Drop Invalid Header Fields for Application Load Balancers

Risk Level: Medium (should be achieved)

Ensure that Drop Invalid Header Fields feature is enabled for your Amazon Application Load Balancers (ALBs) in order to follow security best practices and meet compliance requirements. If Drop Invalid Header Fields security feature is enabled, HTTP headers with header fields that are not valid are removed by the Application Load Balancer instead of being routed to the associated targets.

Security

Amazon Web Services (AWS) considers standard headers to only include alphanumeric characters and hyphens. Therefore, Amazon Elastic Load Balancing (ELB) service introduced the Drop Invalid Header Fields feature (i.e. "routing.http.drop_invalid_header_fields.enabled" flag) to improve security by enabling users to control whether an Application Load Balancer will forward non-standard headers sent in requests by clients to load balancer targets.

Audit

To determine if Drop Invalid Header Fields feature is enabled for your Application Load Balancers (ALBs), perform the following actions:

Using AWS Console

01 Sign in to the AWS Management Console.

02 Navigate to Amazon EC2 console at https://console.aws.amazon.com/ec2/v2/.

03 In the main navigation panel, under Load Balancing, choose Load Balancers.

04 Click inside the Filter by tags and attributes or search by keyword box, select Type and choose application to list the Application Load Balancers available in the current AWS region.

05 Select the Application Load Balancer (ALB) that you want to examine.

06 Select the Description tab from the console bottom panel to view the configuration information available for the selected load balancer.

07 In the Attributes section, check the Drop invalid header fields attribute value. If the Drop invalid header fields attribute value is set to Disabled, the Drop Invalid Header Fields feature is not enabled for the selected Application Load Balancer (ALB).

08 Repeat steps no. 5 – 7 for each Application Load Balancer deployed within the current AWS region.

09 Change the AWS cloud region from the navigation bar and repeat the Audit process for other regions.

Using AWS CLI

01 Run describe-load-balancers command (OSX/Linux/UNIX) with custom query filters to list the Amazon Resource Names (ARNs) of the Application Load Balancers (ALBs) available in the selected AWS region:

aws elbv2 describe-load-balancers
  --region us-east-1
  --query 'LoadBalancers[?(Type == `application`)].LoadBalancerArn'

02 The command output should return an array with the requested ALB ARN(s):

[
	"arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/cc-web-production-alb/abcdabcdabcdabcd",
	"arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/cc-webapp-endpoint-alb/abcd1234abcd1234"
]

03 Run describe-load-balancer-attributes command (OSX/Linux/UNIX) using the ARN of the Application Load Balancer (ALB) that you want to examine as the identifier parameter and custom query filters, to describe the Drop Invalid Header Fields feature status, available for the selected load balancer:

aws elbv2 describe-load-balancer-attributes
  --region us-east-1
  --load-balancer-arn arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/cc-web-production-alb/abcdabcdabcdabcd
  --query 'Attributes[?(Key == `routing.http.drop_invalid_header_fields.enabled`)].Value | []'

04 The command output should return the requested configuration status ("true"for enabled, "false"for disabled):

[
	"false"
]

If the describe-load-balancer-attributes command output returns false, as shown in the output example above, the Drop Invalid Header Fields feature is not enabled for the selected Application Load Balancer (ALB).

05 Repeat steps no. 3 and 4 for each Application Load Balancer deployed in the selected AWS region.

06 Change the AWS cloud region by updating the --region command parameter value and repeat the Audit process for other regions.

Remediation / Resolution

To enable the Drop Invalid Header Fields security feature for your existing Amazon Application Load Balancers (ALBs), perform the following actions:

Using AWS CloudFormation

01 CloudFormation template (JSON):

{
	"AWSTemplateFormatVersion": "2010-09-09",
	"Resources": {
		"ApplicationLoadBalancer": {
			"Type": "AWS::ElasticLoadBalancingV2::LoadBalancer",
			"Properties": {
				"Name": "cc-app-load-balancer",
				"Type": "application",
				"Scheme": "internet-facing",
				"IpAddressType": "ipv4",
				"Subnets": [
					"subnet-01234abcd1234abcd",
					"subnet-0abcd1234abcd1234"
				],
				"SecurityGroups": [
					"sg-0abcd1234abcd1234",
					"sg-01234abcd1234abcd"
				],
				"LoadBalancerAttributes": [
					{
						"Key": "routing.http.drop_invalid_header_fields.enabled",
						"Value": "true"
					}
				]
			}
		}
	}
}

