diff --git a/python/ycm/client/base_request.py b/python/ycm/client/base_request.py
index c9c417f4..c5f37718 100644
--- a/python/ycm/client/base_request.py
+++ b/python/ycm/client/base_request.py
@@ -20,6 +20,7 @@
 import vim
 import requests
 import urlparse
+from base64 import b64decode, b64encode
 from retries import retries
 from requests_futures.sessions import FuturesSession
 from ycm.unsafe_thread_pool_executor import UnsafeThreadPoolExecutor
@@ -124,8 +125,8 @@ class BaseRequest( object ):
     if not request_body:
       request_body = ''
     headers = dict( _HEADERS )
-    headers[ _HMAC_HEADER ] = utils.CreateHexHmac( request_body,
-                                                   BaseRequest.hmac_secret )
+    headers[ _HMAC_HEADER ] = b64encode(
+        utils.CreateHexHmac( request_body, BaseRequest.hmac_secret ) )
     return headers
 
   session = FuturesSession( executor = _EXECUTOR )
@@ -171,9 +172,10 @@ def JsonFromFuture( future ):
 
 
 def _ValidateResponseObject( response ):
-  if not utils.ContentHexHmacValid( response.content,
-                                    response.headers[ _HMAC_HEADER ],
-                                    BaseRequest.hmac_secret ):
+  if not utils.ContentHexHmacValid(
+      response.content,
+      b64decode( response.headers[ _HMAC_HEADER ] ),
+      BaseRequest.hmac_secret ):
     raise RuntimeError( 'Received invalid HMAC for response!' )
   return True
 
diff --git a/python/ycm/server/hmac_plugin.py b/python/ycm/server/hmac_plugin.py
index cf8f475c..fec899bd 100644
--- a/python/ycm/server/hmac_plugin.py
+++ b/python/ycm/server/hmac_plugin.py
@@ -19,6 +19,7 @@
 
 import logging
 import httplib
+from base64 import b64decode, b64encode
 from bottle import request, response, abort
 from ycm import utils
 
@@ -30,6 +31,9 @@ _HMAC_HEADER = 'x-ycm-hmac'
 # We want to ensure that every request coming in has a valid HMAC set in the
 # x-ycm-hmac header and that every response coming out sets such a valid header.
 # This is to prevent security issues with possible remote code execution.
+# The x-ycm-hmac value is encoded as base64 during transport instead of sent raw
+# because https://tools.ietf.org/html/rfc5987 says header values must be in the
+# ISO-8859-1 character set.
 class HmacPlugin( object ):
   name = 'hmac'
   api = 2
@@ -54,9 +58,12 @@ class HmacPlugin( object ):
 
 
 def RequestAuthenticated( body, hmac_secret ):
-  return utils.ContentHexHmacValid( body,
-                                    request.headers[ _HMAC_HEADER ],
-                                    hmac_secret )
+  return utils.ContentHexHmacValid(
+      body,
+      b64decode( request.headers[ _HMAC_HEADER ] ),
+      hmac_secret )
+
 
 def SetHmacHeader( body, hmac_secret ):
-  response.headers[ _HMAC_HEADER ] = utils.CreateHexHmac( body, hmac_secret )
+  response.headers[ _HMAC_HEADER ] = b64encode(
+      utils.CreateHexHmac( body, hmac_secret ) )