := range sessionCache.entries {
+		if now.After(e.expiresAt) {
+			delete(sessionCache.entries, k)
+		}
+	}
+}
+
+// cpSessionVerifyURL builds the upstream /cp/auth/tenant-member URL
+// with the tenant slug attached. Returns "" when the tenant isn't
+// configured for CP verification (CP_UPSTREAM_URL unset).
+func cpSessionVerifyURL(slug string) string {
 	base := strings.TrimRight(os.Getenv("CP_UPSTREAM_URL"), "/")
 	if base == "" {
 		return ""
 	}
-	return base + "/cp/auth/me"
+	return base + "/cp/auth/tenant-member?slug=" + url.QueryEscape(slug)
+}
+
+// tenantSlug returns the slug this platform represents. Pulled from
+// the MOLECULE_ORG_SLUG env at provision time; falls back to empty
+// when unset (self-hosted / dev).
+func tenantSlug() string {
+	return strings.TrimSpace(os.Getenv("MOLECULE_ORG_SLUG"))
 }
 
 // verifiedCPSession returns true when the request carries a cookie
-// that the CP recognizes as a logged-in user. Caches positive results
-// for sessionCacheTTL so burst canvas renders don't fan out to the CP
-// on every admin fetch.
+// that the CP confirms belongs to a MEMBER of THIS tenant's org (not
+// just "someone is logged in"). The difference is the authz boundary:
+// any WorkOS-authed user could hit /cp/auth/me successfully; only
+// actual org members pass /cp/auth/tenant-member?slug=<us>.
 //
-// Returns (false, false) when there is no cookie at all — callers
-// distinguish "no credential presented" (fall through to other tiers)
+// Returns (false, false) when no cookie at all, so callers can
+// distinguish "no credential presented" (fall through to bearer)
 // from "credential presented but invalid" (abort with 401).
+//
+// Also returns (false, false) when MOLECULE_ORG_SLUG isn't configured
+// — fail-safe: better to refuse session auth than to accept it
+// without knowing which tenant we ARE. Deployments that want session
+// auth MUST set both CP_UPSTREAM_URL and MOLECULE_ORG_SLUG.
 func verifiedCPSession(cookieHeader string) (valid, presented bool) {
 	if cookieHeader == "" {
 		return false, false
 	}
-	endpoint := cpSessionEndpointURL()
-	if endpoint == "" {
+	slug := tenantSlug()
+	if slug == "" {
+		return false, false
+	}
+	verifyURL := cpSessionVerifyURL(slug)
+	if verifyURL == "" {
 		return false, true
 	}
 
-	// Cache lookup.
-	if v, ok := sessionCache.Load(cookieHeader); ok {
-		e := v.(sessionCacheEntry)
-		if time.Since(e.verifiedAt) < sessionCacheTTL {
-			return e.ok, true
-		}
-		sessionCache.Delete(cookieHeader)
+	key := cacheKey(slug, cookieHeader)
+	if ok, hit := sessionCacheGet(key); hit {
+		return ok, true
 	}
 
-	// Fetch /cp/auth/me with the presented cookie. Short timeout —
-	// a slow CP mustn't gate every canvas page render.
+	// Short timeout — a slow CP mustn't gate every canvas render.
 	client := &http.Client{Timeout: 3 * time.Second}
-	req, err := http.NewRequest("GET", endpoint, nil)
+	req, err := http.NewRequest("GET", verifyURL, nil)
 	if err != nil {
 		log.Printf("verifiedCPSession: build req: %v", err)
 		return false, true
 	}
 	req.Header.Set("Cookie", cookieHeader)
-	// Browser-style User-Agent so the CP's bot-detection (if any)
-	// doesn't block us; we're a legitimate proxy for the UI.
 	req.Header.Set("User-Agent", "molecule-tenant-platform/session-verifier")
 
 	resp, err := client.Do(req)
 	if err != nil {
 		log.Printf("verifiedCPSession: upstream: %v", err)
+		// NOTE: we deliberately do NOT cache transport failures.
