diff --git a/internal/profile/detectors.go b/internal/profile/detectors.go
index 604f49d..8e245d0 100644
--- a/internal/profile/detectors.go
+++ b/internal/profile/detectors.go
@@ -24,6 +24,11 @@ const (
 	CapSeat       = "seat"
 	CapPrivileged = "privileged"
 
+	// CapUserNamespaces is whether an unprivileged user can make a user namespace: what a build
+	// module's sandbox (bubblewrap) needs (novox/hq ADR 0448 section 5). Proven by making one, as
+	// an unprivileged user, never by a version string or a sysctl alone.
+	CapUserNamespaces = "user-namespaces"
+
 	// The network manager this machine runs, one capability per dialect (novox/hq ADR 0161): the
 	// uplink seat's holder declares its own, so the holder for a manager the machine does not run
 	// is refused the way any missing capability is, naming it. Active, not installed — a machine
@@ -189,6 +194,7 @@ func Default(runner Runner) []Detector {
 		seat{},
 		battery{},
 		powerMeter{},
+		userNamespaces(runner),
 		commandCapability{
 			name: CapContainerRuntime, command: "docker", args: []string{"info", "--format", "{{.ServerVersion}}"},
 			why:    "asks the daemon for its version — a running daemon, not an installed client",
diff --git a/internal/profile/userns.go b/internal/profile/userns.go
new file mode 100644
index 0000000..b648137
--- /dev/null
+++ b/internal/profile/userns.go
@@ -0,0 +1,61 @@
+package profile
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// effectiveUID and readProcFile are replaceable so the unit tests stay off the machine.
+var (
+	effectiveUID = os.Geteuid
+	readProcFile = func(path string) (string, error) {
+		b, err := os.ReadFile(path)
+		return string(b), err
+	}
+)
+
+// unprivilegedID is the uid and gid the probe drops to (nobody on every mesh node).
+const unprivilegedID = "65534"
+
+// userNamespaces builds the detector for CapUserNamespaces: it makes a user namespace as an
+// unprivileged user. The node-engine runs as root, and root can always make one, so as root the
+// probe drops first with setpriv (util-linux). Go's SysProcAttr Cloneflags with a Credential
+// would clone before the uid drops and prove nothing, so it is not used.
+func userNamespaces(runner Runner) Detector {
+	command, args := "unshare", []string{"--user", "true"}
+	if effectiveUID() == 0 {
+		command = "setpriv"
+		args = []string{"--reuid=" + unprivilegedID, "--regid=" + unprivilegedID, "--clear-groups", "unshare", "--user", "true"}
+	}
+	read := readProcFile // bound now, so a probe cut by the timeout never reads a later replacement
+	return commandCapability{
+		name: CapUserNamespaces, command: command, args: args,
+		why: "makes a user namespace as an unprivileged user — what bubblewrap needs, not a kernel setting read",
+		interpret: func(_ string, err error) (bool, string) {
+			if err == nil {
+				return true, "an unprivileged user can make a user namespace"
+			}
+			return false, err.Error() + userNamespaceSettings(read)
+		},
+		runner: runner,
+	}
+}
+
+// userNamespaceSettings names the two kernel settings that switch user namespaces off, when
+// they can be read. It is added to the reason of an absent verdict only.
+func userNamespaceSettings(read func(string) (string, error)) string {
+	var parts []string
+	for _, s := range []struct{ path, name string }{
+		{"/proc/sys/kernel/unprivileged_userns_clone", "unprivileged_userns_clone"},
+		{"/proc/sys/user/max_user_namespaces", "max_user_namespaces"},
+	} {
+		if v, err := read(s.path); err == nil {
+			parts = append(parts, fmt.Sprintf("%s=%s", s.name, strings.TrimSpace(v)))
+		}
+	}
+	if len(parts) == 0 {
+		return ""
+	}
+	return " (" + strings.Join(parts, ", ") + ")"
+}
diff --git a/internal/profile/userns_test.go b/internal/profile/userns_test.go
new file mode 100644
index 0000000..67a27d2
--- /dev/null
+++ b/internal/profile/userns_test.go
@@ -0,0 +1,155 @@
+package profile
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// recorded is a runner that remembers every command it was given and answers the user-namespace
+// probe with the given output and error. Other detectors get "not here".
+type recorded struct {
+	mu    sync.Mutex
+	calls [][]string
+	out   string
+	err   error
+}
+
+func (r *recorded) run(_ context.Context, name string, args ...string) (string, error) {
+	if name != "setpriv" && name != "unshare" {
+		return "", errors.New("not here")
+	}
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.calls = append(r.calls, append([]string{name}, args...))
+	return r.out, r.err
+}
+
+// asUID makes the detector believe it runs as uid, and reads no files from the machine.
