diff --git a/tox.ini b/tox.ini
index 2033a9ee5..ac39e995e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -29,7 +29,10 @@ setenv =
     PYTHONHASHSEED = 0
 # https://testrun.org/tox/latest/example/basic.html#special-handling-of-pythonhas
 
+# cffi<=1.7 is required for oldest tests due to
+# https://bitbucket.org/cffi/cffi/commits/18cdf37d6b2691301a15b0e54f49757ebd4ed0f2?at=default
 deps =
+    py{26,27}-oldest: cffi<=1.7
     py{26,27}-oldest: cryptography==0.8
     py{26,27}-oldest: configargparse==0.10.0
     py{26,27}-oldest: dnspython>=1.12