+		// Caching them would mean a 3s CP blip locks out all users
+		// for the negative-TTL window. Next request retries.
 		return false, true
 	}
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		sessionCache.Store(cookieHeader, sessionCacheEntry{verifiedAt: time.Now(), ok: false})
+		sessionCachePut(key, false)
 		return false, true
 	}
 
-	// Parse minimally to make sure it's actually a session object, not
-	// an HTML error page from an upstream proxy shell.
 	var body struct {
+		Member bool   `json:"member"`
 		UserID string `json:"user_id"`
 	}
-	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil || body.UserID == "" {
-		sessionCache.Store(cookieHeader, sessionCacheEntry{verifiedAt: time.Now(), ok: false})
+	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+		sessionCachePut(key, false)
+		return false, true
+	}
+	if !body.Member || body.UserID == "" {
+		sessionCachePut(key, false)
 		return false, true
 	}
 
-	sessionCache.Store(cookieHeader, sessionCacheEntry{verifiedAt: time.Now(), ok: true})
+	sessionCachePut(key, true)
 	return true, true
 }
diff --git a/workspace-server/internal/middleware/session_auth_test.go b/workspace-server/internal/middleware/session_auth_test.go
new file mode 100644
index 00000000..b60cc7f7
--- /dev/null
+++ b/workspace-server/internal/middleware/session_auth_test.go
@@ -0,0 +1,229 @@
+package middleware
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// resetSessionCache clears global cache state between tests.
+func resetSessionCache() {
+	sessionCache.Lock()
+	defer sessionCache.Unlock()
+	sessionCache.entries = make(map[string]sessionCacheEntry)
+}
+
+// mockCPServer builds an httptest server that returns the given
+// status/body for /cp/auth/tenant-member. Also tracks hit count via
+// the returned atomic so tests can verify cache behavior.
+func mockCPServer(t *testing.T, status int, body string) (*httptest.Server, *atomic.Int64) {
+	t.Helper()
+	hits := &atomic.Int64{}
+	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hits.Add(1)
+		if !strings.HasSuffix(r.URL.Path, "/cp/auth/tenant-member") {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		w.WriteHeader(status)
+		_, _ = w.Write([]byte(body))
+	}))
+	t.Cleanup(s.Close)
+	return s, hits
+}
+
+func TestVerifiedCPSession_EmptyCookie(t *testing.T) {
+	resetSessionCache()
+	ok, presented := verifiedCPSession("")
+	if ok || presented {
+		t.Errorf("empty cookie should be (false, false); got (%v, %v)", ok, presented)
+	}
+}
+
+func TestVerifiedCPSession_NoSlugConfigured(t *testing.T) {
+	resetSessionCache()
+	t.Setenv("CP_UPSTREAM_URL", "https://cp.test")
+	t.Setenv("MOLECULE_ORG_SLUG", "")
+	ok, presented := verifiedCPSession("session=foo")
+	// Without a slug we can't ask about tenant membership. Must
+	// refuse (false, false) — caller falls through to bearer tier.
+	if ok || presented {
+		t.Errorf("no slug should be (false, false); got (%v, %v)", ok, presented)
+	}
+}
+
+func TestVerifiedCPSession_NoCPConfigured(t *testing.T) {
+	resetSessionCache()
+	t.Setenv("CP_UPSTREAM_URL", "")
+	t.Setenv("MOLECULE_ORG_SLUG", "acme")
+	ok, presented := verifiedCPSession("session=foo")
+	// Self-hosted path: CP not configured, but cookie WAS presented.
+	// Presented=true lets the caller know not to fall through to
+	// bearer as if no credential arrived.
+	if ok || !presented {
+		t.Errorf("no CP should be (false, true); got (%v, %v)", ok, presented)
+	}
+}
+
+func TestVerifiedCPSession_MemberTrue(t *testing.T) {
+	resetSessionCache()
+	srv, hits := mockCPServer(t, 200, `{"member":true,"user_id":"u_1","role":"owner","org_id":"org_1"}`)
+	t.Setenv("CP_UPSTREAM_URL", srv.URL)
+	t.Setenv("MOLECULE_ORG_SLUG", "acme")
+
+	ok, presented := verifiedCPSession("session=valid")
+	if !ok || !presented {
+		t.Errorf("valid member should be (true, true); got (%v, %v)", ok, presented)
+	}
+	if hits.Load() != 1 {
+		t.Errorf("expected 1 upstream hit; got %d", hits.Load())
+	}
+
+	// Second call must be served from cache.
