fix: detect and fail on oversized zip entries instead of silent truncation

Replace io.LimitReader with a new readZipEntry helper that reads one extra
byte to detect when content exceeds maxZipEntrySize (500MB). This prevents
silent data corruption where partial file bytes would be stored as if the
upload succeeded.

The import now fails with ErrFileTooLarge instead of accepting truncated
content for attachments and background blobs.

pkg/modules/migration/vikunja-file/vikunja.go

@@ -39,6 +39,25 @@ import (
 const logPrefix = "[Vikunja File Import] "
 const maxZipEntrySize = 500 * 1024 * 1024 // 500 MB
 
+// ErrFileTooLarge is returned when a file in the zip archive exceeds maxZipEntrySize
+var ErrFileTooLarge = fmt.Errorf("file exceeds maximum allowed size of %d bytes", maxZipEntrySize)
+
+// readZipEntry reads from r into a buffer, returning ErrFileTooLarge if the content exceeds maxZipEntrySize.
+// Unlike io.LimitReader which silently truncates, this function explicitly detects overflow.
+func readZipEntry(r io.Reader) (*bytes.Buffer, error) {
+	// Read one extra byte to detect overflow
+	limitedReader := io.LimitReader(r, maxZipEntrySize+1)
+	var buf bytes.Buffer
+	n, err := buf.ReadFrom(limitedReader)
+	if err != nil {
+		return nil, err
+	}
+	if n > maxZipEntrySize {
+		return nil, ErrFileTooLarge
+	}
+	return &buf, nil
+}
+
 type FileMigrator struct {
 }
 
@@ -126,9 +145,10 @@ func (v *FileMigrator) Migrate(user *user.User, file io.ReaderAt, size int64) er
 	if err != nil {
 		return fmt.Errorf("could not open version file: %w", err)
 	}
+	defer vf.Close()
 
-	var bufVersion bytes.Buffer
-	if _, err := bufVersion.ReadFrom(io.LimitReader(vf, maxZipEntrySize)); err != nil {
+	bufVersion, err := readZipEntry(vf)
+	if err != nil {
 		return fmt.Errorf("could not read version file: %w", err)
 	}
 
@@ -158,8 +178,8 @@ func (v *FileMigrator) Migrate(user *user.User, file io.ReaderAt, size int64) er
 	}
 	defer df.Close()
 
-	var bufData bytes.Buffer
-	if _, err := bufData.ReadFrom(io.LimitReader(df, maxZipEntrySize)); err != nil {
+	bufData, err := readZipEntry(df)
+	if err != nil {
 		return fmt.Errorf("could not read data file: %w", err)
 	}
 
@@ -193,8 +213,8 @@ func (v *FileMigrator) Migrate(user *user.User, file io.ReaderAt, size int64) er
 	}
 	defer ff.Close()
 
-	var bufFilter bytes.Buffer
-	if _, err := bufFilter.ReadFrom(io.LimitReader(ff, maxZipEntrySize)); err != nil {
+	bufFilter, err := readZipEntry(ff)
+	if err != nil {
 		return fmt.Errorf("could not read filters file: %w", err)
 	}
 
@@ -254,12 +274,13 @@ func addDetailsToProject(l *models.ProjectWithTasksAndBuckets, storedFiles map[i
 		if err != nil {
 			return fmt.Errorf("could not open project background file %d for reading: %w", l.BackgroundFileID, err)
 		}
-		var buf bytes.Buffer
-		if _, err := buf.ReadFrom(io.LimitReader(bf, maxZipEntrySize)); err != nil {
+		buf, err := readZipEntry(bf)
+		bf.Close()
+		if err != nil {
 			return fmt.Errorf("could not read project background file %d: %w", l.BackgroundFileID, err)
 		}
 
-		l.BackgroundInformation = &buf
+		l.BackgroundInformation = buf
 	}
 
 	for _, t := range l.Tasks {
@@ -280,8 +301,9 @@ func addDetailsToProject(l *models.ProjectWithTasksAndBuckets, storedFiles map[i
 				log.Warningf(logPrefix+"Could not open attachment %d for reading: %v, skipping", attachment.ID, err)
 				continue
 			}
-			var buf bytes.Buffer
-			if _, err := buf.ReadFrom(io.LimitReader(af, maxZipEntrySize)); err != nil {
+			buf, err := readZipEntry(af)
+			af.Close()
+			if err != nil {
 				log.Warningf(logPrefix+"Could not read attachment %d: %v, skipping", attachment.ID, err)
 				continue
 			}