AWS_ATTACH_USER_POLICY

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Summary


Forestall ACL Alias	AWS_ATTACH_USER_POLICY
Edge Type	Attack Path
Affected Object Types	IAM Users, IAM Roles
Exploitation Certainty	High
AWS IAM Action	`iam:AttachUserPolicy` on the target user

Description

iam:AttachUserPolicy lets a principal attach any managed policy to an IAM user. That includes AWS-managed ones like AdministratorAccess.

An attacker with this permission attaches an admin policy to a user they already control. That user becomes admin on the next call. If the target user is someone else, the attacker can escalate that account instead and use it later.

The edge is (Attacker:AWSUser|AWSRole) -[AWS_ATTACH_USER_POLICY]-> (TargetUser:AWSUser).

Identification

List who holds iam:AttachUserPolicy:

aws iam get-account-authorization-details \
  --filter User Role Group LocalManagedPolicy AWSManagedPolicy \
  --query "UserDetailList[?contains(AttachedManagedPolicies[*].PolicyName, 'AttachUserPolicy')]"

Check a specific principal with the policy simulator:

aws iam simulate-principal-policy \
  --policy-source-arn <attacker-user-arn> \
  --action-names iam:AttachUserPolicy \
  --resource-arns "arn:aws:iam::*:user/*"

PowerShell (AWS Tools):

Get-IAMAccountAuthorizationDetail -Filter User | ForEach-Object {
    $_.AttachedManagedPolicies | Where-Object { $_.PolicyName -match "AttachUserPolicy" }
}

Exploitation

Attach an admin policy to a user you control:

aws iam attach-user-policy \
  --user-name attacker-user \
  --policy-arn arn:aws:iam::aws:policy/AdministratorAccess

Confirm it landed:

aws iam list-attached-user-policies --user-name attacker-user

Now use that user’s credentials. It has full admin. If you can’t sign in as the user yet, create an access key or set a login profile on the same user first.

Mitigation

Keep iam:AttachUserPolicy inside a small admin group. It is a direct route to admin.
Scope the permission to specific user ARNs instead of *.
Put a permissions boundary on your users so an attached policy can’t exceed a ceiling.
Alert whenever AdministratorAccess or another high-privilege managed policy gets attached.

Detection

CloudTrail logs every attach call.

Event	Description	Key Fields
`AttachUserPolicy`	Managed policy attached to a user	`requestParameters.userName`, `requestParameters.policyArn`, `userIdentity.arn`

aws cloudtrail lookup-events \
  --lookup-attributes AttributeKey=EventName,AttributeValue=AttachUserPolicy \
  --query 'Events[*].{Time:EventTime,Actor:Username}'

Worth a look:

Any attach of AdministratorAccess to a user that isn’t an admin.
An attach where the actor and the target user are different identities.

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