fix(0128): XSS scheme allowlist + drop dead fileID

review findings:
- MessageBody: only http(s) and relative paths allowed for links;
  data:image/* allowed for inline images. Rejects javascript:,
  data:text/html, vbscript: which would execute via <a href>.
  Unsafe matches fall back to plain text.
- files.go: remove unused fileID var generated then discarded.

backend/files.go

@@ -197,7 +197,6 @@ func handleUploadCardFile(db *DB, workdir string) http.HandlerFunc {
 		}
 
 		fname := safeFilename(header.Filename)
-		fileID := newID()
 		storedPath := filepath.Join(dir, randomFilePrefix()+"__"+fname)
 
 		out, err := os.Create(storedPath)
@@ -237,16 +236,12 @@ func handleUploadCardFile(db *DB, workdir string) http.HandlerFunc {
 
 		actor, _ := infra.UserIDFromContext(r.Context(), userCtxKey)
 
-		// Use the random-prefixed path on disk but a stable file id in the DB.
 		cf, err := db.CreateCardFile(cardID, actor, fname, mimeType, storedPath, source, written)
 		if err != nil {
 			os.Remove(storedPath)
 			serverError(w, err)
 			return
 		}
-		// We generated newID() in CreateCardFile; align the on-disk filename with that id
-		// is not required since stored_path is what we serve from.
-		_ = fileID
 
 		infra.HTTPJSONResponse(w, http.StatusCreated, cf)
 	}

frontend/src/components/MessageBody.tsx

@@ -14,6 +14,24 @@ type Token = ImgToken | LinkToken | TextToken;
 
 const TOKEN_RE = /(!\[([^\]\n]*)\]\(([^)\s]+)\))|(\[([^\]\n]+)\]\(([^)\s]+)\))/g;
 
+// Allow only safe URL schemes. Reject javascript:, data:text/html, vbscript:, etc.
+// Accepts: absolute http(s), protocol-relative //, and same-origin paths (/...).
+function safeURL(url: string): string | null {
+  const u = url.trim();
+  if (u.startsWith("/")) return u;
+  if (/^https?:\/\//i.test(u)) return u;
+  return null;
+}
+
+// data: scheme is allowed only when the MIME prefix is image/.
+function safeImageURL(url: string): string | null {
+  const safe = safeURL(url);
+  if (safe) return safe;
+  const u = url.trim();
+  if (/^data:image\/[a-z0-9.+-]+(;[a-z0-9-]+=[^,]+)*;base64,/i.test(u)) return u;
+  return null;
+}
+
 function tokenize(input: string): Token[] {
   const out: Token[] = [];
   let last = 0;
@@ -24,9 +42,19 @@ function tokenize(input: string): Token[] {
       out.push({ kind: "text", value: input.slice(last, m.index) });
     }
     if (m[1]) {
-      out.push({ kind: "img", alt: m[2] || "", url: m[3] });
+      const url = safeImageURL(m[3]);
+      if (url) {
+        out.push({ kind: "img", alt: m[2] || "", url });
+      } else {
+        out.push({ kind: "text", value: m[0] });
+      }
     } else if (m[4]) {
-      out.push({ kind: "link", label: m[5], url: m[6] });
+      const url = safeURL(m[6]);
+      if (url) {
+        out.push({ kind: "link", label: m[5], url });
+      } else {
+        out.push({ kind: "text", value: m[0] });
+      }
     }
     last = TOKEN_RE.lastIndex;
   }