diff --git a/src/server/request-handler.ts b/src/server/request-handler.ts
index 3f96c56..394e266 100644
--- a/src/server/request-handler.ts
+++ b/src/server/request-handler.ts
@@ -19,8 +19,22 @@ type ShareListingEntry = {
   isCollection: boolean;
 };
 
+function parseHttpUrl(value: string, errorMessage: string): URL {
+  const url = new URL(value.trim());
+
+  if (url.protocol !== "https:" && url.protocol !== "http:") {
+    throw new Error(errorMessage);
+  }
+
+  if (url.username || url.password) {
+    throw new Error("URL credentials are not allowed.");
+  }
+
+  return url;
+}
+
 function resolveDavUrlFromShareUrl(shareUrl: string): string {
-  const url = new URL(shareUrl);
+  const url = parseHttpUrl(shareUrl, "Please enter a public Nextcloud share link.");
   const publicShare = url.pathname.match(/\/s\/([^/?#]+)/);
 
   if (publicShare) {
@@ -36,6 +50,17 @@ function resolveDavUrlFromShareUrl(shareUrl: string): string {
   throw new Error("Please enter a public Nextcloud share link.");
 }
 
+function resolveValidatedBlobUrl(targetUrl: string, shareUrl: string): string {
+  const target = parseHttpUrl(targetUrl, "Invalid image URL.");
+  const shareDavUrl = new URL(resolveDavUrlFromShareUrl(shareUrl));
+
+  if (target.origin !== shareDavUrl.origin || !target.pathname.startsWith(shareDavUrl.pathname)) {
+    throw new Error("Image URL is outside the requested Nextcloud share.");
+  }
+
+  return target.toString();
+}
+
 async function proxyUpstream(url: string, init?: RequestInit) {
   const upstream = await fetch(url, init);
   const body = await upstream.arrayBuffer();
@@ -1018,6 +1043,16 @@ function htmlPage(): string {
         return element;
       }
 
+      function escapeHtml(value) {
+        return String(value ?? "").replace(/[&<>"']/g, (character) => ({
+          "&": "&amp;",
+          "<": "&lt;",
+          ">": "&gt;",
+          '"': "&quot;",
+          "'": "&#39;"
+        })[character]);
+      }
+
       function setTheme(theme) {
         const resolvedTheme = theme === "light" ? "light" : "dark";
         document.body.dataset.theme = resolvedTheme;
@@ -1383,7 +1418,7 @@ function htmlPage(): string {
         return L.divIcon({
           className: "photo-map-marker" + activeClass,
           html:
-            '<img src="' + photo.thumbUrl + '" alt="' + photo.name + '" />',
+            '<img src="' + escapeHtml(photo.thumbUrl) + '" alt="' + escapeHtml(photo.name) + '" />',
           iconSize: [44, 44],
           iconAnchor: [22, 22],
           popupAnchor: [0, -22]
@@ -1588,10 +1623,10 @@ function htmlPage(): string {
 
               marker.bindPopup(
                 '<div class="thumb">' +
-                  '<img src="' + photo.thumbUrl + '" alt="' + photo.name + '" />' +
-                  '<strong>' + photo.name + '</strong>' +
-                  '<small>' + formatDate(photo.capturedAt) + '</small>' +
-                  '<button type="button" data-open="' + photo.id + '">Open</button>' +
+                  '<img src="' + escapeHtml(photo.thumbUrl) + '" alt="' + escapeHtml(photo.name) + '" />' +
+                  '<strong>' + escapeHtml(photo.name) + '</strong>' +
+                  '<small>' + escapeHtml(formatDate(photo.capturedAt)) + '</small>' +
+                  '<button type="button" data-open="' + escapeHtml(photo.id) + '">Open</button>' +
                 '</div>'
               );
 
@@ -1606,8 +1641,10 @@ function htmlPage(): string {
             const item = document.createElement("article");
             item.className = "photo" + (photo.id === state.activePhotoId ? " active" : "");
             item.innerHTML =
-              '<img src="' + photo.thumbUrl + '" alt="' + photo.name + '" />' +
-              '<div><strong>' + photo.name + '</strong><span>' + formatDate(photo.capturedAt) + '</span></div>';
+              '<img src="' + escapeHtml(photo.thumbUrl) + '" alt="' + escapeHtml(photo.name) + '" />' +
+              '<div><strong>' + escapeHtml(photo.name) + '</strong><span>' +
+              escapeHtml(formatDate(photo.capturedAt)) +
+              '</span></div>';
             item.addEventListener("click", () => {
               state.activePhotoId = photo.id;
               renderVisiblePhotos({ fitMap: false });
@@ -1813,8 +1850,12 @@ function htmlPage(): string {
         return parseListing(await response.text(), davBaseUrl);
       }
 
-      async function readRemoteImage(entry, signal) {
-        const response = await fetch("/api/nextcloud/blob?url=" + encodeURIComponent(entry.href), {
+      async function readRemoteImage(entry, shareUrlValue, signal) {
+        const params = new URLSearchParams({
+          share: shareUrlValue.trim(),
+          url: entry.href
+        });
+        const response = await fetch("/api/nextcloud/blob?" + params.toString(), {
           signal
         });
         if (!response.ok) {
@@ -1872,7 +1913,7 @@ function htmlPage(): string {
             }
 
             try {
-              const photo = await readRemoteImage(entry, controller.signal);
+              const photo = await readRemoteImage(entry, importUrlInput.value, controller.signal);
               if (photo.latitude !== null && photo.longitude !== null) {
                 appendPhoto(photo);
                 loaded += 1;
@@ -2005,16 +2046,32 @@ export function createRequestHandler() {
 
     if (url.pathname === "/api/nextcloud/blob") {
       const target = url.searchParams.get("url");
+      const share = url.searchParams.get("share");
 
-      if (!target) {
+      if (!target || !share) {
         res.statusCode = 400;
         res.setHeader("content-type", "application/json; charset=utf-8");
-        res.end(JSON.stringify({ error: "missing `url` parameter" }));
+        res.end(JSON.stringify({ error: "missing `share` or `url` parameter" }));
+        return;
+      }
+
+      let validatedTarget: string;
+
+      try {
+        validatedTarget = resolveValidatedBlobUrl(target, share);
+      } catch (error) {
+        res.statusCode = 400;
+        res.setHeader("content-type", "application/json; charset=utf-8");
+        res.end(
+          JSON.stringify({
+            error: error instanceof Error ? error.message : "invalid image URL"
+          })
+        );
         return;
       }
 
       try {
-        const upstream = await proxyUpstream(target);
+        const upstream = await proxyUpstream(validatedTarget);
         res.statusCode = upstream.status;
         res.setHeader("content-type", upstream.contentType);
         res.end(upstream.body);