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For more information about IAM access:
AWS - IAM, Identity Center & SSO EnumIf you allow an external account (A) to access a role in your account, you will probably have 0 visibility on who can exactly access that external account. This is a problem, because if another external account (B) can access the external account (A) it's possible that B will also be able to access your account.
Therefore, when allowing an external account to access a role in your account it's possible to specify an ExternalId. This is a "secret" string that the external account (A) need to specify in order to assume the role in your organization. As the external account B won't know this string, even if he has access over A he won't be able to access your role.
However, note that this ExternalId "secret" is not a secret, anyone that can read the IAM assume role policy will be able to see it. But as long as the external account A knows it, but the external account B doesn't know it, it prevents B abusing A to access your role.
Example:
For an attacker to exploit a confused deputy he will need to find somehow if principals of the current account can impersonate roles in other accounts.
This policy allows all AWS to assume the role.
This policy allows any account to configure their apigateway to call this Lambda.
If an S3 bucket is given as a principal, because S3 buckets do not have an Account ID, if you deleted your bucket and the attacker created it in their own account, then they could abuse this.
A common way to avoid Confused Deputy problems is the use of a condition with AWS:SourceArn to check the origin ARN. However, some services might not support that (like CloudTrail according to some sources).
Learn & practice AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE) Learn & practice GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)
