fix(security): harden path traversal sanitization in file share API

Replace naive `.replace("..", "_")` with proper sanitization that decodes
percent-encoded bytes (%2e%2e) before stripping traversal sequences.
Handles URL-encoding bypass, backslash normalization, and repeated slashes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui_freenove_allinone/src/app/file_share_service.cpp

@@ -60,6 +60,64 @@ String urlEncodeSimple(const char* text) {
   return out;
 }
 
+// Decode percent-encoded bytes in-place (e.g. %2e → '.').
+String urlDecodeSimple(const String& src) {
+  String out;
+  out.reserve(src.length());
+  for (unsigned int i = 0; i < src.length(); ++i) {
+    if (src[i] == '%' && i + 2 < src.length()) {
+      const char hi = src[i + 1];
+      const char lo = src[i + 2];
+      auto hexVal = [](char c) -> int {
+        if (c >= '0' && c <= '9') return c - '0';
+        if (c >= 'a' && c <= 'f') return c - 'a' + 10;
+        if (c >= 'A' && c <= 'F') return c - 'A' + 10;
+        return -1;
+      };
+      const int h = hexVal(hi);
+      const int l = hexVal(lo);
+      if (h >= 0 && l >= 0) {
+        out += static_cast<char>((h << 4) | l);
+        i += 2;
+        continue;
+      }
+    }
+    out += src[i];
+  }
+  return out;
+}
+
+// Sanitize a user-supplied path: decode percent-encoding, collapse separators,
+// remove all ".." segments, and reject paths that escape the intended root.
+// Returns an empty String on failure.
+String sanitizePath(const String& raw) {
+  // 1. Decode any percent-encoding first so %2e%2e is caught.
+  String path = urlDecodeSimple(raw);
+  path.trim();
+  if (path.isEmpty()) {
+    return String();
+  }
+  // 2. Normalise separators.
+  path.replace("\\", "/");
+  // 3. Reject and strip any ".." segment (handles ../, /../, bare ".." etc.).
+  //    Loop until no more ".." remain to defeat chained encodings.
+  while (path.indexOf("..") >= 0) {
+    path.replace("..", "");
+  }
+  // 4. Collapse repeated slashes.
+  while (path.indexOf("//") >= 0) {
+    path.replace("//", "/");
+  }
+  // 5. Strip leading slash (callers prepend the root).
+  if (path.startsWith("/")) {
+    path.remove(0, 1);
+  }
+  if (path.isEmpty()) {
+    return String();
+  }
+  return path;
+}
+
 bool ensureDir(const char* path) {
   if (path == nullptr || path[0] == '\0') {
     return false;
@@ -152,16 +210,10 @@ bool FileShareService::saveIncomingFile(const char* relative_path,
   if (!ensureSharedDirs()) {
     return false;
   }
-  String path = relative_path;
-  path.trim();
+  String path = sanitizePath(relative_path);
   if (path.isEmpty()) {
     return false;
   }
-  if (path.startsWith("/")) {
-    path = path.substring(1);
-  }
-  path.replace("..", "_");
-  path.replace("\\", "/");
   String full_path = String(kIncomingRoot) + "/" + path;
   const int slash = full_path.lastIndexOf('/');
   if (slash > 0) {
@@ -370,20 +422,10 @@ bool FileShareService::resolveIncomingPath(const char* requested_path, String* o
   if (requested_path == nullptr || requested_path[0] == '\0' || out_full_path == nullptr) {
     return false;
   }
-  String path = requested_path;
-  path.trim();
+  String path = sanitizePath(requested_path);
   if (path.isEmpty()) {
     return false;
   }
-  path.replace("\\", "/");
-  path.replace("..", "_");
-  if (path.startsWith("/apps/shared/incoming/")) {
-    *out_full_path = path;
-    return true;
-  }
-  if (path.startsWith("/")) {
-    path.remove(0, 1);
-  }
   *out_full_path = String(kIncomingRoot) + "/" + path;
   return true;
 }

ui_freenove_allinone/src/app/main.cpp

@@ -6338,8 +6338,33 @@ void setupWebUiImpl() {
       if (g_file_share_service.downloadFile(path.c_str(), &file, &full_path) && file) {
         file.close();
       }
+      // Sanitize: decode percent-encoding, strip "..", collapse slashes.
+      {
+        String clean = path;
+        clean.replace("\\", "/");
+        // Decode %XX sequences to catch encoded traversal (%2e%2e).
+        String decoded;
+        decoded.reserve(clean.length());
+        for (unsigned int ci = 0; ci < clean.length(); ++ci) {
+          if (clean[ci] == '%' && ci + 2 < clean.length()) {
+            auto hv = [](char c) -> int {
+              if (c >= '0' && c <= '9') return c - '0';
+              if (c >= 'a' && c <= 'f') return c - 'a' + 10;
+              if (c >= 'A' && c <= 'F') return c - 'A' + 10;
+              return -1;
+            };
+            int h = hv(clean[ci+1]), l = hv(clean[ci+2]);
+            if (h >= 0 && l >= 0) { decoded += (char)((h<<4)|l); ci += 2; continue; }
+          }
+          decoded += clean[ci];
+        }
+        while (decoded.indexOf("..") >= 0) decoded.replace("..", "");
+        while (decoded.indexOf("//") >= 0) decoded.replace("//", "/");
+        if (decoded.startsWith("/")) decoded.remove(0, 1);
+        path = decoded;
+      }
       saved_path = String("/apps/shared/incoming/") + path;
-      if (saved_path.indexOf("..") >= 0) {
+      if (path.isEmpty()) {
         ok = false;
       } else {
         const char* mode = (offset == 0U) ? "w" : "a";