diff --git a/terraform/environments/aws/compute.tf b/terraform/environments/aws/compute.tf
index 51c2683f..680517c1 100644
--- a/terraform/environments/aws/compute.tf
+++ b/terraform/environments/aws/compute.tf
@@ -2,6 +2,29 @@
 # Compute Platform: Lambda (default)
 # ==============================================
 
+# Lambda Function URL auth type is fully derived from enable_cdn. There is no
+# operator-facing variable for it: the two valid combinations are
+#
+#   enable_cdn = false  -> auth_type = "NONE"
+#     The SPA hits the Function URL directly. Browsers cannot SigV4-sign, so
+#     the URL must be unauthenticated at the AWS-IAM layer; auth is enforced
+#     at the application layer (login session, CSRF, API keys).
+#
+#   enable_cdn = true   -> auth_type = "AWS_IAM"
+#     CloudFront fronts the URL with an OAC that SigV4-signs every request
+#     (see frontend module's aws_cloudfront_origin_access_control.lambda).
+#     The Function URL rejects anything not signed by CloudFront's OAC,
+#     closing the public-internet exposure described in #424.
+#
+# Tying the two flags together prevents two specific mis-configurations:
+#   (a) enable_cdn = true with auth_type = "NONE" -> public Lambda URL behind
+#       a pointless CloudFront (security goal of #424 defeated).
+#   (b) enable_cdn = false with auth_type = "AWS_IAM" -> direct browser hits
+#       to Function URL return 403 (no way to sign), site is unreachable.
+locals {
+  lambda_function_url_auth_type = var.enable_cdn ? "AWS_IAM" : "NONE"
+}
+
 module "compute_lambda" {
   source = "../../modules/compute/aws/lambda"
   count  = var.compute_platform == "lambda" ? 1 : 0
@@ -37,8 +60,7 @@ module "compute_lambda" {
   }
 
   # Function URL
-  enable_function_url    = var.lambda_enable_function_url
-  function_url_auth_type = var.lambda_function_url_auth_type
+  function_url_auth_type = local.lambda_function_url_auth_type
   allowed_origins        = var.lambda_allowed_origins
 
   # Concurrency
@@ -204,3 +226,34 @@ module "compute_fargate" {
   # Build dependency must be explicit — image_uri is computed before docker push completes
   depends_on = [module.networking, module.database, module.secrets, module.build]
 }
+
+# ==============================================
+# Lambda Function URL: CloudFront OAC permission
+# ==============================================
+#
+# This permission lives at the environment layer (not inside the lambda module)
+# because both module.compute_lambda and module.frontend must be fully resolved
+# before the permission can reference both the Lambda function name and the
+# CloudFront distribution ARN. Placing it inside either module would create a
+# dependency cycle (Lambda URL is module.frontend's origin; distribution ARN
+# would need to feed back into the lambda module).
+#
+# Created only when:
+#   - compute platform is lambda (function URL is active)
+#   - enable_cdn is true (CloudFront distribution is deployed and provides the
+#     source ARN); when enable_cdn = true the local also flips auth_type to
+#     AWS_IAM so the IAM gate is enforced on the Function URL.
+#
+# When enable_cdn = false the Function URL stays NONE (no IAM gate) and this
+# resource is count = 0; the SPA hits the Function URL directly and auth is
+# enforced at the application layer.
+resource "aws_lambda_permission" "function_url_cloudfront" {
+  count = var.compute_platform == "lambda" && var.enable_cdn ? 1 : 0
+
+  statement_id           = "FunctionURLAllowCloudFront"
+  action                 = "lambda:InvokeFunctionUrl"
+  function_name          = module.compute_lambda[0].function_name
+  principal              = "cloudfront.amazonaws.com"
+  source_arn             = module.frontend[0].cloudfront_distribution_arn
+  function_url_auth_type = "AWS_IAM"
+}
diff --git a/terraform/environments/aws/dev.tfvars.example b/terraform/environments/aws/dev.tfvars.example
index b7b2e0c6..6457ebf9 100644
--- a/terraform/environments/aws/dev.tfvars.example
+++ b/terraform/environments/aws/dev.tfvars.example
@@ -30,9 +30,7 @@ lambda_timeout          = 60       # Seconds
 lambda_reserved_concurrency = -1   # -1 = no limit
 lambda_log_retention_days   = 30
 
