diff --git a/bin/omarchy-webapp-install b/bin/omarchy-webapp-install
index b97b6ad6..e07354ba 100755
--- a/bin/omarchy-webapp-install
+++ b/bin/omarchy-webapp-install
@@ -54,6 +54,34 @@ download_icon() {
     [[ -s $2 && $(file -b --mime-type "$2") == image/* ]]
 }
 
+# Chromium --app= treats javascript:, file:, and data: as a document to
+# run. Prefix schemeless input with https as before, then refuse anything
+# that is not http(s).
+normalize_webapp_url() {
+  local url=$1
+  if [[ ! $url =~ ^[a-zA-Z][a-zA-Z0-9+.-]*: ]]; then
+    url="https://$url"
+  fi
+  printf '%s' "$url"
+}
+
+# Raw whitespace must be percent-encoded in a URL. Refuse it before serializing
+# the desktop entry; before Exec argument quoting, it also split browser flags
+# and additional URLs into separate arguments. Schemes are case-insensitive.
+require_http_url() {
+  local url=$1
+
+  if [[ $url =~ [[:space:]] ]]; then
+    echo "Error: web app URL must not contain whitespace." >&2
+    exit 1
+  fi
+
+  if [[ ! ${url,,} =~ ^https?:// ]]; then
+    echo "Error: web app URL must be http or https." >&2
+    exit 1
+  fi
+}
+
 fetch_site_icon() {
   local site_url="$1" dest="$2"
   local origin page icon_url
@@ -113,9 +141,8 @@ if (( $# < 3 )); then
   APP_NAME=$(gum input --prompt "Name> " --placeholder "My favorite web app")
   require_plain_name "$APP_NAME"
   APP_URL=$(gum input --prompt "URL> " --placeholder "https://example.com")
-  if [[ ! $APP_URL =~ ^[a-zA-Z][a-zA-Z0-9+.-]*: ]]; then
-    APP_URL="https://$APP_URL"
-  fi
+  APP_URL=$(normalize_webapp_url "$APP_URL")
+  require_http_url "$APP_URL"
 
   # Try to fetch the site's icon automatically first.
   mkdir -p "$ICON_DIR"
@@ -132,10 +159,8 @@ if (( $# < 3 )); then
   INTERACTIVE_MODE=true
 else
   APP_NAME="$1"
-  APP_URL="$2"
-  if [[ ! $APP_URL =~ ^[a-zA-Z][a-zA-Z0-9+.-]*: ]]; then
-    APP_URL="https://$APP_URL"
-  fi
+  APP_URL=$(normalize_webapp_url "$2")
+  require_http_url "$APP_URL"
   ICON_REF="$3"
   CUSTOM_EXEC="$4" # Optional custom exec command
   MIME_TYPES="$5"  # Optional mime types
diff --git a/test/shell.d/webapp-install-test.sh b/test/shell.d/webapp-install-test.sh
new file mode 100644
index 00000000..2db32353
--- /dev/null
+++ b/test/shell.d/webapp-install-test.sh
@@ -0,0 +1,124 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
+
+tmpdir=$(mktemp -d)
+trap 'rm -rf "$tmpdir"' EXIT
+
+home="$tmpdir/home"
+mkdir -p "$home/.local/share/applications"
+
+install_webapp() {
+  HOME="$home" "$ROOT/bin/omarchy-webapp-install" "$@"
+}
+
+desktop_for() {
+  printf '%s' "$home/.local/share/applications/$1.desktop"
+}
+
+if install_webapp "Example" "https://example.com" "webapp" >"$tmpdir/out" 2>"$tmpdir/err"; then
+  :
+else
+  fail "webapp install accepts an https URL" "$(cat "$tmpdir/err")"
+fi
+
+desktop=$(desktop_for Example)
+[[ -f $desktop ]] || fail "webapp install writes a desktop file"
+grep -Fxq 'Name=Example' "$desktop" || fail "webapp install writes the app name"
+grep -Fxq 'Exec=omarchy-launch-webapp "https://example.com"' "$desktop" ||
+  fail "webapp install launches the https URL" "$(cat "$desktop")"
+pass "webapp install writes an https desktop entry"
+
+if install_webapp "Plain" "example.org/app" "webapp" >"$tmpdir/out" 2>"$tmpdir/err"; then
+  :
+else
+  fail "webapp install prefixes a schemeless URL with https" "$(cat "$tmpdir/err")"
+fi
+grep -Fxq 'Exec=omarchy-launch-webapp "https://example.org/app"' "$(desktop_for Plain)" ||
+  fail "webapp install stores the prefixed https URL" "$(cat "$(desktop_for Plain)")"
+pass "webapp install prefixes a schemeless URL with https"
+
+if install_webapp "Local" "https://localhost:47990" "webapp" "omarchy-launch-webapp https://localhost:47990 --ignore-certificate-errors" >"$tmpdir/out" 2>"$tmpdir/err"; then
+  :
+else
+  fail "webapp install keeps a custom https exec" "$(cat "$tmpdir/err")"
+fi
+grep -Fxq 'Exec=omarchy-launch-webapp https://localhost:47990 --ignore-certificate-errors' "$(desktop_for Local)" ||
+  fail "webapp install writes the custom exec" "$(cat "$(desktop_for Local)")"
+pass "webapp install keeps a custom https exec"
+
+for url in "javascript:alert(1)" "file:///etc/passwd" "data:text/html,hi" "ftp://example.com" "ext://x"; do
+  if install_webapp "Bad" "$url" "webapp" >"$tmpdir/out" 2>"$tmpdir/err"; then
+    fail "webapp install refuses '$url'"
+  fi
+  grep -Fq 'must be http or https' "$tmpdir/err" ||
+    fail "webapp install names the scheme refusal for '$url'" "$(cat "$tmpdir/err")"
+  [[ ! -e $(desktop_for Bad) ]] || fail "webapp install does not write a desktop file for '$url'"
+done
+pass "webapp install refuses non-http(s) URLs"
+
+# Raw whitespace is not valid URL data, and before Exec argument quoting it
+# split browser flags or additional URLs into separate arguments.
