Lambda function with privileged role

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Id: terraform-aws-lambda-function-with-privileged-role

Cloud Provider: AWS

Platform: Terraform

Severity: High

Category: Insecure Configurations

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Description

AWS Lambda functions with privileged IAM roles present a significant security risk by violating the principle of least privilege. When Lambda functions are assigned roles with broad permissions such as iam:* or sts:AssumeRole, they become potential vectors for privilege escalation if compromised. Instead, Lambda functions should be granted only the specific permissions needed for their execution.

For example, avoid policies with broad permissions like the following:

policy = jsonencode({
  Statement = [{
    Action = [
      "ec2:Describe*",
      "iam:*"
    ],
    Effect   = "Allow",
    Resource = "*"
  }]
})

Instead, implement least privilege permissions:

policy = jsonencode({
  Statement = [{
    Action = [
      "ec2:Describe*",
      "s3:GetObject"
    ],
    Effect   = "Allow",
    Resource = "*"
  }]
})

Compliant Code Examples

resource "aws_lambda_function" "negativefunction1" {
  filename      = "lambda_function_payload.zip"
  function_name = "lambda_function_name"
  role          = aws_iam_role.negativerole1.arn
  handler       = "exports.test"
  source_code_hash = filebase64sha256("lambda_function_payload.zip")
  runtime = "nodejs12.x"

  tags = {
    Name = "lambda"
  }

  environment = {
    variables = {
      foo = "bar"
    }
  }
}

resource "aws_lambda_function" "negativefunction2" {
  filename      = "lambda_function_payload.zip"
  function_name = "lambda_function_name"
  role          = aws_iam_role.negativerole2.arn
  handler       = "exports.test"
  source_code_hash = filebase64sha256("lambda_function_payload.zip")
  runtime = "nodejs12.x"

  tags = {
    Name = "lambda"
  }

  environment = {
    variables = {
      foo = "bar"
    }
  }
}

resource "aws_iam_role" "negativerole1" {
  name = "negativerole1"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["some:action"],
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": ""
    }
  ]
}
EOF
  tags = {
    tag-key = "tag-value"
  }
}

resource "aws_iam_role" "negativerole2" {
  name = "negativerole2"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["some:action"],
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": ""
    }
  ]
}
EOF
  tags = {
    tag-key = "tag-value"
  }
}


resource "aws_iam_role_policy" "negativeinlinepolicy1" {
  name = "negativeinlinepolicy1"
  role = aws_iam_role.negativerole1.id

  # Terraform's "jsonencode" function converts a
  # Terraform expression result to valid JSON syntax.
  policy = jsonencode({
    Version = "2012-10-17"
    Statement = [
      {
        Action = [
          "ec2:Describe*",
          "s3:GetObject"
        ]
        Effect   = "Allow"
        Resource = "*"
      },
    ]
  })
}

resource "aws_iam_policy" "negativecustomermanagedpolicy1" {
  name        = "negativecustomermanagedpolicy1"

  policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "ec2:Describe*"
      ],
      "Effect": "Allow",
      "Resource": "*"
    }
  ]
}
EOF
}

resource "aws_iam_policy" "negativecustomermanagedpolicy2" {
  name        = "negativecustomermanagedpolicy2"

  policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "lambda:CreateFunction"
      ],
      "Effect": "Allow",
      "Resource": "*"
    }
  ]
}
EOF
}

# Mapping of customer managed policy defined in this template set
resource "aws_iam_role_policy_attachment" "negativerolepolicyattachment1" {
  role       = aws_iam_role.negativerole1.name
  policy_arn = aws_iam_policy.negativecustomermanagedpolicy1.arn
}

resource "aws_iam_policy_attachment" "negativedirectpolicyattachment1" {
  roles       = [aws_iam_role.negativerole1.name]
  policy_arn = aws_iam_policy.negativecustomermanagedpolicy2.arn
}