-# Lambda Function URL
-lambda_enable_function_url    = true
-lambda_function_url_auth_type = "NONE"
+# Lambda Function URL (auth_type is derived from enable_cdn — see compute.tf)
 lambda_allowed_origins        = ["*"]
 
 # Fargate settings (when compute_platform = "fargate")
diff --git a/terraform/environments/aws/frontend.tf b/terraform/environments/aws/frontend.tf
index fa09f511..9a97c974 100644
--- a/terraform/environments/aws/frontend.tf
+++ b/terraform/environments/aws/frontend.tf
@@ -30,6 +30,12 @@ module "frontend" {
   # Don't let module create DNS record if we're managing it in dns_records.tf
   route53_zone_id = var.subdomain_zone_name != "" ? null : var.frontend_route53_zone_id
 
+  # Enable OAC whenever Lambda is fronted by CloudFront (enable_cdn = true,
+  # which also flips the Function URL auth_type to AWS_IAM via the local in
+  # compute.tf, so the OAC's SigV4 signing is the auth the Function URL
+  # expects).
+  enable_oac = var.compute_platform == "lambda" && var.enable_cdn
+
   # CloudFront configuration
   price_class = var.frontend_price_class
 
diff --git a/terraform/environments/aws/github-dev.tfvars b/terraform/environments/aws/github-dev.tfvars
index 7a201826..bef90501 100644
--- a/terraform/environments/aws/github-dev.tfvars
+++ b/terraform/environments/aws/github-dev.tfvars
@@ -18,12 +18,13 @@ compute_platform    = "lambda"
 enable_docker_build = true # Build and push image via terraform apply on the runner
 
 # Lambda Configuration
-lambda_memory_size            = 2048
-lambda_timeout                = 300
-lambda_reserved_concurrency   = -1
-lambda_log_retention_days     = 7
-lambda_enable_function_url    = true
-lambda_function_url_auth_type = "NONE"
+lambda_memory_size          = 2048
+lambda_timeout              = 300
+lambda_reserved_concurrency = -1
+lambda_log_retention_days   = 7
+# Function URL auth_type is derived from enable_cdn (local in compute.tf):
+#   enable_cdn = false -> NONE  (direct browser hits, app-layer auth)
+#   enable_cdn = true  -> AWS_IAM (CloudFront OAC signs every request)
 # Current deployed dev origin (Lambda Function URL) + local Webpack dev server.
 # Wildcard is rejected by the module (allow_credentials=true + * = any-origin CSRF).
 # Update the Lambda Function URL entry when the dev environment is redeployed.
diff --git a/terraform/environments/aws/github-prod.tfvars b/terraform/environments/aws/github-prod.tfvars
index 3af0e89c..99470778 100644
--- a/terraform/environments/aws/github-prod.tfvars
+++ b/terraform/environments/aws/github-prod.tfvars
@@ -18,15 +18,17 @@ compute_platform    = "lambda"
 enable_docker_build = true # Build image via Terraform build module (no separate CI build step)
 
 # Lambda Configuration
-lambda_memory_size            = 1024
-lambda_timeout                = 300
-lambda_reserved_concurrency   = -1
-lambda_log_retention_days     = 30
-lambda_enable_function_url    = true
-lambda_function_url_auth_type = "NONE"
+lambda_memory_size          = 1024
+lambda_timeout              = 300
+lambda_reserved_concurrency = -1
+lambda_log_retention_days   = 30
+# Function URL auth_type is derived from enable_cdn (local in compute.tf):
+#   enable_cdn = false -> NONE  (direct browser hits, app-layer auth)
+#   enable_cdn = true  -> AWS_IAM (CloudFront OAC signs every request)
 # TODO(env-not-deployed): prod environment is not yet provisioned. Update this
 # to the actual prod origin (e.g. the customer-facing dashboard domain) when
-# the env exists. .invalid placeholder fails fast on accidental apply.
+# the env exists. .invalid placeholder ensures any accidental apply fails fast
+# on hostname resolution.
 lambda_allowed_origins = ["https://prod-not-yet-deployed.invalid"]
 
