Default security group behavior in AWS - Mini Project: Build & Apply

Default Security Group Behavior

📖 Scenario: You are setting up a new virtual private cloud (VPC) in AWS. Every VPC comes with a default security group that controls network traffic for resources inside it.Understanding how this default security group behaves helps you manage access and keep your cloud resources safe.

🎯 Goal: Build a simple AWS CloudFormation template that defines a VPC and inspects the default security group behavior by creating a security group with default rules.

📋 What You'll Learn

Create a VPC resource with a specific CIDR block

Create a security group resource without specifying ingress or egress rules (to use default behavior)

Add a tag to the security group with the key 'Name' and value 'DefaultSecurityGroup'

Output the security group ID

💡 Why This Matters

🌍 Real World

Default security groups are automatically created in every AWS VPC. Knowing how they behave helps you secure your cloud network by controlling traffic between resources.

💼 Career

Cloud engineers and architects must understand default security group behavior to design secure network architectures and avoid unintended open access.

1

Create a VPC with CIDR block 10.0.0.0/16

Create a resource called MyVPC of type AWS::EC2::VPC with the property CidrBlock set to "10.0.0.0/16".

AWS

# Your code here

Hint

The VPC resource needs a CidrBlock property with the exact value "10.0.0.0/16".

2

Add a security group resource with default rules

Add a resource called DefaultSG of type AWS::EC2::SecurityGroup with the property VpcId set to { "Ref": "MyVPC" }. Do not add any ingress or egress rules.

AWS

Resources:
  MyVPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: "10.0.0.0/16"
# Your code here

Hint

Security group must reference the VPC using VpcId: { "Ref": "MyVPC" } and have no ingress or egress rules.

3

Add a tag to the security group

Add a Tags property to DefaultSG with a tag that has Key set to Name and Value set to DefaultSecurityGroup.

AWS

Resources:
  MyVPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: "10.0.0.0/16"
  DefaultSG:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: "Default security group"
      VpcId: { "Ref": "MyVPC" }
# Your code here

Hint

Tags are a list of key-value pairs under the Tags property.

4

Output the security group ID

Add an Outputs section with an output called SecurityGroupId that has Value set to { "Ref": "DefaultSG" }.

AWS

Resources:
  MyVPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: "10.0.0.0/16"
  DefaultSG:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: "Default security group"
      VpcId: { "Ref": "MyVPC" }
      Tags:
        - Key: Name
          Value: DefaultSecurityGroup
# Your code here

Hint

Outputs section goes at the root level and references the security group by Ref.

Practice

1. What is the default behavior of the AWS default security group for inbound traffic?

easy

A. It blocks all inbound traffic by default.

B. It allows inbound traffic from any IP address.

C. It allows inbound traffic only from resources assigned to the same security group.

D. It allows inbound traffic only on port 80.

Solution

1. Step 1: Understand default inbound rules

   The default security group allows inbound traffic only from instances assigned to the same security group.

2. Step 2: Compare options with default behavior

   Only It allows inbound traffic only from resources assigned to the same security group. matches: It allows inbound traffic only from resources assigned to the same security group; others allow broader or no inbound traffic.

3. Final Answer:

   It allows inbound traffic only from resources assigned to the same security group. -> Option C

4. Quick Check:

   Inbound traffic limited to same group = A [OK]

Hint: Default inbound allows traffic only from same security group [OK]

Common Mistakes:

• Thinking default allows inbound from anywhere
• Assuming default blocks all inbound traffic
• Believing default allows inbound only on specific ports

2. Which of the following is a correct statement about the AWS default security group syntax when creating a new rule?

easy

A. The default security group automatically allows all outbound traffic.

B. You must specify a CIDR block for inbound rules.

C. You cannot add any rules to the default security group.

D. The default security group blocks all outbound traffic by default.

Solution

1. Step 1: Review default outbound behavior

   The default security group allows all outbound traffic by default without needing extra rules.

2. Step 2: Evaluate each option

   The default security group automatically allows all outbound traffic. correctly states the default outbound allowance; others are incorrect about rules or blocking.

3. Final Answer:

   The default security group automatically allows all outbound traffic. -> Option A

4. Quick Check:

   Default outbound = all allowed [OK]

Hint: Default security group allows all outbound traffic by default [OK]

Common Mistakes:

• Assuming outbound rules must be manually added
• Believing default security group blocks outbound traffic
• Thinking CIDR block is mandatory for all rules

3. Given an EC2 instance assigned to the default security group, which of the following inbound traffic scenarios will be allowed?

medium

A. Inbound traffic from an EC2 instance in a different security group.

B. Inbound traffic from another EC2 instance assigned to the default security group.

C. Inbound traffic from the same EC2 instance itself.

D. Inbound traffic from any IP address on port 22.

Solution

1. Step 1: Recall default inbound rule

   The default security group allows inbound traffic only from instances assigned to the same security group.

2. Step 2: Analyze each option

   Inbound traffic from another EC2 instance assigned to the default security group matches this rule; A is different group, B is self (not inbound from self), D is open to all IPs which is not allowed.

3. Final Answer:

   Inbound traffic from another EC2 instance assigned to the default security group. -> Option B

4. Quick Check:

   Inbound allowed only from same group instances = C [OK]

Hint: Inbound allowed only from instances in same security group [OK]

Common Mistakes:

• Assuming inbound allowed from any IP
• Confusing inbound from self as allowed
• Thinking different security groups allow inbound by default

4. You tried to delete the default security group in your VPC but received an error. What is the most likely reason?

medium

A. Default security groups cannot be deleted.

B. You need to detach all instances before deleting.

C. You must disable all inbound rules first.

D. You need to delete the VPC first.

Solution

1. Step 1: Understand default security group restrictions

   The default security group cannot be deleted by design in AWS.

2. Step 2: Evaluate other options

   Detaching instances or disabling rules is not sufficient; deleting VPC is unrelated to this error.

3. Final Answer:

   Default security groups cannot be deleted. -> Option A

4. Quick Check:

   Default security group deletion blocked = D [OK]

Hint: Default security group cannot be deleted [OK]

Common Mistakes:

• Trying to delete without detaching instances
• Thinking disabling rules allows deletion
• Assuming VPC must be deleted first

5. You want to restrict outbound traffic from an EC2 instance assigned to the default security group. What must you do?

hard

A. Modify the default security group outbound rules to restrict traffic.

B. Outbound traffic cannot be restricted for instances in the default security group.

C. Delete the default security group and create a custom one with restrictions.

D. Create a new security group with restricted outbound rules and assign it to the instance.

Solution

1. Step 1: Understand default security group modification limits

   You can modify rules but cannot delete the default security group; modifying outbound rules is possible but affects all instances assigned.

2. Step 2: Best practice for restricting outbound traffic

   Creating a new security group with specific outbound restrictions and assigning it to the instance is the recommended approach.

3. Final Answer:

   Create a new security group with restricted outbound rules and assign it to the instance. -> Option D

4. Quick Check:

   Use new security group to restrict outbound traffic = B [OK]

Hint: Use a new security group to restrict outbound traffic [OK]

Common Mistakes:

• Trying to delete the default security group
• Modifying default group outbound rules affecting all instances
• Assuming outbound restrictions are impossible