+for url in \
+  " javascript:alert(1)" \
+  " file:///etc/passwd" \
+  "https://example.com data:text/html,hi" \
+  "https://example.com/ --user-agent=INJECTION_PROOF_MARKER_12345"; do
+  if install_webapp "Sneak" "$url" "webapp" >"$tmpdir/out" 2>"$tmpdir/err"; then
+    fail "webapp install refuses whitespace in '$url'" "$(cat "$(desktop_for Sneak)")"
+  fi
+  grep -Fq 'must not contain whitespace' "$tmpdir/err" ||
+    fail "webapp install names the whitespace refusal for '$url'" "$(cat "$tmpdir/err")"
+  [[ ! -e $(desktop_for Sneak) ]] || fail "webapp install writes no desktop file for '$url'"
+done
+pass "webapp install refuses a URL carrying whitespace"
+
+# Schemes are case-insensitive, and HTTPS://example.com installed before the
+# scheme test existed.
+if install_webapp "Upper" "HTTPS://example.com" "webapp" >"$tmpdir/out" 2>"$tmpdir/err"; then
+  :
+else
+  fail "webapp install accepts an uppercase scheme" "$(cat "$tmpdir/err")"
+fi
+grep -Fxq 'Exec=omarchy-launch-webapp "HTTPS://example.com"' "$(desktop_for Upper)" ||
+  fail "webapp install keeps the uppercase scheme" "$(cat "$(desktop_for Upper)")"
+pass "webapp install accepts an uppercase http scheme"
+
+# The interactive prompt fetches the site's icon, so a refused URL must be
+# refused before anything dereferences it.
+stubs="$tmpdir/stubs"
+mkdir -p "$stubs"
+
+cat >"$stubs/gum" <<'GUM'
+#!/bin/bash
+count=$(cat "$GUM_COUNT" 2>/dev/null || echo 0)
+count=$((count + 1))
+printf '%s\n' "$count" >"$GUM_COUNT"
+sed -n "${count}p" "$GUM_ANSWERS"
+GUM
+
+cat >"$stubs/curl" <<'CURL'
+#!/bin/bash
+printf '%s\n' "$*" >>"$CURL_LOG"
+exit 1
+CURL
+
+chmod +x "$stubs/gum" "$stubs/curl"
+
+printf 'Evil\nfile:///etc/passwd\n' >"$tmpdir/answers"
+: >"$tmpdir/gum-count"
+: >"$tmpdir/curl-log"
+
+if GUM_ANSWERS="$tmpdir/answers" GUM_COUNT="$tmpdir/gum-count" CURL_LOG="$tmpdir/curl-log" \
+  PATH="$stubs:$PATH" HOME="$home" "$ROOT/bin/omarchy-webapp-install" \
+  >"$tmpdir/out" 2>"$tmpdir/err"; then
+  fail "interactive webapp install refuses a file: URL" "$(cat "$tmpdir/out")"
+fi
+grep -Fq 'must be http or https' "$tmpdir/err" ||
+  fail "interactive webapp install names the scheme refusal" "$(cat "$tmpdir/err")"
+[[ ! -s $tmpdir/curl-log ]] ||
+  fail "interactive webapp install refuses before fetching the URL" "$(cat "$tmpdir/curl-log")"
+[[ ! -e $(desktop_for Evil) ]] || fail "interactive webapp install writes no desktop file"
+pass "interactive webapp install refuses a bad URL before fetching it"