diff --git a/install.sh b/install.sh
index 443c038..4dc56c6 100755
--- a/install.sh
+++ b/install.sh
@@ -119,6 +119,37 @@ need() {
   fi
 }
 
+normalize_update_cache_path() {
+  case "$1" in
+    /*) printf '%s\n' "$1" ;;
+    *) printf '%s\n' "$(pwd)/$1" ;;
+  esac | sed 's#//*#/#g; s#/$##'
+}
+
+safe_update_cache_path() {
+  raw="$1"
+  if [ -z "$raw" ]; then
+    echo "refusing unsafe update cache path: $raw" >&2
+    exit 2
+  fi
+  case "$raw" in
+    "."|".."|./*|../*|*/.|*/..|*/./*|*/../*)
+      echo "refusing unsafe update cache path: $raw" >&2
+      exit 2
+      ;;
+  esac
+  normalized="$(normalize_update_cache_path "$raw")"
+  home_norm="$(normalize_update_cache_path "$HOME")"
+  home_cache_norm="$(normalize_update_cache_path "$HOME/.cache")"
+  case "$normalized" in
+    ""|"/"|"$home_norm"|"$home_cache_norm")
+      echo "refusing unsafe update cache path: $raw" >&2
+      exit 2
+      ;;
+  esac
+  printf '%s\n' "$normalized"
+}
+
 ensure_cargo() {
   if command -v cargo >/dev/null 2>&1; then
     return
@@ -466,12 +497,7 @@ install_from_source() {
       echo "DOSH_REPO or --repo is required when running the installer from curl" >&2
       exit 2
     fi
-    case "$update_cache" in
-      ""|"/"|"$HOME"|"$HOME/"|"$HOME/.cache")
-        echo "refusing unsafe update cache path: $update_cache" >&2
-        exit 2
-        ;;
-    esac
+    update_cache="$(safe_update_cache_path "$update_cache")"
     need git
     mkdir -p "$(dirname "$update_cache")"
     if [ -d "$update_cache/.git" ]; then
diff --git a/tests/release_scripts.rs b/tests/release_scripts.rs
index 603401a..ff5fdda 100644
--- a/tests/release_scripts.rs
+++ b/tests/release_scripts.rs
@@ -109,6 +109,27 @@ fn windows_installer_reuses_persistent_source_update_cache() {
     );
 }
 
+#[test]
+fn unix_installer_rejects_unsafe_source_update_cache_paths() {
+    let install = include_str!("../install.sh");
+    assert!(install.contains("normalize_update_cache_path()"));
+    assert!(install.contains("safe_update_cache_path()"));
+    assert!(install.contains("update_cache=\"$(safe_update_cache_path \"$update_cache\")\""));
+    for pattern in [
+        "\"\"|\"/\"|\"$home_norm\"|\"$home_cache_norm\"",
+        "\".\"|\"..\"|./*|../*|*/.|*/..|*/./*|*/../*",
+    ] {
+        assert!(
+            install.contains(pattern),
+            "Unix installer must reject unsafe update cache pattern {pattern}"
+        );
+    }
+    assert!(
+        install.contains("rm -rf \"$update_cache\""),
+        "source cache replacement remains the destructive operation guarded by safe_update_cache_path"
+    );
+}
+
 #[test]
 fn package_check_verifies_only_artifacts_it_builds() {
     let makefile = include_str!("../Makefile");