 # Fargate Configuration (when compute_platform = "fargate")
diff --git a/terraform/environments/aws/github-staging.tfvars b/terraform/environments/aws/github-staging.tfvars
index bb3cabad..7e38b86d 100644
--- a/terraform/environments/aws/github-staging.tfvars
+++ b/terraform/environments/aws/github-staging.tfvars
@@ -18,12 +18,13 @@ compute_platform    = "lambda"
 enable_docker_build = true # Build image via Terraform build module (no separate CI build step)
 
 # Lambda Configuration
-lambda_memory_size            = 1024
-lambda_timeout                = 300
-lambda_reserved_concurrency   = -1
-lambda_log_retention_days     = 14
-lambda_enable_function_url    = true
-lambda_function_url_auth_type = "NONE"
+lambda_memory_size          = 1024
+lambda_timeout              = 300
+lambda_reserved_concurrency = -1
+lambda_log_retention_days   = 14
+# Function URL auth_type is derived from enable_cdn (local in compute.tf):
+#   enable_cdn = false -> NONE  (direct browser hits, app-layer auth)
+#   enable_cdn = true  -> AWS_IAM (CloudFront OAC signs every request)
 # TODO(env-not-deployed): staging environment is not yet provisioned. Update this
 # to the actual staging origin when the env exists. Until then the .invalid TLD
 # ensures any accidental terraform apply fails fast on hostname resolution.
diff --git a/terraform/environments/aws/variables.tf b/terraform/environments/aws/variables.tf
index 82310274..a69171c4 100644
--- a/terraform/environments/aws/variables.tf
+++ b/terraform/environments/aws/variables.tf
@@ -234,30 +234,13 @@ variable "lambda_timeout" {
   default     = 30
 }
 
-variable "lambda_enable_function_url" {
-  description = "Enable Lambda Function URL"
-  type        = bool
-  default     = true
-}
-
-variable "lambda_function_url_auth_type" {
-  description = <<-EOT
-    Lambda Function URL auth type — must be "NONE" or "AWS_IAM". Default is
-    "NONE" because CUDly is a browser-served SPA that talks directly to the
-    Function URL; the browser cannot SigV4-sign requests. Auth is enforced at
-    the application layer (login session, JWT, API keys, CSRF) — see
-    internal/api/handler_login.go and middleware in router.go. Override to
-    AWS_IAM only when fronting the URL with a signed-request gateway
-    (CloudFront with Lambda@Edge, API Gateway, etc.).
-  EOT
-  type        = string
-  default     = "NONE"
 
-  validation {
-    condition     = contains(["AWS_IAM", "NONE"], var.lambda_function_url_auth_type)
-    error_message = "lambda_function_url_auth_type must be either \"AWS_IAM\" or \"NONE\"."
-  }
-}
+# Lambda Function URL auth type is derived from var.enable_cdn (see
+# local.lambda_function_url_auth_type in compute.tf). When enable_cdn = true
+# the Function URL is fronted by CloudFront with OAC + SigV4, so auth is
+# AWS_IAM; when enable_cdn = false the browser hits the Function URL directly
+# and the SPA cannot SigV4-sign, so auth is NONE and the application layer
+# (login session, CSRF, API keys) is the gate.
 
