ELB application load balancers should be configured to drop HTTP headers

HTTP header injection and request smuggling attacks rely on malformed headers reaching backend applications. When an ALB forwards invalid headers, attackers can exploit parsing inconsistencies between the load balancer and the application to bypass security controls, poison caches, or exfiltrate data. Enabling drop_invalid_header_fields eliminates this attack surface at the network edge before traffic ever reaches your targets.

The default ALB behavior is to forward all headers, including malformed ones. This means every ALB you deploy without explicitly setting this attribute is silently passing potentially dangerous input through to your applications.

Retrofit consideration

Enabling this on existing ALBs may break applications that depend on non-standard or malformed headers. Test with staging traffic before applying to production load balancers.

Implementation

Choose the approach that matches how you manage Terraform.

Compliance.tfOpen source moduleTerraform resources

Use the compliance.tf module to enforce this control by default. See get started with compliance.tf.

SOC 2HIPAA Omnibus Rule 2013GDPRNIST SP 800-171 Rev 2NYDFS Cybersecurity RegulationRBI Cyber Security Framework for UCBsRBI IT Framework for NBFCsFedRAMP Low Baseline Rev 4HIPAA Security Rule 2003NIST Cybersecurity Framework v1.1NIST SP 800-53 Rev 4PCI DSS v3.2.1

module "alb" {
  source  = "soc2.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "hipaa.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "gdpr.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "nist800171.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "nydfs23.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "rbicybersecurity.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "rbiitfnbfc.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "fedramplow.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "hipaasecurity2003.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "nistcsfv11.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "nist80053rev4.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

module "alb" {
  source  = "pcidssv321.compliance.tf/terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"
}

If you use terraform-aws-modules/alb/aws, set the right module inputs for this control. You can later migrate to the compliance.tf module with minimal changes because it is compatible by design.

module "alb" {
  source  = "terraform-aws-modules/alb/aws"
  version = ">=10.0.0,<11.0.0"

  access_logs = {
    bucket  = "example-bucket-abc123"
    enabled = true
  }
  internal = true
  listeners = {
    https = {
      certificate_arn = "arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012"
      forward = {
        target_group_key = "default"
      }
      protocol   = "HTTPS"
      ssl_policy = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }
  }
  load_balancer_type = "application"
  name               = "abc123"
  security_groups    = ["sg-abc12345"]
  subnets            = ["subnet-abc123", "subnet-def456"]
  target_groups = {
    default = {
      create_attachment = false
      name_prefix       = "def-"
      port              = 443
      protocol          = "HTTPS"
      target_type       = "ip"
    }
  }
  vpc_id = "vpc-12345678"

  drop_invalid_header_fields = true
}

Use AWS provider resources directly. See docs for the resources involved: aws_lb.

resource "aws_lb" "this" {
  access_logs {
    bucket = "pofix-logs-abc123"
  }
  internal = true
  name     = "pofix-abc123"
  subnets  = ["subnet-abc123", "subnet-def456"]

  drop_invalid_header_fields = true
}

What this control checks

The control checks aws_lb resources (and the alias aws_alb) where load_balancer_type is "application" for drop_invalid_header_fields set to true. It fails when the argument is omitted or set to false. The argument lives directly on the aws_lb resource block, not inside a nested block. Only Application Load Balancers are evaluated; Network and Gateway Load Balancers are out of scope.

Common pitfalls

Default value is false
Explicitly set drop_invalid_header_fields = true on every aws_lb resource. Omitting the argument leaves the default of false in place, and Terraform won't surface it as noncompliant without an explicit policy check.
Alias resource type confusion
Terraform treats aws_lb and aws_alb as aliases for the same underlying resource. If your policy engine only checks aws_lb, then aws_alb resources silently escape evaluation. Standardizing on aws_lb across your codebase is the simplest fix.
Attribute only applies to application type
The drop_invalid_header_fields argument only applies to Application Load Balancers. Scope policy evaluation to resources where load_balancer_type is "application" or absent (the default is "application"), otherwise you risk false positives against NLBs and GLBs.
Modules may not expose the argument
Community ALB modules frequently omit drop_invalid_header_fields from their variable interface, which means the ALB gets created with false by default. Audit the module source before assuming it passes this argument through.

Audit evidence

Auditors expect Config rule evaluations for alb-http-drop-invalid-header-enabled showing all applicable ALBs as COMPLIANT. A CSPM report showing this attribute enabled across all ALBs in each account and region is also acceptable. Console verification: EC2 console under Load Balancers, select each ALB, open Attributes, confirm "Drop invalid header fields" is Enabled.

For programmatic verification, aws elbv2 describe-load-balancer-attributes --load-balancer-arn <arn> should return routing.http.drop_invalid_header_fields.enabled with a value of "true" for every Application Load Balancer.

Framework-specific interpretation

SOC 2: CC6.6 calls for controls that restrict access through system boundaries and guard against external threats. Dropping malformed headers at the ALB keeps hostile input out of backend systems. CC6.1 is also relevant, since this contributes to logical access controls over information assets.

HIPAA Omnibus Rule 2013: Malformed HTTP headers are a well-documented request smuggling vector. For workloads handling ePHI, letting those headers reach backend systems creates unauthorized-access risk that the Security Rule's transmission security requirements at 45 CFR 164.312(e)(1) are specifically designed to prevent. Configuring ALBs to drop invalid headers is a concrete technical safeguard against that attack path.

GDPR: Article 25 (data protection by design) and Article 32 (appropriate technical security measures) both apply here. Dropping malformed headers at the ALB prevents header injection attacks that could expose personal data to unauthorized access or exfiltration. This is a network-layer implementation of those obligations.

Tool mappings

Use these identifiers to cross-reference this control across tools, reports, and evidence.

Compliance.tf Control: elb_application_lb_drop_http_headers
AWS Config Managed Rule: ALB_HTTP_DROP_INVALID_HEADER_ENABLED
Checkov Check: CKV_AWS_131
Powerpipe Controls: aws_compliance.control.elb_application_lb_drop_http_headers, aws_compliance.control.elb_application_lb_http_drop_invalid_header_enabled
Prowler Check: elbv2_desync_mitigation_mode
AWS Security Hub Control: ELB.4
KICS Query: 6e3fd2ed-5c83-4c68-9679-7700d224d379
Trivy Check: AWS-0052

Last reviewed: 2026-03-09