# Mapping of pre-existing policy arns
resource "aws_iam_role_policy_attachment" "negativerolepolicyattachment2" {
  role       = aws_iam_role.negativerole2.name
  policy_arn = "arn:aws:iam::policy/negativepreexistingpolicyarn1"
}

resource "aws_iam_policy_attachment" "negativedirectpolicyattachment2" {
  roles       = [aws_iam_role.negativerole2.name]
  policy_arn = "arn:aws:iam::policy/DenyAll"
}

Non-Compliant Code Examples

resource "aws_lambda_function" "positivefunction1" {
  filename      = "lambda_function_payload.zip"
  function_name = "lambda_function_name"
  role          = aws_iam_role.positiverole1.arn
  handler       = "exports.test"
  source_code_hash = filebase64sha256("lambda_function_payload.zip")
  runtime = "nodejs12.x"

  tags = {
    Name = "lambda"
  }

  environment = {
    variables = {
      foo = "bar"
    }
  }
}

resource "aws_lambda_function" "positivefunction2" {
  filename      = "lambda_function_payload.zip"
  function_name = "lambda_function_name"
  role          = aws_iam_role.positiverole2.arn
  handler       = "exports.test"
  source_code_hash = filebase64sha256("lambda_function_payload.zip")
  runtime = "nodejs12.x"

  tags = {
    Name = "lambda"
  }

  environment = {
    variables = {
      foo = "bar"
    }
  }
}

resource "aws_iam_role" "positiverole1" {
  name = "positiverole1"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["some:action"],
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": ""
    }
  ]
}
EOF
  tags = {
    tag-key = "tag-value"
  }
}

resource "aws_iam_role" "positiverole2" {
  name = "positiverole2"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["some:action"],
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": ""
    }
  ]
}
EOF
  tags = {
    tag-key = "tag-value"
  }
}


resource "aws_iam_role_policy" "positiveinlinepolicy1" {
  name = "positiveinlinepolicy1"
  role = aws_iam_role.positiverole1.id

  # Terraform's "jsonencode" function converts a
  # Terraform expression result to valid JSON syntax.
  policy = jsonencode({
    Version = "2012-10-17"
    Statement = [
      {
        Action = [
          "ec2:Describe*",
          "iam:*"
        ]
        Effect   = "Allow"
        Resource = "*"
      },
    ]
  })
}

resource "aws_iam_policy" "positivecustomermanagedpolicy1" {
  name        = "positivecustomermanagedpolicy1"

  policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "ec2:Describe*",
        "sts:AssumeRole"
      ],
      "Effect": "Allow",
      "Resource": "*"
    },
    {
      "Action": [
        "iam:CreateLoginProfile"
      ],
      "Effect": "Allow",
      "Resource": "*"
    }
  ]
}
EOF
}

resource "aws_iam_policy" "positivecustomermanagedpolicy2" {
  name        = "positivecustomermanagedpolicy2"

  policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "ec2:Describe*",
        "sts:AssumeRole"
      ],
      "Effect": "Allow",
      "Resource": "*"
    }
  ]
}
EOF
}

# Mapping of customer managed policy defined in this template set
resource "aws_iam_role_policy_attachment" "positiverolepolicyattachment1" {
  role       = aws_iam_role.positiverole1.name
  policy_arn = aws_iam_policy.positivecustomermanagedpolicy1.arn
}

resource "aws_iam_policy_attachment" "positivedirectpolicyattachment1" {
  roles       = [aws_iam_role.positiverole1.name]
  policy_arn = aws_iam_policy.positivecustomermanagedpolicy2.arn
}

# Mapping of pre-existing policy arns
resource "aws_iam_role_policy_attachment" "positiverolepolicyattachment2" {
  role       = aws_iam_role.positiverole2.name
  policy_arn = "arn:aws:iam::policy/positivepreexistingpolicyarn1"
}

resource "aws_iam_policy_attachment" "positivedirectpolicyattachment2" {
  roles       = [aws_iam_role.positiverole2.name]
  policy_arn = "arn:aws:iam::policy/AmazonPersonalizeFullAccess"
}