diff --git a/rhodecode/lib/middleware/simplesvn.py b/rhodecode/lib/middleware/simplesvn.py
--- a/rhodecode/lib/middleware/simplesvn.py
+++ b/rhodecode/lib/middleware/simplesvn.py
@@ -31,7 +31,7 @@ from rhodecode.lib import rc_cache
 from rhodecode.lib.middleware import simplevcs
 from rhodecode.lib.middleware.utils import get_path_info
 from rhodecode.lib.utils import is_valid_repo
-from rhodecode.lib.str_utils import safe_str, safe_int
+from rhodecode.lib.str_utils import safe_str, safe_int, safe_bytes
 from rhodecode.lib.type_utils import str2bool
 from rhodecode.lib.ext_json import json
 from rhodecode.lib.hooks_daemon import store_txn_id_data
@@ -51,8 +51,8 @@ class SimpleSvnApp(object):
 
     def __call__(self, environ, start_response):
         request_headers = self._get_request_headers(environ)
-        data = environ['wsgi.input']
-        req_method = environ['REQUEST_METHOD']
+        data_io = environ['wsgi.input']
+        req_method: str = environ['REQUEST_METHOD']
         has_content_length = 'CONTENT_LENGTH' in environ
 
         path_info = self._get_url(
@@ -66,37 +66,38 @@ class SimpleSvnApp(object):
         if req_method in ['MKCOL'] or has_content_length:
             data_processed = False
             # read chunk to check if we have txn-with-props
-            initial_data = data.read(1024)
-            if initial_data.startswith('(create-txn-with-props'):
-                data = initial_data + data.read()
+            initial_data: bytes = data_io.read(1024)
+            if initial_data.startswith(b'(create-txn-with-props'):
+                data_io = initial_data + data_io.read()
                 # store on-the-fly our rc_extra using svn revision properties
                 # those can be read later on in hooks executed so we have a way
                 # to pass in the data into svn hooks
                 rc_data = base64.urlsafe_b64encode(json.dumps(self.rc_extras))
-                rc_data_len = len(rc_data)
+                rc_data_len = str(len(rc_data))
                 # header defines data length, and serialized data
-                skel = ' rc-scm-extras {} {}'.format(rc_data_len, rc_data)
-                data = data[:-2] + skel + '))'
+                skel = b' rc-scm-extras %b %b' % (safe_bytes(rc_data_len), safe_bytes(rc_data))
+                data_io = data_io[:-2] + skel + b'))'
                 data_processed = True
 
             if not data_processed:
                 # NOTE(johbo): Avoid that we end up with sending the request in chunked
                 # transfer encoding (mainly on Gunicorn). If we know the content
                 # length, then we should transfer the payload in one request.
-                data = initial_data + data.read()
+                data_io = initial_data + data_io.read()
 
         if req_method in ['GET', 'PUT'] or transfer_encoding == 'chunked':
-            # NOTE(marcink): when getting/uploading files we want to STREAM content
+            # NOTE(marcink): when getting/uploading files, we want to STREAM content
             # back to the client/proxy instead of buffering it here...
             stream = True
 
         stream = stream
         log.debug('Calling SVN PROXY at `%s`, using method:%s. Stream: %s',
                   path_info, req_method, stream)
+
         try:
             response = requests.request(
                 req_method, path_info,
-                data=data, headers=request_headers, stream=stream)
+                data=data_io, headers=request_headers, stream=stream)
         except requests.ConnectionError:
             log.exception('ConnectionError occurred for endpoint %s', path_info)
             raise
@@ -119,9 +120,7 @@ class SimpleSvnApp(object):
             port = safe_int(self.rc_extras['hooks_uri'].split(':')[-1])
             store_txn_id_data(txn_id, {'port': port})
 
-        start_response(
-            '{} {}'.format(response.status_code, response.reason),
-            response_headers)
+        start_response(f'{response.status_code} {response.reason}', response_headers)
         return response.iter_content(chunk_size=1024)
 
     def _get_url(self, svn_http_server, path):