sec(iac/aws): pin cross-account sts:AssumeRole to declared account IDs - #1876
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The hub's Lambda and Fargate roles, and the shipped CloudFormation hub, granted sts:AssumeRole on arn:aws:iam::*:role/CUDly* -- any CUDly-prefixed role in any AWS account in the partition. The role-name prefix narrows WHICH role but never WHOSE account, and the customer-side template defaults the role name to literally "CUDly", so the pattern matched every linked account by construction and every un-linked one as well. The sts:ExternalId StringLike "*" condition on the two Terraform sites could not cover for that: it is a presence check, and the role ARN and external ID are read off the same CloudAccount record, so a mis-selected account supplies both consistently. IAM therefore authorised the whole namespace and the only thing standing between an account-selection bug and an irreversible purchase in the wrong account was the application layer. The CloudFormation copy carried no Condition at all. All three sites now carry a StringEquals aws:ResourceAccount condition holding the account IDs the operator declared, so an undeclared account is an AccessDenied rather than a successful purchase. The ExternalId presence check is retained where it existed and added to CloudFormation for parity. Empty means no reach, never all accounts: the environment derives enable_cross_account_sts from the list, CloudFormation drops the statement via Fn::If, and a module consumer that enables the grant with an empty list fails at plan time on a precondition rather than rendering an unconstrained policy. The permissions boundary needs no change -- a boundary caps by intersection, so narrowing an identity policy can never exceed the existing CrossAccountAssumeRoleCeiling. Guarded by terraform/modules/compute/aws/cross_account_sts_guard_test.go, which discovers policy sites by walking terraform/ and cloudformation/stacks/ rather than opening a list of three paths, and fails loudly if it inspects nothing or misses a known site. Proven against 17 mutations, each asserted on its specific failure message: dropping the condition at any of the three sites, weakening StringEquals to StringLike or ForAllValues:StringEquals, dropping the ExternalId check, hardcoding the account list, moving the condition into a decoy locals block, hiding a grant in an inline_policy block, in another module, or beside the permissions boundary, leaving the conditions behind as // comments, and emptying the walk itself. Two of the cases are inverse: a service-principal trust policy and an ARN glob containing /* must leave the guard green, because a guard that fires on correct code gets deleted rather than fixed. BREAKING: multi-account deployments must declare their accounts. Terraform: cross_account_target_account_ids. CloudFormation: CrossAccountTargetAccountIds. Without it the grant is not created and cross-account collection fails with AccessDenied. The three checked-in github-*.tfvars are set to [] explicitly so the removal is reviewed rather than silent. For bastion-mode accounts the BASTION's account ID is the one to list: only the first hop runs on the deployment's own identity. Closes #1636
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📝 WalkthroughWalkthroughThe change adds explicit cross-account target account configuration to Terraform and CloudFormation. It conditionally creates STS permissions, restricts them with ChangesCross-account STS authorization
Estimated code review effort: 4 (Complex) | ~60 minutes Merge Risk: 🔵 Low · up to This PR narrows cross-account role access to explicitly declared AWS account IDs and disables the grant when none are declared. If an environment has runtime-linked accounts that are not added to the checked-in configuration, the next deployment can stop cross-account collection with AccessDenied until those IDs are supplied; merge is reasonable with explicit owner confirmation of the environment configuration. Sequence Diagram(s)sequenceDiagram
participant DeploymentConfig
participant TerraformModules
participant CloudFormationStack
participant IAM
DeploymentConfig->>TerraformModules: provide target account IDs
TerraformModules->>IAM: create conditional AssumeRole policy
DeploymentConfig->>CloudFormationStack: provide CrossAccountTargetAccountIds
CloudFormationStack->>IAM: create conditional AssumeRole policy
IAM->>IAM: enforce aws:ResourceAccount and sts:ExternalId
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In `@terraform/modules/compute/aws/cross_account_sts_guard_test.go`:
- Around line 1-19: Split the oversized test file by moving grantRoots,
walkPolicyFiles, countIdentityGrants, isTrustGrant, lastMarkerIndex, and
stripCommentLines into a second file in the same package. Keep all test
functions and their behavior in the current file, preserving helper visibility
and existing test references.
