Researchers Detail AppSync Cross-Tenant Vulnerability in Amazon Web Services

Amazon Web Services (AWS) has resolved a cross-tenant
vulnerability in its platform that could be weaponized by an
attacker to gain unauthorized access to resources.

The issue relates to a confused deputy problem^[1], a type of privilege
escalation where a program that doesn’t have permission to perform
an action can coerce a more-privileged entity to perform the
action.

The shortcoming was reported by Datadog to AWS on September 1,
2022, following which a patch was shipped on September 6.

“This attack abuses the AppSync service to assume [identity and
access management] roles^[2]
in other AWS accounts, which allows an attacker to pivot into a
victim organization and access resources in those accounts,”
Datadog researcher Nick Frichette said^[3]
in a report published last week.

In a coordinated disclosure, Amazon said^[4]
that no customers were affected by the vulnerability and that no
customer action is required.

It described it as a “case-sensitivity parsing issue within AWS
AppSync, which could potentially be used to bypass the service’s
cross-account role usage validations and take action as the service
across customer accounts.”

AWS AppSync offers^[5]
developers GraphQL APIs to retrieve or modify data from multiple
data sources as well as automatically sync data between mobile and
web applications and the cloud.

The service can also be used to integrate with other AWS
services through specific roles designed to perform the necessary
API calls with the required IAM permissions.

While AWS does have safeguards in place to prevent AppSync from
assuming arbitrary roles by validating the role’s Amazon Resource
Name (ARN), the problem stems from the fact that the check could be
trivially bypassed by passing the “serviceRoleArn^[6]” parameter in a lower
case.

This behavior could then be exploited to provide the identifier
of a role in a different AWS account.

“This vulnerability in AWS AppSync allowed attackers to cross
account boundaries and execute AWS API calls in victim accounts via
IAM roles that trusted the AppSync service,” Frichette said.

“By using this method, attackers could breach organizations that
used AppSync and gain access to resources associated with those
roles.”