+	ok, _ = verifiedCPSession("session=valid")
+	if !ok {
+		t.Errorf("cached call should still be true")
+	}
+	if hits.Load() != 1 {
+		t.Errorf("cache miss: expected still 1 upstream hit; got %d", hits.Load())
+	}
+}
+
+func TestVerifiedCPSession_MemberFalse(t *testing.T) {
+	resetSessionCache()
+	// CP returns 200 but member=false — user is authed but not in this org
+	srv, hits := mockCPServer(t, 200, `{"member":false}`)
+	t.Setenv("CP_UPSTREAM_URL", srv.URL)
+	t.Setenv("MOLECULE_ORG_SLUG", "acme")
+
+	ok, presented := verifiedCPSession("session=wrong-tenant")
+	if ok || !presented {
+		t.Errorf("non-member should be (false, true); got (%v, %v)", ok, presented)
+	}
+	if hits.Load() != 1 {
+		t.Fatalf("expected 1 upstream hit")
+	}
+	// Cached negatively.
+	_, _ = verifiedCPSession("session=wrong-tenant")
+	if hits.Load() != 1 {
+		t.Errorf("negative result should cache too; got %d hits", hits.Load())
+	}
+}
+
+func TestVerifiedCPSession_Upstream401(t *testing.T) {
+	resetSessionCache()
+	srv, _ := mockCPServer(t, 401, ``)
+	t.Setenv("CP_UPSTREAM_URL", srv.URL)
+	t.Setenv("MOLECULE_ORG_SLUG", "acme")
+
+	ok, presented := verifiedCPSession("session=expired")
+	if ok || !presented {
+		t.Errorf("401 upstream should be (false, true); got (%v, %v)", ok, presented)
+	}
+}
+
+func TestVerifiedCPSession_MalformedJSON(t *testing.T) {
+	resetSessionCache()
+	srv, _ := mockCPServer(t, 200, `not-json`)
+	t.Setenv("CP_UPSTREAM_URL", srv.URL)
+	t.Setenv("MOLECULE_ORG_SLUG", "acme")
+
+	ok, presented := verifiedCPSession("session=broken")
+	if ok || !presented {
+		t.Errorf("malformed body should be (false, true); got (%v, %v)", ok, presented)
+	}
+}
+
+func TestVerifiedCPSession_TransportErrorNotCached(t *testing.T) {
+	resetSessionCache()
+	// Point at a port that's definitely refused.
+	t.Setenv("CP_UPSTREAM_URL", "http://127.0.0.1:1")
+	t.Setenv("MOLECULE_ORG_SLUG", "acme")
+
+	ok, presented := verifiedCPSession("session=whatever")
+	if ok || !presented {
+		t.Errorf("transport error should be (false, true); got (%v, %v)", ok, presented)
+	}
+	// Transport errors must NOT be cached — otherwise a 3s CP blip
+	// locks every user out for the negative-TTL window.
+	sessionCache.Lock()
+	n := len(sessionCache.entries)
+	sessionCache.Unlock()
+	if n != 0 {
+		t.Errorf("transport error cached %d entries; want 0", n)
+	}
+}
+
+func TestVerifiedCPSession_CrossTenantIsolation(t *testing.T) {
+	resetSessionCache()
+	// Even if we have a valid session for tenant A, asking for
+	// tenant B's membership must hit the CP separately. Same cookie
+	// with different tenant slug → different cache key.
+	reqs := []string{}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		reqs = append(reqs, r.URL.RawQuery)
+		// Return member=true for slug=acme, member=false for slug=bob
+		if strings.Contains(r.URL.RawQuery, "slug=acme") {
+			_, _ = w.Write([]byte(`{"member":true,"user_id":"u_1"}`))
+			return
+		}
+		_, _ = w.Write([]byte(`{"member":false}`))
+	}))
+	defer srv.Close()
+	t.Setenv("CP_UPSTREAM_URL", srv.URL)
+
+	cookie := "session=shared-auth"
+
+	t.Setenv("MOLECULE_ORG_SLUG", "acme")
+	if ok, _ := verifiedCPSession(cookie); !ok {
+		t.Errorf("acme should say member=true")
+	}
+
+	t.Setenv("MOLECULE_ORG_SLUG", "bob")
+	if ok, _ := verifiedCPSession(cookie); ok {
+		t.Errorf("bob tenant must NOT accept acme cookie despite same session bytes")
+	}
+	if len(reqs) != 2 {
+		t.Errorf("cross-tenant should issue 2 upstream calls; got %d", len(reqs))
+	}
+}
+
+func TestSessionCache_BoundedEviction(t *testing.T) {
+	resetSessionCache()
+	// Fill beyond cap and verify size stays roughly bounded.
