diff --git a/fixed-requirements.txt b/fixed-requirements.txt
index 99c6a7b165..ed29375589 100644
--- a/fixed-requirements.txt
+++ b/fixed-requirements.txt
@@ -13,7 +13,7 @@ amqp==2.3.2
 oslo.config>=1.12.1,<1.13
 oslo.utils>=3.36.2,<=3.37.0
 six==1.12.0
-pyyaml==3.13
+pyyaml>=4.2b4,<5.2
 requests[security]<2.15,>=2.14.1
 apscheduler==3.5.3
 gitpython==2.1.11
@@ -41,7 +41,6 @@ prompt-toolkit==1.0.15
 tooz==1.63.1
 zake==0.2.2
 routes==2.4.1
-flex==6.13.2
 webob==1.8.4
 prance==0.9.0
 pywinrm==0.3.0
diff --git a/requirements.txt b/requirements.txt
index 27b053a9ff..1b9bb5f1e6 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,7 +6,7 @@ argcomplete
 bcrypt
 cryptography==2.4.2
 eventlet==0.24.1
-flex==6.13.2
+flex>=6.13.2,<6.15.0
 git+https://github.com/Kami/logshipper.git@stackstorm_patched#egg=logshipper
 git+https://github.com/StackStorm/orquesta.git@v0.3#egg=orquesta
 git+https://github.com/StackStorm/python-mistralclient.git#egg=python-mistralclient
@@ -43,7 +43,7 @@ python-json-logger
 python-statsd==2.1.0
 pytz==2018.7
 pywinrm==0.3.0
-pyyaml==3.13
+pyyaml>=4.2b4,<5.2
 rednose
 requests[security]<2.15,>=2.14.1
 retrying==1.3.3
diff --git a/st2client/requirements.txt b/st2client/requirements.txt
index 037fa0cffc..e544f4f936 100644
--- a/st2client/requirements.txt
+++ b/st2client/requirements.txt
@@ -8,7 +8,7 @@ prompt-toolkit==1.0.15
 python-dateutil==2.7.5
 python-editor==1.0.3
 pytz==2018.7
-pyyaml==3.13
+pyyaml>=4.2b4,<5.2
 requests[security]<2.15,>=2.14.1
 six==1.12.0
 sseclient==0.0.19
diff --git a/st2common/requirements.txt b/st2common/requirements.txt
index dc10f6f8a8..5b545f655e 100644
--- a/st2common/requirements.txt
+++ b/st2common/requirements.txt
@@ -3,7 +3,7 @@ amqp==2.3.2
 apscheduler==3.5.3
 cryptography==2.4.2
 eventlet==0.24.1
-flex==6.13.2
+flex>=6.13.2,<6.15.0
 git+https://github.com/StackStorm/orquesta.git@v0.3#egg=orquesta
 greenlet==0.4.15
 ipaddr
@@ -18,7 +18,7 @@ paramiko==2.4.2
 pymongo==3.7.2
 python-dateutil==2.7.5
 python-statsd==2.1.0
-pyyaml==3.13
+pyyaml>=4.2b4,<5.2
 requests[security]<2.15,>=2.14.1
 retrying==1.3.3
 routes==2.4.1
diff --git a/test-requirements.txt b/test-requirements.txt
index 97d6be145b..f898b88685 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -19,7 +19,7 @@ nose-timer>=0.7.4,<0.8
 # splitting tests run on a separate CI machines
 nose-parallel==0.3.1
 # Required by st2client tests
-pyyaml<4.0,>=3.12
+pyyaml>=4.2b4,<5.2
 RandomWords
 gunicorn==19.9.0
 psutil==5.4.8