Index: subversion/libsvn_subr/config_auth.c
===================================================================
--- subversion/libsvn_subr/config_auth.c        (revision 1615184)
+++ subversion/libsvn_subr/config_auth.c        (working copy)
@@ -90,6 +90,7 @@ svn_config_read_auth_data(apr_hash_t **hash,
   if (kind == svn_node_file)
     {
       svn_stream_t *stream;
+      svn_string_t *stored_realm;

       SVN_ERR_W(svn_stream_open_readonly(&stream, auth_path, pool, pool),
                 _("Unable to open auth file for reading"));
@@ -100,6 +101,12 @@ svn_config_read_auth_data(apr_hash_t **hash,
                 apr_psprintf(pool, _("Error parsing '%s'"),
                              svn_dirent_local_style(auth_path, pool)));

+      stored_realm = apr_hash_get(*hash, SVN_CONFIG_REALMSTRING_KEY,
+                                  APR_HASH_KEY_STRING);
+
+      if (!stored_realm || strcmp(stored_realm->data, realmstring) != 0)
+        *hash = NULL; /* Hash collision, or somebody tampering with storage */
+
       SVN_ERR(svn_stream_close(stream));
     }

]]]

  Patch against 1.8.9:

[[[
Index: subversion/libsvn_subr/config_auth.c
===================================================================
--- subversion/libsvn_subr/config_auth.c        (revision 1605943)
+++ subversion/libsvn_subr/config_auth.c        (revision 1605944)
@@ -94,6 +94,7 @@ svn_config_read_auth_data(apr_hash_t **hash,
   if (kind == svn_node_file)
     {
       svn_stream_t *stream;
+      svn_string_t *stored_realm;

       SVN_ERR_W(svn_stream_open_readonly(&stream, auth_path, pool, pool),
                 _("Unable to open auth file for reading"));
@@ -104,6 +105,11 @@ svn_config_read_auth_data(apr_hash_t **hash,
                 apr_psprintf(pool, _("Error parsing '%s'"),
                              svn_dirent_local_style(auth_path, pool)));

+      stored_realm = svn_hash_gets(*hash, SVN_CONFIG_REALMSTRING_KEY);
+
+      if (!stored_realm || strcmp(stored_realm->data, realmstring) != 0)
+        *hash = NULL; /* Hash collision, or somebody tampering with storage */
+
       SVN_ERR(svn_stream_close(stream));
     }