+	// Not testing exact eviction policy (random) — just that we
+	// don't grow unbounded.
+	for i := 0; i < sessionCacheMax+500; i++ {
+		sessionCachePut(fmt.Sprintf("k%d", i), true)
+	}
+	sessionCache.Lock()
+	n := len(sessionCache.entries)
+	sessionCache.Unlock()
+	if n > sessionCacheMax {
+		t.Errorf("cache grew to %d, exceeds cap %d", n, sessionCacheMax)
+	}
+}
+
+func TestSessionCache_ExpiredEntryIgnored(t *testing.T) {
+	resetSessionCache()
+	key := "k-expired"
+	sessionCache.Lock()
+	sessionCache.entries[key] = sessionCacheEntry{
+		expiresAt: time.Now().Add(-1 * time.Second),
+		ok:        true,
+	}
+	sessionCache.Unlock()
+	if ok, hit := sessionCacheGet(key); ok || hit {
+		t.Errorf("expired entry must not hit; got ok=%v hit=%v", ok, hit)
+	}
+}
+
+func TestCacheKey_SlugSeparator(t *testing.T) {
+	// ("a","bc") and ("ab","c") must not collide.
+	if cacheKey("a", "bc") == cacheKey("ab", "c") {
+		t.Errorf("cacheKey collides on ambiguous splits")
+	}
+}
diff --git a/workspace-server/internal/middleware/tenant_guard.go b/workspace-server/internal/middleware/tenant_guard.go
index 6692c5c3..cbf5e90a 100644
--- a/workspace-server/internal/middleware/tenant_guard.go
+++ b/workspace-server/internal/middleware/tenant_guard.go
@@ -72,6 +72,16 @@ func TenantGuardWithOrgID(configuredOrgID string) gin.HandlerFunc {
 		// doesn't need to attach org identity here. Bypassing the guard
 		// avoids blocking the proxy with a 404 that would then look
 		// like the CP is down.
+		//
+		// SECURITY NOTE: this pass-through is only safe because:
+		//   (a) cp_proxy enforces its own explicit path allowlist
+		//       (see router/cp_proxy.go cpProxyAllowedPrefixes) so
+		//       traversal to admin-surface endpoints is blocked.
+		//   (b) tenant SG has no :8080 inbound; only the Cloudflare
+		//       tunnel reaches the platform. A future SG change that
+		//       opens :8080 to the VPC would also open this path to
+		//       unauthenticated /cp/* probing — tighten cp_proxy's
+		//       allowlist OR remove this bypass if that happens.
 		if strings.HasPrefix(c.Request.URL.Path, "/cp/") {
 			c.Next()
 			return
diff --git a/workspace-server/internal/router/cp_proxy.go b/workspace-server/internal/router/cp_proxy.go
index 94e3555f..f8daa3cc 100644
--- a/workspace-server/internal/router/cp_proxy.go
+++ b/workspace-server/internal/router/cp_proxy.go
@@ -5,10 +5,62 @@ import (
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+	"strings"
 
 	"github.com/gin-gonic/gin"
 )
 
+// cpProxyAllowedPrefixes is the explicit list of /cp/* paths the
+// tenant will forward to the CP. Anything else 404s BEFORE the cookie
+// and Authorization headers leave the tenant.
+//
+// Why an allowlist, not a denylist: /cp/admin/* endpoints accept a
+// WorkOS session cookie (scoped to .moleculesai.app) as one of their
+// auth tiers. A tenant-authed user visiting <tenant>.moleculesai.app
+// and crafting a request to /cp/admin/tenants/other-slug/diagnostics
+// would have the tenant happily forward their cookie upstream. The CP
+// would then see a legitimate admin session and honor the request —
+// effectively turning any tenant into an admin-access lateral-
+// movement hop. (Observed as a theoretical risk in today's review.)
