diff --git a/tools/sandbox-lint/src/sandbox_lint/__init__.py b/tools/sandbox-lint/src/sandbox_lint/__init__.py
index e9cf173bc..a617166c7 100644
--- a/tools/sandbox-lint/src/sandbox_lint/__init__.py
+++ b/tools/sandbox-lint/src/sandbox_lint/__init__.py
@@ -121,15 +121,13 @@
 
 
 def _normalise(path: str) -> str:
-    """Strip a single trailing slash so '~/.aws' and '~/.aws/' compare equal.
+    """Strip all trailing slashes so '~/.aws' and '~/.aws//' compare equal.
 
-    The sandbox treats both forms as the same directory; the validator
-    must too, otherwise the forbidden-list could be bypassed by a
-    trailing-slash variant.
+    The sandbox treats every trailing-slash count as the same directory;
+    the validator must too, otherwise the forbidden-list could be bypassed
+    by a multi-slash variant.
     """
-    if path.endswith("/") and len(path) > 1:
-        return path[:-1]
-    return path
+    return path.rstrip("/") or "/"
 
 
 def _normalised_set(paths: list[str]) -> set[str]:
diff --git a/tools/sandbox-lint/tests/test_validator.py b/tools/sandbox-lint/tests/test_validator.py
index 791664835..497d95bdb 100644
--- a/tools/sandbox-lint/tests/test_validator.py
+++ b/tools/sandbox-lint/tests/test_validator.py
@@ -163,6 +163,7 @@ def test_invariant_missing_deny_read_root(baseline: dict[str, Any]) -> None:
         "~/.config/gcloud",
         "/",
         "~/",
+        "~/.ssh//",
     ],
 )
 def test_invariant_allow_read_rejects_credential_paths(baseline: dict[str, Any], forbidden: str) -> None:
@@ -181,6 +182,7 @@ def test_invariant_allow_read_rejects_credential_paths(baseline: dict[str, Any],
         "~/.gnupg",
         "~/.ssh",
         "~/.aws/",
+        "~/.aws//",
     ],
 )
 def test_invariant_allow_write_rejects_credential_paths(baseline: dict[str, Any], forbidden: str) -> None: