diff --git a/client/server/ownership.go b/client/server/ownership.go
index 7791f1f4d..244b702bc 100644
--- a/client/server/ownership.go
+++ b/client/server/ownership.go
@@ -127,6 +127,60 @@ func (s *Server) claimForCallerLocked(id ipcauth.Identity, cfg *profilemanager.C
 	return nil
 }
 
+// authorizeTargetProfile authorizes a caller to operate on a specific target
+// profile. It MUST be called after bindCallerUsername, which enforces the legacy
+// per-username-directory guard. this layers the collision-free Owners field on
+// top of it:
+//
+//   - Privileged callers (root / elevated-admin) may operate on any profile.
+//   - If the target has Owners (or is Shared), they are authoritative. This
+//     disambiguates users whose sanitized usernames collide.
+//   - If the target is unowned (a legacy profile predating ownership), passing
+//     the username guard is sufficient and then the profile is claimed.
+//
+// Caller must hold s.mutex (it may persist an ownership claim).
+func (s *Server) authorizeTargetProfile(ctx context.Context, target *profilemanager.Profile, claim bool) error {
+	id, ok := ipcauth.IdentityFromContext(ctx)
+	if !ok {
+		return gstatus.Error(codes.PermissionDenied, "caller identity could not be verified")
+	}
+	if id.IsPrivileged() {
+		return nil
+	}
+
+	path, err := target.FilePath()
+	if err != nil {
+		return fmt.Errorf("resolve target profile path: %w", err)
+	}
+	cfg, err := profilemanager.GetConfig(path)
+	if err != nil {
+		return fmt.Errorf("load target profile config: %w", err)
+	}
+
+	ownership := ipcauth.Ownership{Owners: cfg.Owners, Shared: cfg.Shared}
+
+	// Owned or shared: the Owners field is authoritative (collision-free).
+	if len(ownership.Owners) > 0 || ownership.Shared {
+		if ipcauth.Authorize(ownership, id, s.groupResolver) {
+			return nil
+		}
+		return gstatus.Errorf(codes.PermissionDenied,
+			"not authorized to operate on profile %q (owned by another principal)", target.Name)
+	}
+
+	// Unowned legacy profile: the username guard authorizes. Stamp the caller
+	// as owner so future access is collision-free.
+	if claim {
+		principal := ipcauth.OwnerPrincipalForIdentity(id)
+		cfg.Owners = []string{principal}
+		if err := util.WriteJson(context.Background(), path, cfg); err != nil {
+			return fmt.Errorf("persist profile ownership claim: %w", err)
+		}
+		log.Infof("profile %q (%s) claimed by %s on first access (trust-on-first-use)", target.Name, target.ID, id)
+	}
+	return nil
+}
+
 // AddOwner adds a principal to the active profile's owner list. The interceptor
 // has already confirmed the caller is an owner or privileged, the handler just
 // validates and persists.
diff --git a/client/server/ownership_test.go b/client/server/ownership_test.go
new file mode 100644
index 000000000..4163f7b94
--- /dev/null
+++ b/client/server/ownership_test.go
@@ -0,0 +1,88 @@
+package server
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc/codes"
+	gstatus "google.golang.org/grpc/status"
+
+	"github.com/netbirdio/netbird/client/internal/ipcauth"
+	"github.com/netbirdio/netbird/client/internal/profilemanager"
+	"github.com/netbirdio/netbird/util"
+)
+
+// writeTargetProfile writes a profile JSON with the given ownership and returns
+// a Profile handle pointing at it (Path set, so FilePath() resolves directly).
+func writeTargetProfile(t *testing.T, dir, id string, owners []string, shared bool) *profilemanager.Profile {
+	t.Helper()
+	path := filepath.Join(dir, id+".json")
+	cfg := &profilemanager.Config{Owners: owners, Shared: shared}
+	require.NoError(t, util.WriteJson(context.Background(), path, cfg))
+	return &profilemanager.Profile{ID: profilemanager.ID(id), Name: id, Path: path}
+}
+
+func readOwners(t *testing.T, path string) ([]string, bool) {
+	t.Helper()
+	cfg, err := profilemanager.GetConfig(path)
+	require.NoError(t, err)
+	return cfg.Owners, cfg.Shared
+}
+
+func TestAuthorizeTargetProfile(t *testing.T) {
+	s := &Server{groupResolver: ipcauth.NewDefaultGroupResolver()}
+	owner := ipcauth.Identity{UID: 1000}
+	other := ipcauth.Identity{UID: 1001}
+	root := ipcauth.Identity{UID: 0}
+
+	t.Run("no identity denies", func(t *testing.T) {
+		p := writeTargetProfile(t, t.TempDir(), "p", []string{"uid:1000"}, false)
+		err := s.authorizeTargetProfile(context.Background(), p, true)
+		assert.Equal(t, codes.PermissionDenied, gstatus.Code(err))
+	})
+
+	t.Run("privileged allowed on another's profile", func(t *testing.T) {
+		p := writeTargetProfile(t, t.TempDir(), "p", []string{"uid:1000"}, false)
+		assert.NoError(t, s.authorizeTargetProfile(ctxWithIdentity(root), p, true))
+	})
+
+	t.Run("owner allowed", func(t *testing.T) {
+		p := writeTargetProfile(t, t.TempDir(), "p", []string{"uid:1000"}, false)
+		assert.NoError(t, s.authorizeTargetProfile(ctxWithIdentity(owner), p, true))
+	})
+
+	t.Run("non-owner denied", func(t *testing.T) {
+		p := writeTargetProfile(t, t.TempDir(), "p", []string{"uid:1000"}, false)
+		err := s.authorizeTargetProfile(ctxWithIdentity(other), p, true)
+		assert.Equal(t, codes.PermissionDenied, gstatus.Code(err))
+	})
+
+	t.Run("shared allows any caller", func(t *testing.T) {
+		p := writeTargetProfile(t, t.TempDir(), "p", nil, true)
+		assert.NoError(t, s.authorizeTargetProfile(ctxWithIdentity(other), p, true))
+	})
+
+	t.Run("unowned claim stamps owner", func(t *testing.T) {
+		p := writeTargetProfile(t, t.TempDir(), "p", nil, false)
+		require.NoError(t, s.authorizeTargetProfile(ctxWithIdentity(other), p, true))
+
+		owners, shared := readOwners(t, p.Path)
+		assert.Equal(t, []string{"uid:1001"}, owners)
+		assert.False(t, shared)
+
+		// A different caller is now locked out of the claimed profile.
+		err := s.authorizeTargetProfile(ctxWithIdentity(owner), p, true)
+		assert.Equal(t, codes.PermissionDenied, gstatus.Code(err))
+	})
+
+	t.Run("unowned without claim leaves profile unowned", func(t *testing.T) {
+		p := writeTargetProfile(t, t.TempDir(), "p", nil, false)
+		require.NoError(t, s.authorizeTargetProfile(ctxWithIdentity(other), p, false))
+
+		owners, _ := readOwners(t, p.Path)
+		assert.Empty(t, owners)
+	})
+}
diff --git a/client/server/profile_authz.go b/client/server/profile_authz.go
index b1a3876ae..a53b3ee4b 100644
--- a/client/server/profile_authz.go
+++ b/client/server/profile_authz.go
@@ -13,14 +13,12 @@ import (
 )
 
