IAM Roles vs. Instance Profiles: Misconfigurations, Security Gaps, and Hardening Strategies

IAM (Identity and Access Management) roles are a cornerstone of secure AWS deployments. They allow you to grant temporary permissions to services and applications running on AWS resources, without embedding long-term credentials. One common and powerful use case is attaching an IAM role to an EC2 instance via an?instance profile. However, misconfigurations in these areas can create significant security vulnerabilities. This post will explore the risks of overly permissive roles, IMDS vulnerabilities, and provide concrete command-line solutions to implement best practices.

What are IAM Roles and Instance Profiles?

• IAM Roles:?Think of an IAM role as a temporary access key that you can assign to an AWS resource, like an EC2 instance. The role defines?what?the resource is allowed to do. It specifies permissions, such as reading S3 buckets, writing to DynamoDB, or invoking Lambda functions.
• Instance Profiles:?An instance profile is a?container?for an IAM role specifically designed to be used with EC2 instances. When you launch an EC2 instance, you can associate it with an instance profile. This automatically assigns the IAM role within the profile to the EC2 instance, granting it the defined permissions.

Risks of Overly Permissive Roles Attached to EC2 Instance Profiles

The most common, and potentially devastating, misconfiguration is assigning an IAM role with overly broad permissions to an EC2 instance. If an attacker gains access to the instance (through a vulnerable application, compromised SSH keys, or other means), they inherit?all?the permissions granted by the attached role.

Scenario:?Let's say you grant an EC2 instance the?AdministratorAccess?managed policy to simplify development. While convenient, this grants the instance (and anyone who compromises it) full control over your entire AWS account.

The Danger Zone:?An attacker who compromises this instance can:

• Steal sensitive data:?Access S3 buckets containing customer information, code, or database backups.
• Launch new resources:?Create new EC2 instances, databases, or other services to further their attack.
• Delete resources:?Destroy critical infrastructure, causing significant downtime and data loss.
• Escalate Privileges:?Create new IAM users and roles with even broader permissions, making it extremely difficult to regain control.

Mitigation: Principle of Least Privilege (POLP)

The Principle of Least Privilege (POLP) is the golden rule of security. Grant only the minimum permissions necessary for a resource to perform its intended function.

Command-Line Instructions (AWS CLI):

1. Identify the overly permissive role:?List the IAM roles in your account and examine their attached policies.

aws iam list-roles        

Pay close attention to roles with names like "AdminRole", "DevRole", or roles attached to EC2 instance profiles in your development/staging environments.

1. List the attached policies for a specific role:?Replace?<role-name>?with the actual name.

aws iam list-attached-role-policies --role-name <role-name>        

If you see?AdministratorAccess,?PowerUserAccess, or similar broad-managed policies, your role is likely overly permissive.

1. Create a custom policy that grants only the necessary permissions:?Let's say your application running on the EC2 instance needs to read from a specific S3 bucket and write logs to CloudWatch Logs.

First, create a JSON file (e.g.,?limited-policy.json) with the following content, replacing?<your-bucket-name>?and?<your-region>?with the appropriate values:

{

    "Version": "2012-10-17",

    "Statement": [

        {

            "Effect": "Allow",

            "Action": [

                "s3:GetObject",

                "s3:ListBucket"

            ],

            "Resource": [

                "arn:aws:s3:::<your-bucket-name>",

                "arn:aws:s3:::<your-bucket-name>/*"

            ]

        },

        {

            "Effect": "Allow",

            "Action": [

                "logs:CreateLogGroup",

                "logs:CreateLogStream",

                "logs:PutLogEvents",

                "logs:DescribeLogGroups",

                "logs:DescribeLogStreams"

            ],

            "Resource": "arn:aws:logs:<your-region>:<your-account-id>:*"

        }

    ]

}        

Remember to replace?<your-account-id>?with your AWS account ID. You can get this from the AWS Console or by running?aws sts get-caller-identity.

Then, create the IAM policy:

aws iam create-policy --policy-name LimitedEC2Access --policy-document file://limited-policy.json --description "Policy granting limited S3 read and CloudWatch Logs write access"        

This command will output the ARN (Amazon Resource Name) of the newly created policy. Copy this ARN; you'll need it in the next step.

1. Detach the overly permissive managed policy (e.g., AdministratorAccess):

aws iam detach-role-policy --role-name <role-name> --policy-arn arn:aws:iam::aws:policy/AdministratorAccess        

Replace?<role-name>?with the actual name of the IAM role. Replace?arn:aws:iam::aws:policy/AdministratorAccess?with the ARN of the managed policy you want to detach.

2. Attach the new, limited policy:

aws iam attach-role-policy --role-name <role-name> --policy-arn <arn-of-limited-policy>        

Replace?<role-name>?with the IAM role name and?<arn-of-limited-policy>?with the ARN of the policy you created in step 3.

3. Test thoroughly:?Verify that your application still functions correctly with the new, limited permissions. This is crucial to avoid breaking your application. Increase permissions incrementally if needed.

