diff --git a/src/calibre/ebooks/conversion/plugins/html_input.py b/src/calibre/ebooks/conversion/plugins/html_input.py
index eb26b5c6ed..5bc4f2577c 100644
--- a/src/calibre/ebooks/conversion/plugins/html_input.py
+++ b/src/calibre/ebooks/conversion/plugins/html_input.py
@@ -86,7 +86,7 @@ class HTMLInput(InputFormatPlugin):
         if hasattr(stream, 'name'):
             basedir = os.path.dirname(stream.name)
             fname = os.path.basename(stream.name)
-        self.root_dir_of_input = os.path.abspath(basedir) + os.sep
+        self.root_dir_of_input = os.path.normcase(os.path.abspath(basedir) + os.sep)
 
         if file_ext != 'opf':
             if opts.dont_package:
@@ -262,7 +262,7 @@ class HTMLInput(InputFormatPlugin):
         if not link:
             return None, None
         link = os.path.abspath(os.path.realpath(link))
-        if not link.startswith(self.root_dir_of_input):
+        if not os.path.normcase(link).startswith(self.root_dir_of_input):
             if not self.opts.allow_local_files_outside_root:
                 self.log.warn('Not adding {} as it is outside the document root: {}'.format(link, self.root_dir_of_input))
                 return None, None
diff --git a/src/calibre/ebooks/conversion/plugins/txt_input.py b/src/calibre/ebooks/conversion/plugins/txt_input.py
index 6aa3c30ed4..550e066524 100644
--- a/src/calibre/ebooks/conversion/plugins/txt_input.py
+++ b/src/calibre/ebooks/conversion/plugins/txt_input.py
@@ -107,12 +107,13 @@ class TXTInput(InputFormatPlugin):
         from html5_parser import parse
         root = parse(html)
         changed = False
+        base_dir = os.path.normcase(os.path.abspath(base_dir)) + os.sep
         for img in root.xpath('//img[@src]'):
             src = img.get('src')
             prefix = src.split(':', 1)[0].lower()
             if src and prefix not in ('file', 'http', 'https', 'ftp') and not os.path.isabs(src):
                 src = os.path.join(base_dir, src)
-                if os.path.isfile(src) and os.access(src, os.R_OK):
+                if os.path.normcase(src).startswith(base_dir) and os.path.isfile(src) and os.access(src, os.R_OK):
                     with open(src, 'rb') as f:
                         data = f.read()
                     f = self.shift_file(os.path.basename(src), data)