diff --git a/src/lib/containers/install.go b/src/lib/containers/install.go
index 753711e..ccdf09f 100644
--- a/src/lib/containers/install.go
+++ b/src/lib/containers/install.go
@@ -30,6 +30,7 @@ import (
 	"syscall"
 	"time"
 
+	"git.dcntrlized.org/Dcntrlized/Maestro/lib/authmgmt"
 	"git.dcntrlized.org/Dcntrlized/Maestro/lib/docker"
 	"git.dcntrlized.org/Dcntrlized/Maestro/lib/logger"
 	"git.dcntrlized.org/Dcntrlized/Maestro/lib/types"
@@ -70,6 +71,20 @@ func InstallContainer(ctx context.Context, item types.RegistryContainer) error {
 		RegistryContainer: item,
 		InstalledAt:       time.Now().UTC().Format(time.RFC3339),
 	}
+
+	// 2.6. Pin the auth service's signing key (security theme M2 §5): fetch its
+	// attestation over mTLS and store the fingerprint in the install record so
+	// the container gets AUTH_PINNED_FINGERPRINT at start. Best-effort — when
+	// the auth service is not reachable yet the pin stays empty and is retried
+	// on start.
+	if att, err := fetchAuthAttestation(ctx); err == nil {
+		metadata.AuthFingerprint = att.Fingerprint
+		metadata.AuthKid = att.Kid
+		metadata.AuthAttestedAt = att.IssuedAt
+	} else {
+		logger.Warn("Install %s: auth attestation not pinned (will retry at start): %v", item.Name, err)
+	}
+
 	metadataPath := filepath.Join(targetDir, "metadata.json")
 	if err := writeMetadata(metadataPath, metadata); err != nil {
 		return fmt.Errorf("failed to write metadata: %w", err)
@@ -453,3 +468,23 @@ func writeMetadata(path string, metadata types.InstalledMetadata) error {
 	}
 	return os.WriteFile(path, data, 0644)
 }
+
+// fetchAuthAttestation fetches the auth service's public key statement over
+// mTLS (Maestro's own identity certificate). It backs the M2 §5 TOFU pin: the
+// returned fingerprint is what consumer containers are pinned to.
+func fetchAuthAttestation(ctx context.Context) (*authmgmt.Attestation, error) {
+	client, err := authmgmt.New()
+	if err != nil {
+		return nil, fmt.Errorf("auth management client: %w", err)
+	}
+	fetchCtx, cancel := context.WithTimeout(ctx, 8*time.Second)
+	defer cancel()
+	att, err := client.Attestation(fetchCtx)
+	if err != nil {
+		return nil, err
+	}
+	if att.Fingerprint == "" {
+		return nil, fmt.Errorf("attestation carried an empty fingerprint")
+	}
+	return att, nil
+}
diff --git a/src/lib/containers/start.go b/src/lib/containers/start.go
index c277fc3..ff050a4 100644
--- a/src/lib/containers/start.go
+++ b/src/lib/containers/start.go
@@ -18,6 +18,7 @@ package containers
 import (
 	"context"
 	"fmt"
+	"path/filepath"
 	"strings"
 
 	"git.dcntrlized.org/Dcntrlized/Maestro/lib/docker"
@@ -64,7 +65,7 @@ func StartContainer(ctx context.Context, item types.RegistryContainer) error {
 	// downloaded from git never declares these mounts — Maestro injects them
 	// (security theme M1 §2). Failing to provision identity is fatal: a peer
 	// without a cert cannot authenticate on the mTLS edges.
-	if err := injectServiceIdentity(project, serviceName); err != nil {
+	if err := injectServiceIdentity(ctx, project, serviceName); err != nil {
 		return err
 	}
 