 // bindCallerUsername enforces that a non-privileged caller may only operate on
-// its OWN user's profiles. Profiles live in per-username directories, but the
-// username is a client-supplied gRPC field the server historically never
-// checked; binding it to the caller's kernel identity closes the "pass another
-// user's username" hole. Privileged callers (root / elevated-admin) may manage
-// any user's profiles.
+// its OWN user's profiles. This binds the client-supplied gRPC field to the
+// caller's kernel identity.
+// Privileged callers (root / elevated-admin) may manage any user's profiles.
 func (s *Server) bindCallerUsername(ctx context.Context, requested string) error {
 	if requested == "" {
-		return nil // handlers validate emptiness themselves; nothing to bind
+		return nil
 	}
 
 	id, ok := ipcauth.IdentityFromContext(ctx)
diff --git a/client/server/server.go b/client/server/server.go
index ebe96c2c2..e040c2337 100644
--- a/client/server/server.go
+++ b/client/server/server.go
@@ -127,6 +127,12 @@ type Server struct {
 	updateManager *updater.Manager
 
 	jwtCache *jwtCache
+
+	// groupResolver resolves a Unix caller's supplementary group membership
+	// (NSS/getent) so gid:/group: owner principals authorize correctly. Nil on
+	// Windows (SID group membership travels in the identity itself); ipcauth
+	// treats a nil resolver as "no group matching".
+	groupResolver ipcauth.GroupResolver
 }
 
 type oauthAuthFlow struct {
@@ -151,6 +157,7 @@ func New(ctx context.Context, logFile string, configFile string, profilesDisable
 		jwtCache:               newJWTCache(),
 		extendAuthSessionFlow:  auth.NewPendingFlow(),
 		probeThrottle:          newProbeThrottle(probeThreshold),
+		groupResolver:          ipcauth.NewDefaultGroupResolver(),
 	}
 	agent := &serverAgent{s}
 	s.sleepHandler = sleephandler.New(agent)
@@ -1080,6 +1087,19 @@ func (s *Server) SwitchProfile(callerCtx context.Context, msg *proto.SwitchProfi
 		if err := s.bindCallerUsername(callerCtx, targetUsername); err != nil {
 			return nil, err
 		}
+		// Authorize against the target profile's owners, claiming an unowned
+		// legacy target for the caller.
+		resolveUsername := targetUsername
+		if *msg.ProfileName == profilemanager.DefaultProfileName {
+			resolveUsername = ""
+		}
+		resolvedTarget, err := s.resolveProfileHandle(*msg.ProfileName, resolveUsername)
+		if err != nil {
+			return nil, err
+		}
+		if err := s.authorizeTargetProfile(callerCtx, resolvedTarget, true); err != nil {
+			return nil, err
+		}
 		if _, err := s.switchProfileIfNeeded(*msg.ProfileName, msg.Username, activeProf); err != nil {
 			log.Errorf("failed to switch profile: %v", err)
 			return nil, err
@@ -2080,6 +2100,10 @@ func (s *Server) RenameProfile(ctx context.Context, msg *proto.RenameProfileRequ
 		return nil, err
 	}
 
+	if err := s.authorizeTargetProfile(ctx, resolved, true); err != nil {
+		return nil, err
+	}
+
 	err = s.profileManager.RenameProfile(resolved.ID, msg.Username, msg.NewProfileName)
 	if err != nil {
 		log.Errorf("failed to rename profile: %v", err)
@@ -2113,6 +2137,11 @@ func (s *Server) RemoveProfile(ctx context.Context, msg *proto.RemoveProfileRequ
 		return nil, err
 	}
 
+	// claim=false: don't stamp ownership on a profile we're about to delete.
+	if err := s.authorizeTargetProfile(ctx, resolved, false); err != nil {
+		return nil, err
+	}
+
 	if err := s.logoutFromProfile(ctx, resolved); err != nil {
 		log.Warnf("failed to logout from profile %s before removal: %v", resolved.ID, err)
 	}