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📒 Files selected for processing (13)
cloudformation/stacks/CUDly/template.yamldocs/DEPLOYMENT.mdterraform/environments/aws/compute.tfterraform/environments/aws/dev.tfvars.exampleterraform/environments/aws/github-dev.tfvarsterraform/environments/aws/github-prod.tfvarsterraform/environments/aws/github-staging.tfvarsterraform/environments/aws/variables.tfterraform/modules/compute/aws/cross_account_sts_guard_test.goterraform/modules/compute/aws/fargate/main.tfterraform/modules/compute/aws/fargate/variables.tfterraform/modules/compute/aws/lambda/main.tfterraform/modules/compute/aws/lambda/variables.tf
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CI's golangci-lint (misspell, locale US) failed on `recognise` in the guard test. Fixed, along with two `authorises` in the Terraform descriptions and the five em-dashes the changeset introduced. The guard test had grown to 521 lines against the project's 500-line rule, so the discovery and trust/identity classification move to cross_account_sts_discovery_test.go and the assertions stay put. Same package, no behaviour change: 262 and 268 lines. Mutation harness re-run and retargeted at the new file, since a mutation result stops being evidence once the tree underneath it moves. 18/18 still bite.
1 participant
Current state on
mainThe hub's execution role may
sts:AssumeRoleinto anyCUDly*role in any AWS account in thepartition. Three independently maintained copies of the grant, all live:
Resourceas it exists todayConditionterraform/modules/compute/aws/lambda/main.tf:424arn:aws:iam::*:role/${var.cross_account_role_name_prefix}*StringLike sts:ExternalId = "*"terraform/modules/compute/aws/fargate/main.tf:260cloudformation/stacks/CUDly/template.yaml:488arn:aws:iam::*:role/CUDly*cross_account_role_name_prefixis hardcoded to"CUDly"at both call sites(
terraform/environments/aws/compute.tf:104,:225), so the rendered resource on a real deploymentis literally
arn:aws:iam::*:role/CUDly*.What that admits. The role-name prefix narrows which role, never whose account.
TestAssumeRoleResourcePatternDoesNotRestrictTheAccountcompiles the pattern into the regex IAMevaluates it as and asserts that
arn:aws:iam::999999999999:role/CUDly— an account nobody onboardedand the operator does not control — matches. The customer-side template defaults
RoleNametoliterally
CUDly(cloudformation/stacks/CUDly-CrossAccount/template.yaml:40-46), so the patternmatches every linked account by construction and every un-linked one as well.
Why the existing ExternalId condition does not cover for this.
StringLike "*"is a presencecheck.
AWSRoleARNandAWSExternalIDare read off the sameCloudAccountrecord(
internal/credentials/resolver.go:190-195), so a mis-selected account supplies both consistently —a well-formed, IAM-satisfying request. It defends against an app-layer bug that omits the field; it
is structurally incapable of detecting one that picks the wrong record. So IAM authorised the whole
namespace and the only thing between an account-selection bug and an irreversible purchase in the
wrong account was the application layer.
The narrowed form
One mechanism at all three sites — a
StringEqualscondition onaws:ResourceAccountcarrying theaccount IDs the operator declared:
The ExternalId presence check is retained where it existed and added to CloudFormation, which had
no
Conditionblock at all.Chosen over rendering one
arn:aws:iam::<id>:role/CUDly*per account because plain CloudFormationcannot map over a
CommaDelimitedListinto aResourcelist. That would have forced a differentmechanism on the CFN site, and three copies of one grant already drifted into two different shapes.
A
CommaDelimitedListdrops straight into a multi-valuedStringEquals. It also sidesteps the IAMinline-policy size limit.
Empty means no reach, never all accounts — three layers, all fail-closed:
compute.tfderivesenable_cross_account_sts = length(var.cross_account_target_account_ids) > 0,so an undeclared deployment gets no policy resource rather than an unconstrained one.