+//
+// Only paths that are legitimately used by the canvas browser bundle
+// go in this list. If a new UI fetch needs a new /cp/ prefix, add it
+// here — fail-closed is the default.
+var cpProxyAllowedPrefixes = []string{
+	"/cp/auth/",     // me, tenant-member, login/signup/callback for return flows
+	"/cp/orgs",      // list / get / provision-status / export
+	"/cp/billing/",  // checkout + portal
+	"/cp/templates", // template registry reads
+	"/cp/legal/",    // terms document (served on CP)
+}
+
+// isCPProxyAllowedPath enforces the allowlist. Prefix match with an
+// optional trailing slash tolerance (/cp/orgs matches /cp/orgs AND
+// /cp/orgs/acme). Rejects any path that doesn't start with /cp/ so
+// the handler isn't inadvertently mounted on other prefixes.
+func isCPProxyAllowedPath(p string) bool {
+	if !strings.HasPrefix(p, "/cp/") {
+		return false
+	}
+	for _, prefix := range cpProxyAllowedPrefixes {
+		if p == prefix || strings.HasPrefix(p, prefix+"/") || strings.HasPrefix(p, prefix) && prefixMatches(p, prefix) {
+			return true
+		}
+	}
+	return false
+}
+
+// prefixMatches handles the case where the allowlist entry itself ends
+// in a slash (e.g. /cp/auth/): that means "anything under /cp/auth/".
+// Entries without a trailing slash (/cp/orgs) match both the exact path
+// and any subpath. Separate function so the intent is readable.
+func prefixMatches(path, prefix string) bool {
+	if strings.HasSuffix(prefix, "/") {
+		return strings.HasPrefix(path, prefix)
+	}
+	return path == prefix || strings.HasPrefix(path, prefix+"/")
+}
+
 // newCPProxy returns a Gin handler that reverse-proxies /cp/* requests
 // to the control plane. Lives beside newCanvasProxy because they solve
 // the same problem — tenant browser fetches targeted at a single
@@ -70,6 +122,13 @@ func newCPProxy(targetURL string) gin.HandlerFunc {
 	}
 
 	return func(c *gin.Context) {
+		// Allowlist enforcement: block anything outside the browser-
+		// canvas-facing /cp/* surface. Returns 404 (not 403) to avoid
+		// leaking which paths exist on the CP side.
+		if !isCPProxyAllowedPath(c.Request.URL.Path) {
+			c.AbortWithStatus(http.StatusNotFound)
+			return
+		}
 		proxy.ServeHTTP(c.Writer, c.Request)
 	}
 }
diff --git a/workspace-server/internal/router/cp_proxy_test.go b/workspace-server/internal/router/cp_proxy_test.go
new file mode 100644
index 00000000..01701207
--- /dev/null
+++ b/workspace-server/internal/router/cp_proxy_test.go
@@ -0,0 +1,151 @@
+package router
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestIsCPProxyAllowedPath(t *testing.T) {
+	cases := []struct {
+		path string
+		want bool
+		why  string
+	}{
+		// Allowed — canvas UI needs these
+		{"/cp/auth/me", true, "auth check"},
+		{"/cp/auth/tenant-member", true, "membership check"},
+		{"/cp/auth/login", true, "return-flow login"},
+		{"/cp/orgs", true, "list orgs"},
+		{"/cp/orgs/acme", true, "get one org"},
+		{"/cp/orgs/acme/provision-status", true, "provision poll"},
+		{"/cp/billing/checkout", true, "Stripe checkout"},
+		{"/cp/templates", true, "template registry"},
+		{"/cp/templates/starter", true, "template detail"},
+		{"/cp/legal/terms", true, "ToS document"},
+
+		// Blocked — admin surface must not traverse the tenant proxy
+		{"/cp/admin/orgs", false, "cross-tenant admin list (lateral movement)"},
+		{"/cp/admin/tenants/other/diagnostics", false, "admin tenant probe"},
+		{"/cp/admin/beta-allowlist", false, "beta admin"},
+		{"/cp/workspaces/provision", false, "CP provisioning (shared-secret gate)"},
+		{"/cp/internal/usage", false, "internal usage ingest"},
+		{"/cp/tenants/config", false, "tenant-bootstrap config (admin_token gated)"},
+		{"/cp/tenants/backup-report", false, "tenant-bootstrap backup (admin_token gated)"},
+
+		// Edge cases
+		{"/cp/", false, "empty suffix"},
+		{"/cp", false, "no trailing slash"},
+		{"/something-else", false, "not under /cp/"},
+		{"/cp/auth", false, "prefix trailing-slash entries require subpath"},
+		{"/cp/authsomething", false, "substring match defense"},
+		{"/cp/orgsabc", false, "prefix match needs / or exact"},
+	}
+	for _, tc := range cases {
+		got := isCPProxyAllowedPath(tc.path)
+		if got != tc.want {
+			t.Errorf("path %q: want %v (%s); got %v", tc.path, tc.want, tc.why, got)
+		}
+	}
+}
+
+func TestCPProxy_Allowlist_Blocks404(t *testing.T) {
+	// Allowlist should return 404 before any upstream call.