 variable "lambda_allowed_origins" {
   description = "CORS allowed origins"
diff --git a/terraform/modules/compute/aws/lambda/main.tf b/terraform/modules/compute/aws/lambda/main.tf
index 6700a924..21ab8877 100644
--- a/terraform/modules/compute/aws/lambda/main.tf
+++ b/terraform/modules/compute/aws/lambda/main.tf
@@ -115,8 +115,6 @@ resource "aws_lambda_function" "main" {
 # ==============================================
 
 resource "aws_lambda_function_url" "main" {
-  count = var.enable_function_url ? 1 : 0
-
   function_name      = aws_lambda_function.main.function_name
   authorization_type = var.function_url_auth_type
 
@@ -132,9 +130,9 @@ resource "aws_lambda_function_url" "main" {
 # Resource-based policy grant for the Function URL. Without this, every request
 # is rejected at the Lambda edge with HTTP 403 AccessDeniedException, even when
 # authorization_type = "NONE". The AWS provider used to create this implicitly
-# but stopped doing so in recent versions — it must be declared explicitly.
+# but stopped doing so in recent versions -- it must be declared explicitly.
 resource "aws_lambda_permission" "function_url" {
-  count = var.enable_function_url && var.function_url_auth_type == "NONE" ? 1 : 0
+  count = var.function_url_auth_type == "NONE" ? 1 : 0
 
   statement_id           = "FunctionURLAllowPublicAccess"
   action                 = "lambda:InvokeFunctionUrl"
diff --git a/terraform/modules/compute/aws/lambda/outputs.tf b/terraform/modules/compute/aws/lambda/outputs.tf
index 97b3f7ac..94e7ff63 100644
--- a/terraform/modules/compute/aws/lambda/outputs.tf
+++ b/terraform/modules/compute/aws/lambda/outputs.tf
@@ -10,7 +10,7 @@ output "function_arn" {
 
 output "function_url" {
   description = "Lambda Function URL"
-  value       = var.enable_function_url ? aws_lambda_function_url.main[0].function_url : null
+  value       = aws_lambda_function_url.main.function_url
 }
 
 output "function_invoke_arn" {
diff --git a/terraform/modules/compute/aws/lambda/variables.tf b/terraform/modules/compute/aws/lambda/variables.tf
index 131c9c67..6c062a44 100644
--- a/terraform/modules/compute/aws/lambda/variables.tf
+++ b/terraform/modules/compute/aws/lambda/variables.tf
@@ -83,16 +83,25 @@ variable "vpc_config" {
   default = null
 }
 
-variable "enable_function_url" {
-  description = "Enable Lambda Function URL"
-  type        = bool
-  default     = true
-}
 
 variable "function_url_auth_type" {
-  description = "Function URL authorization type (NONE or AWS_IAM)"
+  description = <<-EOT
+    Function URL authorization type. Use "AWS_IAM" (default, recommended) when a
+    CloudFront distribution with an OAC is deployed in front of the Function URL —
+    AWS then enforces SigV4 signing on every request so the Function URL is
+    unreachable without a valid AWS identity.
+    Use "NONE" only in environments where direct access without CloudFront is
+    acceptable (e.g. local dev, or environments not yet provisioned with CloudFront).
+    Pair with cloudfront_distribution_arn when switching to AWS_IAM so the module can
+    create the necessary resource-based policy granting CloudFront invocation rights.
+  EOT
   type        = string
-  default     = "NONE"
+  default     = "AWS_IAM"
+
+  validation {
+    condition     = contains(["NONE", "AWS_IAM"], var.function_url_auth_type)
+    error_message = "function_url_auth_type must be \"NONE\" or \"AWS_IAM\"."
+  }
 }
 
 variable "allowed_origins" {
diff --git a/terraform/modules/frontend/aws/main.tf b/terraform/modules/frontend/aws/main.tf
index 6e93f4fd..14689fd5 100644
--- a/terraform/modules/frontend/aws/main.tf
+++ b/terraform/modules/frontend/aws/main.tf
@@ -12,6 +12,22 @@ terraform {
   }
 }
 