@@ -108,7 +109,12 @@ func StartContainer(ctx context.Context, item types.RegistryContainer) error {
 // cert/key, and the mTLS env vars into every non-oneoff compose service. The
 // env vars follow the shared-module contract (AUTH_CA_BUNDLE/AUTH_CLIENT_CERT/
 // AUTH_CLIENT_KEY) so consumers need no per-service config.
-func injectServiceIdentity(project *composetypes.Project, serviceName string) error {
+//
+// It also injects AUTH_PINNED_FINGERPRINT (M2 §5 TOFU): the auth signing key's
+// fingerprint recorded at install. A service that already has the pin skips the
+// fetch; one installed before this feature (or whose install could not reach
+// the auth service) backfills it here, best-effort.
+func injectServiceIdentity(ctx context.Context, project *composetypes.Project, serviceName string) error {
 	dataRoot, err := getDataRoot()
 	if err != nil {
 		return fmt.Errorf("resolve data root: %w", err)
@@ -135,6 +141,14 @@ func injectServiceIdentity(project *composetypes.Project, serviceName string) er
 		"AUTH_CLIENT_KEY":  identityMountDir + "/tls.key",
 	}
 
+	// The auth service is the attester itself — nothing to pin there. For every
+	// other peer, pin its signing key (fetch + persist once).
+	if serviceName != AuthServiceName {
+		if pinned := pinAuthFingerprint(ctx, serviceName); pinned != "" {
+			env["AUTH_PINNED_FINGERPRINT"] = pinned
+		}
+	}
+
 	for name, s := range project.Services {
 		if isOneoff(s) {
 			continue
@@ -147,6 +161,41 @@ func injectServiceIdentity(project *composetypes.Project, serviceName string) er
 	return nil
 }
 
+// pinAuthFingerprint returns the service's pinned auth fingerprint, fetching
+// and persisting it when the install record does not have one yet (M2 §5).
+// Best-effort: when the auth service is unreachable, it returns "" and the
+// next start retries.
+func pinAuthFingerprint(ctx context.Context, serviceName string) string {
+	dir, err := getContainerDataDir(serviceName)
+	if err != nil {
+		return ""
+	}
+	metadata, err := readInstalledMetadata(dir)
+	if err != nil {
+		return ""
+	}
+	if metadata.AuthFingerprint != "" {
+		return metadata.AuthFingerprint
+	}
+
+	att, err := fetchAuthAttestation(ctx)
+	if err != nil {
+		logger.Warn("Auth pin for %s: attestation fetch failed (will retry at next start): %v", serviceName, err)
+		return ""
+	}
+	if att.Fingerprint == "" {
+		return ""
+	}
+	metadata.AuthFingerprint = att.Fingerprint
+	metadata.AuthKid = att.Kid
+	metadata.AuthAttestedAt = att.IssuedAt
+	if err := writeMetadata(filepath.Join(dir, "metadata.json"), *metadata); err != nil {
+		logger.Warn("Auth pin for %s: persist failed: %v", serviceName, err)
+	}
+	logger.Info("Pinned auth fingerprint %s for %s", att.Fingerprint, serviceName)
+	return att.Fingerprint
+}
+
 // composeVolumes converts "src:dst:ro" mount strings to compose volume configs.
 func composeVolumes(mounts []string) []composetypes.ServiceVolumeConfig {
 	out := make([]composetypes.ServiceVolumeConfig, 0, len(mounts))
diff --git a/src/lib/types/registry.go b/src/lib/types/registry.go
index c1d2739..f937e95 100644
--- a/src/lib/types/registry.go
+++ b/src/lib/types/registry.go
@@ -58,4 +58,14 @@ type InstalledMetadata struct {
 	// services keep advertising auth consumption and its key.
 	HasAuth   bool   `json:"has_auth,omitempty"`
 	PublicKey string `json:"public_key,omitempty"`
+	// AuthFingerprint pins the auth service's signing key after first contact
+	// (security theme M2 §5, TOFU/D3). It is fetched from the auth service's
+	// attestation at install time and injected into consumer containers as
+	// AUTH_PINNED_FINGERPRINT, so a substituted key is refused instead of
+	// silently trusted.
+	AuthFingerprint string `json:"auth_fingerprint,omitempty"`
+	// AuthKid and AuthAttestedAt record which key was pinned and when, so a
+	// rotation (M3) can be surfaced as an explicit re-acceptance.
+	AuthKid        string `json:"auth_kid,omitempty"`
+	AuthAttestedAt string `json:"auth_attested_at,omitempty"`
 }