diff --git a/main.go b/main.go
index e7a31f3..823a691 100644
--- a/main.go
+++ b/main.go
@@ -369,8 +369,8 @@ func allowedRoots(home string) (roots []string, def bool, err error) {
 			return nil, false, fmt.Errorf("%s: %q is a dot-directory of $HOME", allowedPathsFile(), l)
 		}
 		for _, bad := range forbiddenRoots {
-			if inside(r, bad) {
-				return nil, false, fmt.Errorf("%s: %q is under %s", allowedPathsFile(), l, bad)
+			if bad = realpath(bad); inside(r, bad) || inside(bad, r) {
+				return nil, false, fmt.Errorf("%s: %q overlaps %s", allowedPathsFile(), l, bad)
 			}
 		}
 		for _, d := range pathDirs() {
@@ -398,9 +398,9 @@ func pathDirs() []string {
 }
 
 // checkLaunchDir returns pwd resolved if it is strictly under an allowed root,
-// never $HOME itself or anything holding the sandbox's config, the API socket
-// or a $PATH entry. Under the default $HOME root, dot-dirs of $HOME are
-// refused too.
+// never $HOME itself, anything holding the sandbox's config, the API socket or
+// a $PATH entry, nor anything overlapping a forbidden system dir. Under the
+// default $HOME root, dot-dirs of $HOME are refused too.
 func checkLaunchDir(pwd, allowlist string) (string, error) {
 	pwd = realpath(pwd)
 	home := realpath(os.Getenv("HOME"))
@@ -416,11 +416,16 @@ func checkLaunchDir(pwd, allowlist string) (string, error) {
 	}
 	paths := allowedPathsFile()
 	for _, dir := range append([]string{configDir(), filepath.Dir(allowlist), filepath.Dir(realpath(allowlist)),
-		filepath.Dir(paths), filepath.Dir(realpath(paths)), runtimeDir()}, pathDirs()...) {
+		filepath.Dir(realpath(paths)), runtimeDir()}, pathDirs()...) {
 		if dir = realpath(dir); inside(dir, pwd) {
 			return "", fmt.Errorf("refusing to mount %s: it contains %s", pwd, dir)
 		}
 	}
+	for _, bad := range forbiddenRoots {
+		if bad = realpath(bad); inside(pwd, bad) || inside(bad, pwd) {
+			return "", fmt.Errorf("refusing to mount %s: it overlaps %s", pwd, bad)
+		}
+	}
 	for _, r := range roots {
 		if pwd != r && inside(pwd, r) {
 			return pwd, nil
diff --git a/main_test.go b/main_test.go
index db257ca..753e279 100644
--- a/main_test.go
+++ b/main_test.go
@@ -325,6 +325,7 @@ func TestLaunchRefusals(t *testing.T) {
 			writePaths(t, e, "~/src")
 		}},
 		{"invalid-root", "home/proj", func(t *testing.T, e *env) { writePaths(t, e, "/run/x") }},
+		{"var-tmp-under-var-root", "/var/tmp", func(t *testing.T, e *env) { writePaths(t, e, "/var") }},
 		{"default-root-dot-dir", "home/.local/src/x", func(t *testing.T, e *env) {
 			_ = os.MkdirAll(e.work+"/home/.local/src/x", 0o755)
 		}},
@@ -359,6 +360,11 @@ func TestCheckLaunchDirRuntimeFallback(t *testing.T) {
 	}
 	t.Setenv("HOME", "/tmp/home")
 	_, err = checkLaunchDir("/tmp/home/proj", "/nonexistent/hosts")
+	check(t, "forbidden default root", true, err != nil && strings.Contains(err.Error(), "overlaps "+realpath("/tmp")))
+	origRoots := forbiddenRoots
+	forbiddenRoots = []string{"/run"}
+	t.Cleanup(func() { forbiddenRoots = origRoots })
+	_, err = checkLaunchDir("/tmp/home/proj", "/nonexistent/hosts")
 	check(t, "subdir", nil, err)
 }
 
@@ -591,11 +597,14 @@ func TestCheckLaunchDirRoots(t *testing.T) {
 	}
 	for _, bad := range []string{"/", "/home", "~", "~/", "~/.local", "~/.local/share", "~/.ssh", "~/.ssh/x",
 		"~/.gnupg", "~/.config/foo", cfg, cfg + "/agent-sandbox/x", "~/.local/share/containers/s", "relative/x", "/run/user/1000", "/tmp/x", "/var/tmp", "/proc/1",
-		"/sys", "/dev/shm", "/usr", "/srv/tools"} {
+		"/sys", "/dev/shm", "/usr", "/srv/tools", "/var"} {
 		write("/srv/work\n" + bad)
 		_, _, err := allowedRoots("/home/u")
 		check(t, "invalid root "+bad, true, strings.Contains(fmt.Sprint(err), "allowed-paths"))
 	}
+	write("/var")
+	_, err := checkLaunchDir("/var/tmp/x", "/nowhere/hosts")
+	check(t, "/var/tmp under /var root", true, err != nil)
 }
 
 func TestLaunchPassesArgsVerbatim(t *testing.T) {