feat: Add optional policy to deny ssec encrypted uploads

There have been incidents where leaked credentials have led to attackers encrypting S3 buckets with their own keys in
a form of ransomware attack. AWS recommends implementing policy to prevent using SSEC if that is not a feature that
you use in a given bucket.

Reference:
https://aws.amazon.com/blogs/security/preventing-unintended-encryption-of-amazon-s3-objects/

Author: mbainter

README.md

@@ -172,6 +172,7 @@ No modules.
 | [aws_iam_policy_document.deny_incorrect_encryption_headers](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document) | data source |
 | [aws_iam_policy_document.deny_incorrect_kms_key_sse](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document) | data source |
 | [aws_iam_policy_document.deny_insecure_transport](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document) | data source |
+| [aws_iam_policy_document.deny_ssec_encrypted_object_uploads](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document) | data source |
 | [aws_iam_policy_document.deny_unencrypted_object_uploads](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document) | data source |
 | [aws_iam_policy_document.elb_log_delivery](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document) | data source |
 | [aws_iam_policy_document.inventory_and_analytics_destination_policy](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document) | data source |
@@ -198,6 +199,7 @@ No modules.
 | <a name="input_attach_deny_incorrect_encryption_headers"></a> [attach\_deny\_incorrect\_encryption\_headers](#input\_attach\_deny\_incorrect\_encryption\_headers) | Controls if S3 bucket should deny incorrect encryption headers policy attached. | `bool` | `false` | no |
 | <a name="input_attach_deny_incorrect_kms_key_sse"></a> [attach\_deny\_incorrect\_kms\_key\_sse](#input\_attach\_deny\_incorrect\_kms\_key\_sse) | Controls if S3 bucket policy should deny usage of incorrect KMS key SSE. | `bool` | `false` | no |
 | <a name="input_attach_deny_insecure_transport_policy"></a> [attach\_deny\_insecure\_transport\_policy](#input\_attach\_deny\_insecure\_transport\_policy) | Controls if S3 bucket should have deny non-SSL transport policy attached | `bool` | `false` | no |
+| <a name="input_attach_deny_ssec_encrypted_object_uploads"></a> [attach\_deny\_ssec\_encrypted\_object\_uploads](#input\_attach\_deny\_ssec\_encrypted\_object\_uploads) | Controls if S3 bucket should deny SSEC encrypted object uploads. | `bool` | `false` | no |
 | <a name="input_attach_deny_unencrypted_object_uploads"></a> [attach\_deny\_unencrypted\_object\_uploads](#input\_attach\_deny\_unencrypted\_object\_uploads) | Controls if S3 bucket should deny unencrypted object uploads policy attached. | `bool` | `false` | no |
 | <a name="input_attach_elb_log_delivery_policy"></a> [attach\_elb\_log\_delivery\_policy](#input\_attach\_elb\_log\_delivery\_policy) | Controls if S3 bucket should have ELB log delivery policy attached | `bool` | `false` | no |
 | <a name="input_attach_inventory_destination_policy"></a> [attach\_inventory\_destination\_policy](#input\_attach\_inventory\_destination\_policy) | Controls if S3 bucket should have bucket inventory destination policy attached. | `bool` | `false` | no |

examples/complete/main.tf

@@ -139,14 +139,15 @@ module "s3_bucket" {
   }
 
   # Bucket policies
-  attach_policy                            = true
-  policy                                   = data.aws_iam_policy_document.bucket_policy.json
-  attach_deny_insecure_transport_policy    = true
-  attach_require_latest_tls_policy         = true
-  attach_deny_incorrect_encryption_headers = true
-  attach_deny_incorrect_kms_key_sse        = true
-  allowed_kms_key_arn                      = aws_kms_key.objects.arn
-  attach_deny_unencrypted_object_uploads   = true
+  attach_policy                             = true
+  policy                                    = data.aws_iam_policy_document.bucket_policy.json
+  attach_deny_insecure_transport_policy     = true
+  attach_require_latest_tls_policy          = true
+  attach_deny_incorrect_encryption_headers  = true
+  attach_deny_incorrect_kms_key_sse         = true
+  allowed_kms_key_arn                       = aws_kms_key.objects.arn
+  attach_deny_unencrypted_object_uploads    = true
+  attach_deny_ssec_encrypted_object_uploads = true
 
   # S3 bucket-level Public Access Block configuration (by default now AWS has made this default as true for S3 bucket-level block public access)
   # block_public_acls       = true

main.tf

@@ -12,7 +12,7 @@ locals {
 
   create_bucket_acl = (var.acl != null && var.acl != "null") || length(local.grants) > 0
 
