diff --git a/terraform/modules/compute/aws/lambda/main.tf b/terraform/modules/compute/aws/lambda/main.tf
index 727f666b..90516d3d 100644
--- a/terraform/modules/compute/aws/lambda/main.tf
+++ b/terraform/modules/compute/aws/lambda/main.tf
@@ -252,9 +252,14 @@ resource "aws_iam_role_policy" "secrets_access" {
         Action = [
           "secretsmanager:GetSecretValue"
         ]
+        # Every "${arn}*" entry is guarded: an empty ARN would otherwise render
+        # the bare "*", which compact() keeps, granting GetSecretValue on every
+        # secret in the account (#291). var.database_password_secret_arn is
+        # also validated non-empty; the guard keeps this fail-closed if that
+        # input is ever made optional.
         Resource = compact([
           var.database_password_secret_arn,
-          "${var.database_password_secret_arn}*",
+          var.database_password_secret_arn != "" ? "${var.database_password_secret_arn}*" : "",
           var.admin_password_secret_arn,
           var.admin_password_secret_arn != "" ? "${var.admin_password_secret_arn}*" : "",
           var.credential_encryption_key_secret_arn,
diff --git a/terraform/modules/compute/aws/lambda/tests/secrets_policy.tftest.hcl b/terraform/modules/compute/aws/lambda/tests/secrets_policy.tftest.hcl
new file mode 100644
index 00000000..62312c6b
--- /dev/null
+++ b/terraform/modules/compute/aws/lambda/tests/secrets_policy.tftest.hcl
@@ -0,0 +1,124 @@
+# Lambda secrets policy must fail closed on the database secret ARN (#291).
+# The GetSecretValue resource list adds "${arn}*" for each secret; the
+# database entry was unguarded, so an empty ARN rendered the bare "*" (which
+# compact() keeps) and granted read on every secret in the account. The input
+# is now validated, and the entry is guarded like its optional siblings.
+mock_provider "aws" {}
+
+variables {
+  stack_name                   = "cudly-test"
+  permissions_boundary_arn     = "arn:aws:iam::123456789012:policy/cudly-boundary"
+  environment                  = "dev"
+  region                       = "us-east-1"
+  image_uri                    = "123456789012.dkr.ecr.us-east-1.amazonaws.com/cudly:test"
+  database_host                = "db.example.com"
+  database_name                = "cudly"
+  database_username            = "cudly"
+  database_password_secret_arn = "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-db-AbCdEf"
+  admin_email                  = "admin@example.com"
+  allowed_origins              = ["https://app.example.com"]
+}
+
+run "db_secret_only_grants_that_secret" {
+  command = plan
+
+  assert {
+    condition = jsondecode(aws_iam_role_policy.secrets_access.policy).Statement[0].Resource == [
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-db-AbCdEf",
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-db-AbCdEf*",
+    ]
+    error_message = "With only the database secret set, GetSecretValue must cover exactly that ARN and its suffix pattern."
+  }
+
+  assert {
+    condition = alltrue([
+      for s in jsondecode(aws_iam_role_policy.secrets_access.policy).Statement :
+      !contains(flatten([s.Resource]), "*")
+    ])
+    error_message = "No secrets policy statement may grant Resource \"*\"."
+  }
+}
+
+run "optional_secrets_add_their_own_patterns_only" {
+  command = plan
+
+  variables {
+    admin_password_secret_arn            = "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-admin-AbCdEf"
+    credential_encryption_key_secret_arn = "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-credkey-AbCdEf"
+  }
+
+  assert {
+    condition = jsondecode(aws_iam_role_policy.secrets_access.policy).Statement[0].Resource == [
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-db-AbCdEf",
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-db-AbCdEf*",
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-admin-AbCdEf",
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-admin-AbCdEf*",
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-credkey-AbCdEf",
+      "arn:aws:secretsmanager:us-east-1:123456789012:secret:cudly-dev-credkey-AbCdEf*",
+    ]
+    error_message = "Each configured secret must contribute exactly its ARN and suffix pattern."
+  }
+}
+
+run "accepts_govcloud_partition" {
+  command = plan
+
+  variables {
+    database_password_secret_arn = "arn:aws-us-gov:secretsmanager:us-gov-west-1:123456789012:secret:cudly/db_pass-AbCdEf"
+  }
+
+  assert {
+    condition     = contains(jsondecode(aws_iam_role_policy.secrets_access.policy).Statement[0].Resource, "arn:aws-us-gov:secretsmanager:us-gov-west-1:123456789012:secret:cudly/db_pass-AbCdEf*")
+    error_message = "A GovCloud secret ARN must be accepted and granted."
+  }
+}
+
+run "rejects_empty_db_secret_arn" {
+  command = plan
+
+  variables {
+    database_password_secret_arn = ""
+  }
+
+  expect_failures = [var.database_password_secret_arn]
+}
+
+run "rejects_bare_wildcard" {
+  command = plan
+
+  variables {
+    database_password_secret_arn = "*"
+  }
+
+  expect_failures = [var.database_password_secret_arn]
+}
+
+run "rejects_wildcard_secret_name" {
+  command = plan
+
+  variables {
+    database_password_secret_arn = "arn:aws:secretsmanager:us-east-1:123456789012:secret:*"
+  }
+
+  expect_failures = [var.database_password_secret_arn]
+}
+
+run "rejects_wildcard_account" {
+  command = plan
+
+  variables {
+    database_password_secret_arn = "arn:aws:secretsmanager:us-east-1:*:secret:cudly-dev-db"
+  }
+
+  expect_failures = [var.database_password_secret_arn]
+}
+
+run "rejects_non_secretsmanager_arn" {
+  command = plan
+
+  variables {
+    database_password_secret_arn = "arn:aws:ssm:us-east-1:123456789012:parameter/cudly-dev-db"
+  }
+
+  expect_failures = [var.database_password_secret_arn]
+}
diff --git a/terraform/modules/compute/aws/lambda/variables.tf b/terraform/modules/compute/aws/lambda/variables.tf
index 8a8cf1ed..791e1a36 100644
--- a/terraform/modules/compute/aws/lambda/variables.tf
+++ b/terraform/modules/compute/aws/lambda/variables.tf
@@ -85,6 +85,15 @@ variable "database_username" {
 variable "database_password_secret_arn" {
   description = "ARN of Secrets Manager secret containing database password"
   type        = string
+
+  # The secrets policy grants GetSecretValue on this ARN and "${arn}*". An
+  # empty value would make that the bare "*" (every secret in the account),
+  # and a wildcard anywhere in it widens the grant the same way, so require a
+  # concrete Secrets Manager secret ARN (#291).
+  validation {
+    condition     = can(regex("^arn:aws(-[a-z]+)*:secretsmanager:[a-z0-9-]+:[0-9]{12}:secret:[A-Za-z0-9/_+=.@-]+$", var.database_password_secret_arn))
+    error_message = "database_password_secret_arn must be a concrete Secrets Manager secret ARN (arn:<partition>:secretsmanager:<region>:<account-id>:secret:<name>) with no wildcards. An empty value would render the secrets policy resource as \"*\" and grant GetSecretValue on every secret in the account."
+  }
 }
 
 variable "admin_email" {