+# Origin Access Control for Lambda Function URL
+# Instructs CloudFront to SigV4-sign every forwarded request so the Lambda
+# Function URL (authorization_type = "AWS_IAM") only accepts traffic that
+# originates from this distribution. The signing happens at the CloudFront edge
+# -- the browser never sees an AWS credential.
+# Only created when enable_oac = true (Lambda + AWS_IAM deployments).
+resource "aws_cloudfront_origin_access_control" "lambda" {
+  count = var.enable_oac ? 1 : 0
+
+  name                              = "${var.project_name}-${var.environment}-lambda-oac"
+  description                       = "OAC for ${var.project_name} Lambda Function URL (${var.environment})"
+  origin_access_control_origin_type = "lambda"
+  signing_behavior                  = "always"
+  signing_protocol                  = "sigv4"
+}
+
 # CloudFront distribution with single compute origin
 resource "aws_cloudfront_distribution" "frontend" {
   enabled         = true
@@ -26,6 +42,11 @@ resource "aws_cloudfront_distribution" "frontend" {
     domain_name = var.origin_domain_name
     origin_id   = "compute"
 
+    # Attach OAC when Lambda Function URL uses authorization_type = "AWS_IAM".
+    # CloudFront will SigV4-sign every forwarded request. Null when enable_oac = false
+    # (Fargate ALB or Lambda with auth_type = "NONE").
+    origin_access_control_id = var.enable_oac ? aws_cloudfront_origin_access_control.lambda[0].id : null
+
     custom_origin_config {
       http_port              = 80
       https_port             = 443
diff --git a/terraform/modules/frontend/aws/variables.tf b/terraform/modules/frontend/aws/variables.tf
index 093f2df6..5b64391a 100644
--- a/terraform/modules/frontend/aws/variables.tf
+++ b/terraform/modules/frontend/aws/variables.tf
@@ -69,6 +69,20 @@ variable "alarm_sns_topic_arn" {
   default     = ""
 }
 
+variable "enable_oac" {
+  description = <<-EOT
+    When true, create a CloudFront Origin Access Control (OAC) of type "lambda"
+    and attach it to the compute origin in the distribution. Required when the
+    Lambda Function URL uses authorization_type = "AWS_IAM": CloudFront will
+    SigV4-sign every forwarded request so the Function URL only accepts traffic
+    from this distribution.
+    Set to false when the origin is a Fargate ALB or when the Lambda uses
+    authorization_type = "NONE".
+  EOT
+  type        = bool
+  default     = false
+}
+
 variable "tags" {
   description = "Additional tags for resources"
   type        = map(string)
diff --git a/terraform/profiles/aws/dev.tfvars.example b/terraform/profiles/aws/dev.tfvars.example
index 642ad8e5..3e90b835 100644
--- a/terraform/profiles/aws/dev.tfvars.example
+++ b/terraform/profiles/aws/dev.tfvars.example
@@ -18,8 +18,7 @@ lambda_memory_size            = 512
 lambda_timeout                = 60
 lambda_reserved_concurrency   = -1
 lambda_log_retention_days     = 7
-lambda_enable_function_url    = true
-lambda_function_url_auth_type = "NONE"
+# Function URL auth_type is derived from enable_cdn — see compute.tf local
 lambda_allowed_origins        = ["*"]
 
 # Docker build settings
diff --git a/terraform/profiles/aws/prod.tfvars.example b/terraform/profiles/aws/prod.tfvars.example
index 9df5cb66..a61adacf 100644
--- a/terraform/profiles/aws/prod.tfvars.example
+++ b/terraform/profiles/aws/prod.tfvars.example
@@ -18,8 +18,7 @@ lambda_memory_size            = 1024 # More memory for production
 lambda_timeout                = 60
 lambda_reserved_concurrency   = -1
 lambda_log_retention_days     = 30
-lambda_enable_function_url    = true
-lambda_function_url_auth_type = "NONE"
+# Function URL auth_type is derived from enable_cdn — see compute.tf local
 lambda_allowed_origins        = ["*"] # TODO: restrict to production domain
 
 # Docker build settings