Fn::If/AWS::NoValue.lifecycle.precondition.No boundary change and no bootstrap re-run: a permissions boundary caps by intersection, so
narrowing an identity policy can never exceed the existing
CrossAccountAssumeRoleCeiling.TestBoundaryMatchesCrossAccountRolePrefixstill passes.What a caller can still reach after this
Stated precisely, because "closed" would be too strong:
CUDly*role inside a declared account: still reachable, subject to that role's trustpolicy. The prefix is unchanged and intentional.
arn:aws:iam::*:role/CUDly*still matchesarn:aws:iam::<declared>:role/CUDly-x/EvilRole, because*does not stop at/. Not closed here,but subsumed — a path-bearing impostor must now live in an account the operator explicitly
declared. sec(iac/aws): role-prefix ARN scopes are satisfiable via the IAM path (arn:aws:iam::*:role/cudly-*) cloud-commitments-platform#197 targets a different policy and stays open on its own terms.
identity policy in the bastion account. Documented, not fixed.
from every AWS account to the declared set; it does not go to zero.
Guard
terraform/modules/compute/aws/cross_account_sts_guard_test.godiscovers policy sites by walkingterraform/andcloudformation/stacks/rather than opening a list of paths, and fails loudly if itinspects zero files or misses a known site. Proven against 17 mutations, each asserted on its
specific failure message, not on a non-zero exit.
Deliberate blind spot, so reviewers know the guard's reach rather than assuming it is total: an
identity policy expressed as
data "aws_iam_policy_document"is not detected. That marker was onthe identity list and had to come off — the same data source is the canonical way to write a trust
policy, and the repo already uses it that way at
terraform/modules/secrets/aws/main.tf:423. Tellingthe two apart needs a real HCL parse. The condition-proximity check is likewise an explicit
heuristic, not a parse. Both are documented in the file.
Two mutation cases are inverse cases — a service-principal trust policy and an ARN glob
containing
/*must leave the guard green, because a guard that fires on correct code gets deletedrather than fixed.
Verification
Executed: guard suite (16 pass),
ci-cd-permissionssuite (34 pass),go vet,go build ./...,terraform fmt -check -recursive,terraform validateon both modules and the environment, CFNtemplate parsed and the statement located in the policy document, full pre-commit hook set.
Variable validation proven both directions with real
terraform plan: rejects["*"],["12345678901"],["1234567890 12"]; accepts two real IDs and[]. The module precondition provento fire, against a harness with a local STS stub. The rendered policy extracted from a real plan file
shows the
StringEqualscarrying both declared accounts.No AWS credentials were used and no
applywas run. Theaws:ResourceAccountbehaviour isestablished by documentation and reasoning, not by observation:
(Audit Manager, several
Accept*Invitationactions, all EBS actions, six EC2 accept actions,EventBridge
PutEvents, two Route 53 actions, OpenSearchAcceptInboundConnection). STS is not onit.
any IAM roles outside of your own AWS account by configuring an IAM policy to deny access to AWS
STS assume role actions unless
aws:ResourceAccountmatches your unique AWS account ID".The
aws iam simulate-custom-policyassertion the issue asks for —implicitDenyforarn:aws:iam::999999999999:role/CUDly,allowedfor a declared account — has not been run andshould be, once, against a real account before this is considered settled.
Note that
scripts/check-aws-iam-parity.shreturning OK is vacuous here: it explicitly excludesthe
stsnamespace. It is not evidence for this change.Note on the issue's framing
The substance holds up. The cross-tenant framing does not: CUDly is self-hosted only
(
README.md:583-589), so the accounts a hub reaches are one operator's own estate, not unrelatedcustomers. The real exposure is (a) a purchase landing in the wrong account of your own organisation,
and (b) the genuinely unbounded half — the grant reaches a
CUDly*role in any AWS account on earth,including an attacker-controlled one whose role trusts the hub. (b) is the stronger argument and the
issue understates it.
BREAKING: multi-account deployments must declare their accounts.
Terraform: cross_account_target_account_ids. CloudFormation:
CrossAccountTargetAccountIds. Without it the grant is not created and
cross-account collection fails with AccessDenied. The three checked-in
github-*.tfvars are set to [] explicitly so the removal is reviewed rather than
silent. For bastion-mode accounts the BASTION's account ID is the one to list:
only the first hop runs on the deployment's own identity.
On those tfvars:
[]is the honest transcription of what Terraform declares today — none of thethree has ever listed a linked account. But accounts are onboarded at runtime through the API and
org discovery, and that state is not visible from the repo. If any environment has had a linked
account added through the app, list it here before merging — otherwise its first apply after this
lands stops collection with
AccessDenied. I did not guess account IDs.Closes #1636
Summary by CodeRabbit
Security Enhancements
Documentation
Tests