-  attach_policy = var.attach_require_latest_tls_policy || var.attach_access_log_delivery_policy || var.attach_elb_log_delivery_policy || var.attach_lb_log_delivery_policy || var.attach_deny_insecure_transport_policy || var.attach_inventory_destination_policy || var.attach_deny_incorrect_encryption_headers || var.attach_deny_incorrect_kms_key_sse || var.attach_deny_unencrypted_object_uploads || var.attach_policy
+  attach_policy = var.attach_require_latest_tls_policy || var.attach_access_log_delivery_policy || var.attach_elb_log_delivery_policy || var.attach_lb_log_delivery_policy || var.attach_deny_insecure_transport_policy || var.attach_inventory_destination_policy || var.attach_deny_incorrect_encryption_headers || var.attach_deny_incorrect_kms_key_sse || var.attach_deny_unencrypted_object_uploads || var.attach_deny_ssec_encrypted_object_uploads || var.attach_policy
 
   # Variables with type `any` should be jsonencode()'d when value is coming from Terragrunt
   grants               = try(jsondecode(var.grant), var.grant)
@@ -558,6 +558,7 @@ data "aws_iam_policy_document" "combined" {
     var.attach_require_latest_tls_policy ? data.aws_iam_policy_document.require_latest_tls[0].json : "",
     var.attach_deny_insecure_transport_policy ? data.aws_iam_policy_document.deny_insecure_transport[0].json : "",
     var.attach_deny_unencrypted_object_uploads ? data.aws_iam_policy_document.deny_unencrypted_object_uploads[0].json : "",
+    var.attach_deny_ssec_encrypted_object_uploads ? data.aws_iam_policy_document.deny_ssec_encrypted_object_uploads[0].json : "",
     var.attach_deny_incorrect_kms_key_sse ? data.aws_iam_policy_document.deny_incorrect_kms_key_sse[0].json : "",
     var.attach_deny_incorrect_encryption_headers ? data.aws_iam_policy_document.deny_incorrect_encryption_headers[0].json : "",
     var.attach_inventory_destination_policy || var.attach_analytics_destination_policy ? data.aws_iam_policy_document.inventory_and_analytics_destination_policy[0].json : "",
@@ -908,6 +909,34 @@ data "aws_iam_policy_document" "deny_unencrypted_object_uploads" {
   }
 }
 
+data "aws_iam_policy_document" "deny_ssec_encrypted_object_uploads" {
+  count = local.create_bucket && var.attach_deny_ssec_encrypted_object_uploads ? 1 : 0
+
+  statement {
+    sid    = "denySSECEncryptedObjectUploads"
+    effect = "Deny"
+
+    actions = [
+      "s3:PutObject"
+    ]
+
+    resources = [
+      "${aws_s3_bucket.this[0].arn}/*"
+    ]
+
+    principals {
+      identifiers = ["*"]
+      type        = "*"
+    }
+
+    condition {
+      test     = "Null"
+      variable = "s3:x-amz-server-side-encryption-customer-algorithm"
+      values   = [false]
+    }
+  }
+}
+
 resource "aws_s3_bucket_public_access_block" "this" {
   count = local.create_bucket && var.attach_public_policy ? 1 : 0
 

variables.tf

@@ -82,6 +82,12 @@ variable "attach_deny_unencrypted_object_uploads" {
   default     = false
 }
 
+variable "attach_deny_ssec_encrypted_object_uploads" {
+  description = "Controls if S3 bucket should deny SSEC encrypted object uploads."
+  type        = bool
+  default     = false
+}
+
 variable "bucket" {
   description = "(Optional, Forces new resource) The name of the bucket. If omitted, Terraform will assign a random, unique name."
   type        = string

wrappers/main.tf

@@ -17,6 +17,7 @@ module "wrapper" {
   attach_deny_incorrect_encryption_headers   = try(each.value.attach_deny_incorrect_encryption_headers, var.defaults.attach_deny_incorrect_encryption_headers, false)
   attach_deny_incorrect_kms_key_sse          = try(each.value.attach_deny_incorrect_kms_key_sse, var.defaults.attach_deny_incorrect_kms_key_sse, false)
   attach_deny_insecure_transport_policy      = try(each.value.attach_deny_insecure_transport_policy, var.defaults.attach_deny_insecure_transport_policy, false)
+  attach_deny_ssec_encrypted_object_uploads  = try(each.value.attach_deny_ssec_encrypted_object_uploads, var.defaults.attach_deny_ssec_encrypted_object_uploads, false)
   attach_deny_unencrypted_object_uploads     = try(each.value.attach_deny_unencrypted_object_uploads, var.defaults.attach_deny_unencrypted_object_uploads, false)
   attach_elb_log_delivery_policy             = try(each.value.attach_elb_log_delivery_policy, var.defaults.attach_elb_log_delivery_policy, false)
   attach_inventory_destination_policy        = try(each.value.attach_inventory_destination_policy, var.defaults.attach_inventory_destination_policy, false)