How Attackers Can Abuse Instance Metadata Service (IMDSv1 vs. IMDSv2)

EC2 instances have access to metadata about themselves through the Instance Metadata Service (IMDS). This service is accessible via HTTP at?https://169.254.169.254/latest/meta-data/. Among other things, IMDS provides access to the temporary security credentials associated with the IAM role attached to the instance.

IMDSv1 (Legacy):

• Vulnerable to SSRF (Server-Side Request Forgery) and other attacks:?IMDSv1 is susceptible to SSRF vulnerabilities in web applications running on the EC2 instance. An attacker who can inject a malicious HTTP request can potentially retrieve the IAM role credentials from the instance metadata service.

Example SSRF Payload (in a GET request parameter):

https://example.com/?url=https://169.254.169.254/latest/meta-data/iam/security-credentials/<role-name>/?        

• Simple HTTP requests:?IMDSv1 uses simple HTTP GET requests. This makes it easy to exploit with various techniques.

IMDSv2 (Latest):

• Requires a token:?IMDSv2 mitigates many of the vulnerabilities of IMDSv1 by requiring a?session token?to be obtained before accessing the metadata. This token must be included in subsequent requests. This is also known as "hop-limit".
• Protects against SSRF:?The session token requirement makes it much harder to exploit SSRF vulnerabilities.

Mitigation: Enforce IMDSv2

Disabling IMDSv1 and requiring IMDSv2 is a critical security measure.

Command-Line Instructions:

Launch a new EC2 instance with IMDSv2 enforced:?When launching an instance, specify the?MetadataOptions?parameter.

aws ec2 run-instances \

    --image-id ami-xxxxxxxxxxxxxxxxx \

    --instance-type t2.micro \

    --subnet-id subnet-xxxxxxxxxxxxxxxxx \

    --iam-instance-profile Arn=arn:aws:iam::<your-account-id>:instance-profile/<instance-profile-name> \

    --metadata-options "HttpTokens=required,HttpPutResponseHopLimit=1,HttpEndpoint=enabled"        

?

1. HttpTokens=required: Enforces the use of IMDSv2 tokens.
2. HttpPutResponseHopLimit=1: Sets the hop limit for HTTP requests to 1. This helps prevent attackers from using the instance as a proxy to access the metadata service. A hop limit of 1 means that the HTTP request can only travel one hop from the instance itself.
3. HttpEndpoint=enabled: Enables the metadata endpoint.

Replace the?ami-xxxxxxxxxxxxxxxxx,?subnet-xxxxxxxxxxxxxxxxx,?<your-account-id>, and?<instance-profile-name>?with your actual values.

Update existing instances to require IMDSv2:

aws ec2 modify-instance-metadata-options \

    --instance-id <instance-id> \

    --http-tokens required \

    --http-put-response-hop-limit 1 \

    --http-endpoint enabled        

Replace?<instance-id>?with the ID of the EC2 instance you want to update.

Verify IMDSv2 is enforced:?SSH into the instance and try to access the instance metadata without a token. You should receive an error.

Attempt to access instance metadata using IMDSv1 (This should fail):

curl https://169.254.169.254/latest/meta-data/iam/security-credentials/<role-name>/        

Get the IMDSv2 token:

TOKEN=`curl -X PUT "https://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600"`        

Now, use the token to access the metadata:

curl -H "X-aws-ec2-metadata-token: $TOKEN" https://169.254.169.254/latest/meta-data/iam/security-credentials/<role-name>/?        

This last command should succeed.

Further Hardening Tips:

• Use Instance Metadata Service Version 3 (IMDSv3):?This is an enhancement to IMDSv2, with added capabilities for token rotation and improved request validation. While not always available depending on your AWS Region and instance types, using it when possible provides even stronger protection. The CLI commands remain the same as enforcing IMDSv2. The framework and libraries will internally determine how to best leverage the enhancements.
• Network Configuration:?Ensure your EC2 instances are placed in private subnets and only accessible through a controlled interface (e.g., a load balancer) to minimize external access.
• Security Groups:?Restrict inbound and outbound traffic to only the necessary ports and protocols. Don't allow unrestricted access.
• Regular Audits:?Periodically review your IAM roles, instance profiles, and security configurations to identify and remediate any potential weaknesses. AWS IAM Access Analyzer can help identify unused access.
• Use AWS Config:?AWS Config can continuously monitor your AWS resources and configurations, alerting you to any deviations from your defined security policies. Create Config rules to detect if IMDSv1 is enabled on EC2 instances.
• Monitor CloudTrail Logs:?Regularly monitor CloudTrail logs for suspicious activity related to IAM role usage and instance metadata access.

Conclusion:

Securing your AWS environment requires a proactive and layered approach. Properly configuring IAM roles and instance profiles, combined with enforcing IMDSv2 (or IMDSv3) and implementing the Principle of Least Privilege, significantly reduces your attack surface and protects your data from unauthorized access. Regular auditing and monitoring are crucial for maintaining a secure posture in the ever-evolving cloud landscape. Remember that security is a journey, not a destination!

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