AWS EC2 instance cannot see another instance on a specific port - amazon-web-services

Question

I have created two EC2 instances using CloudFormation - one for the Apache web-server, another one for the PostgreSQL DB. For some reason the web-server cannot telnet into the DB instance on port 5432 even though the DB instance can telnet into the web-server instance on port 80. When I check the DB instance from localhost, it is working fine and telnetting to the localhost 5432 successfully. There are two security groups for each instance:
"TheWebServerSecurityGroup" : {
"Type" : "AWS::EC2::SecurityGroup",
"Properties" : {
"GroupDescription" : "Security Group for The web-server instance",
"SecurityGroupIngress" : [
{ "IpProtocol" : "tcp", "FromPort" : "80", "ToPort" : "80", "CidrIp" : "0.0.0.0/0" },
{ "IpProtocol" : "tcp", "FromPort" : "22", "ToPort" : "22", "CidrIp" : "0.0.0.0/0" }
],
"VpcId" : { "Ref": "TheVPC" },
"Tags" : [ { "Key": "Name", "Value": "TheWebServerSecurityGroup" } ]
}
},
"TheDBSecurityGroup" : {
"Type" : "AWS::EC2::SecurityGroup",
"Properties" : {
"GroupDescription" : "Security Group for The DB instance",
"SecurityGroupIngress" : [
{ "IpProtocol" : "tcp", "FromPort" : "22", "ToPort" : "22", "CidrIp" : "0.0.0.0/0" },
{ "IpProtocol" : "tcp", "FromPort" : "5432", "ToPort" : "5432", "CidrIp" : "0.0.0.0/0" }
],
"VpcId" : { "Ref": "TheVPC" },
"Tags" : [ { "Key": "Name", "Value": "TheDBSecurityGroup" } ]
}
},
What might be wrong with the configuration? Any help is appreciated.
UPD: I tried adding the following inbound/outbound rules, but with that it becomes impossible to install packages via yum:
"TheOutboundRule": {
"Type": "AWS::EC2::SecurityGroupEgress",
"Properties":{
"IpProtocol": "tcp",
"FromPort": 0,
"ToPort": 5432,
"DestinationSecurityGroupId": {
"Fn::GetAtt": [
"TheDBSecurityGroup",
"GroupId"
]
},
"GroupId": {
"Fn::GetAtt": [
"TheWebServerSecurityGroup",
"GroupId"
]
}
}
},
"TheInboundRule": {
"Type": "AWS::EC2::SecurityGroupIngress",
"Properties":{
"IpProtocol": "tcp",
"FromPort": 0,
"ToPort": 5432,
"SourceSecurityGroupId": {
"Fn::GetAtt": [
"TheWebServerSecurityGroup",
"GroupId"
]
},
"GroupId": {
"Fn::GetAtt": [
"TheDBSecurityGroup",
"GroupId"
]
}
}
},
I also tried adding just the inbound rule (without an outbound one), but it does not work either.