diff --git a/requirements.txt b/requirements.txt
index 9f4914a39a..7340d07e8f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,6 +9,8 @@ click>=7.0
 pyserial>=3.0
 future>=0.15.2
 cryptography>=2.1.4,<35
+# cffi 1.15 supports Python 3.6+ only
+cffi<1.15;python_version<'3.6'
 pyparsing>=2.0.3,<2.4.0
 pyelftools>=0.22