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		t.Errorf("upstream must NOT be called for blocked paths")
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer upstream.Close()
+
+	handler := newCPProxy(upstream.URL)
+	r := gin.New()
+	r.Any("/cp/*path", handler)
+
+	w := newTestRecorder()
+	req := httptest.NewRequest("GET", "/cp/admin/orgs", nil)
+	r.ServeHTTP(w, req)
+
+	if w.Code != http.StatusNotFound {
+		t.Errorf("blocked path should 404; got %d", w.Code)
+	}
+}
+
+func TestCPProxy_AllowedPathsForward(t *testing.T) {
+	var receivedPath string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		receivedPath = r.URL.Path
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`{"ok":true}`))
+	}))
+	defer upstream.Close()
+
+	handler := newCPProxy(upstream.URL)
+	r := gin.New()
+	r.Any("/cp/*path", handler)
+
+	w := newTestRecorder()
+	req := httptest.NewRequest("GET", "/cp/auth/me", nil)
+	r.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("allowed path should forward; got %d", w.Code)
+	}
+	if receivedPath != "/cp/auth/me" {
+		t.Errorf("path not forwarded cleanly; got %q", receivedPath)
+	}
+}
+
+func TestCPProxy_ForwardsCookiesAndAuth(t *testing.T) {
+	// Cookie + Authorization must reach the CP — that's how
+	// session verification + bearer auth work upstream.
+	var gotCookie, gotAuth string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotCookie = r.Header.Get("Cookie")
+		gotAuth = r.Header.Get("Authorization")
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer upstream.Close()
+
+	handler := newCPProxy(upstream.URL)
+	r := gin.New()
+	r.Any("/cp/*path", handler)
+
+	w := newTestRecorder()
+	req := httptest.NewRequest("GET", "/cp/auth/me", nil)
+	req.Header.Set("Cookie", "session=abc123")
+	req.Header.Set("Authorization", "Bearer xyz")
+	r.ServeHTTP(w, req)
+
+	if gotCookie != "session=abc123" {
+		t.Errorf("Cookie not forwarded: got %q", gotCookie)
+	}
+	if gotAuth != "Bearer xyz" {
+		t.Errorf("Authorization not forwarded: got %q", gotAuth)
+	}
+}
+
+func TestCPProxy_HostRewrittenToUpstream(t *testing.T) {
+	var gotHost string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotHost = r.Host
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer upstream.Close()
+
+	handler := newCPProxy(upstream.URL)
+	r := gin.New()
+	r.Any("/cp/*path", handler)
+
+	w := newTestRecorder()
+	req := httptest.NewRequest("GET", "/cp/auth/me", nil)
+	req.Host = "acme.moleculesai.app" // the tenant hostname the browser used
+	r.ServeHTTP(w, req)
+
+	// Host should be rewritten to the upstream's host so CP's
+	// CORS + cookie-domain logic sees itself.
+	if gotHost == "acme.moleculesai.app" {
+		t.Errorf("Host was not rewritten; upstream still saw tenant Host: %q", gotHost)
+	}
+}