02 CloudFormation template (YAML):

AWSTemplateFormatVersion: '2010-09-09'
	Resources:
	ApplicationLoadBalancer:
		Type: AWS::ElasticLoadBalancingV2::LoadBalancer
		Properties:
		Name: cc-app-load-balancer
		Type: application
		Scheme: internet-facing
		IpAddressType: ipv4
		Subnets:
			- subnet-01234abcd1234abcd
			- subnet-0abcd1234abcd1234
		SecurityGroups:
			- sg-0abcd1234abcd1234
			- sg-01234abcd1234abcd
		LoadBalancerAttributes:
			- Key: routing.http.drop_invalid_header_fields.enabled
			Value: 'true'

Using Terraform (AWS Provider)

01 Terraform configuration file (.tf):

terraform {
	required_providers {
		aws = {
			source  = "hashicorp/aws"
			version = "~> 4.0"
		}
	}

	required_version = ">= 0.14.9"
}

provider "aws" {
	profile = "default"
	region  = "us-east-1"
}

resource "aws_lb" "application-load-balancer" {
	name                       = "cc-app-load-balancer"
	load_balancer_type         = "application"
	internal                   = false
	ip_address_type            = "ipv4"
	subnets                    = ["subnet-01234abcd1234abcd","subnet-0abcd1234abcd1234"]
	security_groups            = ["sg-0abcd1234abcd1234","sg-01234abcd1234abcd"]
	drop_invalid_header_fields = true
}

Using AWS Console

01 Sign in to the AWS Management Console.

02 Navigate to Amazon EC2 console at https://console.aws.amazon.com/ec2/v2/.

03 In the main navigation panel, under Load Balancing, choose Load Balancers.

04 Click inside the Filter by tags and attributes or search by keyword box, select Type and choose application to list the Application Load Balancers available in the current AWS region.

05 Select the Application Load Balancer (ALB) that you want to reconfigure.

06 Select the Description tab and click on the Edit attributes button available in the Attributes section.

07 In the Edit load balancer attributes configuration box, select Enable next to Drop invalid header fields to enable the Drop Invalid Header Fields security feature for the selected Application Load Balancer (ALB). Choose Save to apply the configuration changes.

08 Repeat steps no. 5 – 7 for each Application Load Balancer that you want to reconfigure, available within the current AWS region.

09 Change the AWS cloud region from the navigation bar and repeat the Remediation process for other regions.

Using AWS CLI

01 Run modify-load-balancer-attributes command (OSX/Linux/UNIX) using the ARN of the Application Load Balancer (ALB) that you want to reconfigure as the identifier parameter, to enable the Drop Invalid Header Fields security feature for the selected load balancer:

aws elbv2 modify-load-balancer-attributes
  --region us-east-1
  --load-balancer-arn arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/cc-web-production-alb/abcdabcdabcdabcd
  --attributes Key=routing.http.drop_invalid_header_fields.enabled,Value=true

02 The command output should return the configuration attributes available for the modified load balancer:

{
	"Attributes": [
		{
			"Value": "true",
			"Key": "routing.http.drop_invalid_header_fields.enabled"
		},
		{
			"Value": "false",
			"Key": "access_logs.s3.enabled"
		},
		{
			"Value": "",
			"Key": "access_logs.s3.bucket"
		},
		{
			"Value": "",
			"Key": "access_logs.s3.prefix"
		},
		{
			"Value": "30",
			"Key": "idle_timeout.timeout_seconds"
		},
		{
			"Value": "true",
			"Key": "deletion_protection.enabled"
		},
		{
			"Value": "true",
			"Key": "routing.http2.enabled"
		},
		{
			"Value": "strictest",
			"Key": "routing.http.desync_mitigation_mode"
		}
	]
}

03 Repeat steps no. 1 and 2 for each Application Load Balancer that you want to reconfigure, available in the selected AWS region.

04 Change the AWS cloud region by updating the --region command parameter value and repeat the Remediation process for other regions.

References

AWS Command Line Interface (CLI) Documentation
describe-load-balancers
describe-load-balancer-attributes
modify-load-balancer-attributes

Publication date Aug 1, 2021

Audit

Using AWS Console

Using AWS CLI

Remediation / Resolution

Using AWS CloudFormation

Using Terraform (AWS Provider)

Using AWS Console

Using AWS CLI

References

Related ELBv2 rules