+func asUID(t *testing.T, uid int, procs map[string]string) {
+	t.Helper()
+	oldUID, oldRead := effectiveUID, readProcFile
+	effectiveUID = func() int { return uid }
+	readProcFile = func(path string) (string, error) {
+		if v, ok := procs[path]; ok {
+			return v, nil
+		}
+		return "", errors.New("no such file")
+	}
+	t.Cleanup(func() { effectiveUID, readProcFile = oldUID, oldRead })
+}
+
+func verdictOf(t *testing.T, p Profile, name string) Verdict {
+	t.Helper()
+	for _, v := range p.Capabilities {
+		if v.Name == name {
+			return v
+		}
+	}
+	t.Fatalf("no verdict named %s in the profile", name)
+	return Verdict{}
+}
+
+// unshare's own words when user namespaces are off, from util-linux's unshare.c:
+// err(EXIT_FAILURE, _("unshare failed")), which prints "unshare: unshare failed: " and strerror(EPERM).
+// ExecRunner wraps stderr as "<name> exited <code>: <stderr>".
+const unshareRefused = "unshare exited 1: unshare: unshare failed: Operation not permitted"
+
+func TestUserNamespacesAsRootDropsToAnUnprivilegedUIDFirst(t *testing.T) {
+	// 1308/AC1. Root can always make a user namespace, so a probe run as root proves nothing: the
+	// command must drop to an unprivileged uid before unshare runs.
+	asUID(t, 0, nil)
+	r := &recorded{}
+	got := Detect(context.Background(), Default(r.run), time.Second)
+
+	v := verdictOf(t, got, CapUserNamespaces)
+	if !v.Present {
+		t.Fatalf("a successful probe was reported absent: %+v", v)
+	}
+	want := "setpriv --reuid=65534 --regid=65534 --clear-groups unshare --user true"
+	if len(r.calls) != 1 || strings.Join(r.calls[0], " ") != want {
+		t.Fatalf("the probe ran %v, want one call: %s", r.calls, want)
+	}
+}
+
+func TestUserNamespacesAbsentCarriesTheCommandsWords(t *testing.T) {
+	// 1308/AC2. The failing probe gives absent, and the detail is what unshare said.
+	asUID(t, 0, nil)
+	r := &recorded{err: errors.New(unshareRefused)}
+	got := Detect(context.Background(), Default(r.run), time.Second)
+
+	v := verdictOf(t, got, CapUserNamespaces)
+	if v.Present {
+		t.Fatal("a probe that failed was reported present")
+	}
+	if !strings.Contains(v.Detail, "unshare failed: Operation not permitted") {
+		t.Fatalf("the command's words were lost; detail was %q", v.Detail)
+	}
+	if strings.TrimSpace(v.How) == "" {
+		t.Fatal("an absent verdict with no stated method")
+	}
+}
+
+func TestUserNamespacesAbsentSaysWhyFromTheKernelSettings(t *testing.T) {
+	// 1308/AC2, the reasons: the two kernel settings that switch user namespaces off are read
+	// through a replaceable function and named in the detail.
+	asUID(t, 0, map[string]string{
+		"/proc/sys/user/max_user_namespaces":         "0\n",
+		"/proc/sys/kernel/unprivileged_userns_clone": "0\n",
+	})
+	r := &recorded{err: errors.New(unshareRefused)}
+	v := verdictOf(t, Detect(context.Background(), Default(r.run), time.Second), CapUserNamespaces)
+
+	if v.Present {
+		t.Fatal("reported present")
+	}
+	// The detail is one line (firstLine), so the settings must be on that line.
+	if !strings.Contains(v.Detail, "max_user_namespaces=0") || !strings.Contains(v.Detail, "unprivileged_userns_clone=0") {
+		t.Fatalf("the kernel settings are not in the detail: %q", v.Detail)
+	}
+}
+
+func TestUserNamespacesNotAsRootUnsharesDirectly(t *testing.T) {
+	// 1308/AC3. An unprivileged node-engine is already the unprivileged user: no setpriv.
+	asUID(t, 1000, nil)
+	r := &recorded{}
+	v := verdictOf(t, Detect(context.Background(), Default(r.run), time.Second), CapUserNamespaces)
+
+	if !v.Present {
+		t.Fatalf("a successful probe was reported absent: %+v", v)
+	}
+	if len(r.calls) != 1 || strings.Join(r.calls[0], " ") != "unshare --user true" {
+		t.Fatalf("the probe ran %v, want: unshare --user true", r.calls)
+	}
+	if strings.TrimSpace(v.Detail) == "" || strings.TrimSpace(v.How) == "" {
+		t.Fatalf("present verdict lacks detail or how: %+v", v)
+	}
+}
+
+func TestUserNamespacesProbeThatHangsIsCutAndTheRestStillReport(t *testing.T) {
+	// 1308/AC4. A wedged unshare is cut by Detect's timeout; the other detectors still answer.
+	asUID(t, 0, nil)
+	blocking := func(ctx context.Context, name string, args ...string) (string, error) {
+		if name == "setpriv" {
+			<-ctx.Done()
+			return "", ctx.Err()
+		}
+		return "ok", nil
+	}
+	done := make(chan Profile, 1)
+	go func() { done <- Detect(context.Background(), Default(blocking), 50*time.Millisecond) }()
+	select {
+	case got := <-done:
+		if verdictOf(t, got, CapUserNamespaces).Present {
+			t.Error("a probe that never answered was reported present")
+		}
+		if !got.Has(CapPackageManager) {
+			t.Error("the other capabilities were lost behind the wedged probe")
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("detection did not return")
+	}
+}