diff --git a/ChangeLog b/ChangeLog
index 6f5d24b..d8b1061 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -22,6 +22,8 @@
      cause buffer bound checks to be bypassed. Found by Eyal Itkin.
    * Fixed potential arithmetic overflow in mbedtls_md2_update() that could
      cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+   * Fixed potential arithmetic overflow in mbedtls_base64_decode() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
 
 = mbed TLS 1.3.18 branch 2016-10-17
 
diff --git a/library/base64.c b/library/base64.c
index 7de87e5..3de67f0 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -198,7 +198,7 @@
         return( 0 );
     }
 
-    n = ( ( n * 6 ) + 7 ) >> 3;
+    n = ( 6 * ( n >> 3 ) ) + ( ( 6 * ( n & 0x7 ) + 7 ) >> 3 );
     n -= j;
 
     if( dst == NULL || *dlen < n )