Fix arbitrary file write vulnerability by moving file dialog to backend

Moved the file selection dialog (`tauri-plugin-dialog`) into the Rust
backend commands (`write_image_file` and `write_text_file`) instead of
having the frontend supply arbitrary raw paths to write to. This ensures
that the OS-native dialog handles secure path resolution and prevents
potential remote code execution (RCE) via XSS.

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

patch_returns.diff

@@ -0,0 +1,52 @@
+--- src-tauri/src/main.rs
++++ src-tauri/src/main.rs
+@@ -40,40 +40,42 @@
+ }
+
+ #[tauri::command]
+-fn write_text_file(app: AppHandle, contents: String, is_sum: bool) -> Result<(), String> {
++fn write_text_file(app: AppHandle, contents: String, is_sum: bool) -> Result<bool, String> {
+     let (name, ext) = if is_sum {
+         ("Summarum Sheet", "sum")
+     } else {
+         ("Text file", "txt")
+     };
+     if let Some(p) = app
+         .dialog()
+         .file()
+         .add_filter(name, &[ext])
+         .blocking_save_file()
+     {
+         write_atomic(&p.into_path().map_err(|e| e.to_string())?, &contents)?;
++        Ok(true)
++    } else {
++        Ok(false)
+     }
+-    Ok(())
+ }
+
+ #[tauri::command]
+-fn write_image_file(app: AppHandle, data_base64: String) -> Result<(), String> {
++fn write_image_file(app: AppHandle, data_base64: String) -> Result<bool, String> {
+     if let Some(p) = app
+         .dialog()
+         .file()
+         .add_filter("PNG Image", &["png"])
+         .blocking_save_file()
+     {
+         let path = p.into_path().map_err(|e| e.to_string())?;
+         use std::io::Write;
+         let bytes = base64_decode(&data_base64)?;
+         let tmp = path.with_extension("tmp");
+         std::fs::File::create(&tmp)
+             .and_then(|mut f| f.write_all(&bytes))
+             .map_err(|e| e.to_string())?;
+         fs::rename(&tmp, path).map_err(|e| e.to_string())?;
++        Ok(true)
++    } else {
++        Ok(false)
+     }
+-    Ok(())
+ }
+
+ fn base64_decode(s: &str) -> Result<Vec<u8>, String> {

src-tauri/src/main.rs

@@ -9,6 +9,7 @@ use std::time::{SystemTime, UNIX_EPOCH};
 use tauri::menu::{Menu, MenuItem};
 use tauri::tray::TrayIconBuilder;
 use tauri::{AppHandle, Emitter, Manager, WindowEvent};
+use tauri_plugin_dialog::DialogExt;
 use tauri_plugin_opener::OpenerExt;
 
 const RATES_TTL_SECS: u64 = 3600;
@@ -38,20 +39,45 @@ fn safe_name(name: &str) -> bool {
 }
 
 #[tauri::command]
-fn write_text_file(path: String, contents: String) -> Result<(), String> {
-    write_atomic(std::path::Path::new(&path), &contents)
+fn write_text_file(app: AppHandle, contents: String, is_sum: bool) -> Result<bool, String> {
+    let (name, ext) = if is_sum {
+        ("Summarum Sheet", "sum")
+    } else {
+        ("Text file", "txt")
+    };
+    if let Some(p) = app
+        .dialog()
+        .file()
+        .add_filter(name, &[ext])
+        .blocking_save_file()
+    {
+        write_atomic(&p.into_path().map_err(|e| e.to_string())?, &contents)?;
+        Ok(true)
+    } else {
+        Ok(false)
+    }
 }
 
 #[tauri::command]
-fn write_image_file(path: String, data_base64: String) -> Result<(), String> {
-    use std::io::Write;
-    let bytes = base64_decode(&data_base64)?;
-    let p = std::path::Path::new(&path);
-    let tmp = p.with_extension("tmp");
-    std::fs::File::create(&tmp)
-        .and_then(|mut f| f.write_all(&bytes))
-        .map_err(|e| e.to_string())?;
-    fs::rename(&tmp, p).map_err(|e| e.to_string())
+fn write_image_file(app: AppHandle, data_base64: String) -> Result<bool, String> {
+    if let Some(p) = app
+        .dialog()
+        .file()
+        .add_filter("PNG Image", &["png"])
+        .blocking_save_file()
+    {
+        let path = p.into_path().map_err(|e| e.to_string())?;
+        use std::io::Write;
+        let bytes = base64_decode(&data_base64)?;
+        let tmp = path.with_extension("tmp");
+        std::fs::File::create(&tmp)
+            .and_then(|mut f| f.write_all(&bytes))
+            .map_err(|e| e.to_string())?;
+        fs::rename(&tmp, path).map_err(|e| e.to_string())?;
+        Ok(true)
+    } else {
+        Ok(false)
+    }
 }
 
 fn base64_decode(s: &str) -> Result<Vec<u8>, String> {

src/main.ts

@@ -907,18 +907,16 @@ async function boot(): Promise<void> {
     } else if (action === "print") {
       window.print();
     } else if (isTauri()) {
-      const { save } = await import("@tauri-apps/plugin-dialog");
       const { invoke } = await import("@tauri-apps/api/core");
       const isSum = action === "save-sum";
-      const path = await save({
-        filters: isSum
-          ? [{ name: "Summarum Sheet", extensions: ["sum"] }]
-          : [{ name: "Text file", extensions: ["txt"] }],
-      });
-      if (path) {
-        const content = isSum ? editor.getText() : editor.getSheetWithResults();
-        await invoke("write_text_file", { path, contents: content });
-        toast(t("saved"));
+      const content = isSum ? editor.getText() : editor.getSheetWithResults();
+      try {
+        const saved = await invoke<boolean>("write_text_file", { contents: content, isSum });
+        if (saved) {
+          toast(t("saved"));
+        }
+      } catch {
+        toast(t("imageFailed"));
       }
     }
   });

src/storage.ts

@@ -234,9 +234,9 @@ export async function fetchHistoricalRates(date: string): Promise<Record<string,
 
 // ---------- image export
 
-export async function writeImageFile(path: string, dataBase64: string): Promise<void> {
-  if (!isTauri()) return;
-  await invoke("write_image_file", { path, dataBase64 });
+export async function writeImageFile(dataBase64: string): Promise<boolean> {
+  if (!isTauri()) return false;
+  return invoke("write_image_file", { dataBase64 });
 }
 
 // ---------- market data

src/ui/export.ts

@@ -58,12 +58,9 @@ export async function exportSheetImage(deps: ExportImageDeps): Promise<void> {
   } catch {
     // clipboard API failed — save to file
     try {
-      const { save } = await import("@tauri-apps/plugin-dialog");
-      const path = await save({ filters: [{ name: "PNG Image", extensions: ["png"] }] });
-      if (path) {
-        const buf = await blob.arrayBuffer();
-        const b64 = bytesToBase64(new Uint8Array(buf));
-        await writeImageFile(path, b64);
+      const buf = await blob.arrayBuffer();
+      const b64 = bytesToBase64(new Uint8Array(buf));
+      if (await writeImageFile(b64)) {
         toast(t("saved"));
       }
     } catch {