Exploiting AWS IAM’s Eventual Consistency: A New Persistence Threat
In the realm of cloud security, a recent discovery has unveiled a critical persistence technique within Amazon Web Services’ (AWS) Identity and Access Management (IAM) system. This technique exploits IAM’s eventual consistency model, enabling attackers to maintain unauthorized access even after defenders have deleted compromised access keys.
Understanding Eventual Consistency in AWS IAM
AWS IAM, like many distributed systems, employs an eventual consistency model to ensure scalability across various regions and replicas. In this model, updates to resources such as access keys or policies do not propagate instantaneously but instead take a predictable delay—approximately 3 to 4 seconds, as confirmed by security firm OFFENSAI’s testing across regions like us-east-1 and eu-central-1.
During this brief window, deleted access keys remain valid for API calls. This allows attackers to exploit the delay by listing keys (which returns an empty array) or generating new ones before the invalidation process completes.
Exploitation Scenario
In a simulated attack scenario, a defender executes the command `aws iam delete-access-key –access-key-id AKIA… –user-name bob` to delete a compromised access key. However, an attacker, aware of the eventual consistency delay, rapidly follows with `aws iam create-access-key –user-name bob` to generate a new access key.
While AWS CloudTrail logs accurately record both the deletion and subsequent actions, the consistency lag permits the attacker to maintain persistence. This issue extends beyond access keys to other IAM resources, including policy attachments, role deletions, and login profiles, thereby amplifying risks during incident response efforts.
Challenges with Traditional Mitigation Strategies
Traditional incident response playbooks may prove ineffective against this persistence technique. For instance, attaching deny-all policies like `AWSDenyAll` during the delay window allows attackers to detect and detach them via polling `ListAccessKeys` or similar APIs.
AWS’s own Credential Cleanup Procedure, published on re:Post, advises waiting for full propagation periods. However, this approach may be inefficient against proactive attackers who can preempt policy enforcement during the delay window.
AWS’s Response and Recommendations
AWS has acknowledged the findings and applied development fixes and documentation updates without classifying the issue as a vulnerability. Post-disclosure testing revealed partial fixes; for example, a deleted key now blocks new key creation. However, gaps persist, as attackers can still detect changes and deploy assumable roles with `AdministratorAccess` from external accounts.
AWS recommends implementing security best practices and designing applications to account for these delays. Specifically, customers should avoid long-term IAM access keys due to their indefinite validity and potential for theft or accidental disclosure. Instead, AWS advises using temporary credentials generated via the AWS Security Token Service (STS) or leveraging IAM roles and federation for programmatic access to AWS services. These methods offer time-limited access that automatically expires, reducing the risk of unauthorized access.
Mitigation Strategies
To mitigate the risks associated with this persistence technique, organizations should consider the following strategies:
1. Implement Account-Level Service Control Policies (SCPs): Utilize AWS Organizations to enforce SCPs that deny all actions for compromised principals. Attackers typically lack control over SCPs, making this an effective measure.
2. Use Temporary Credentials: Favor IAM roles and STS temporary credentials over long-term access keys to minimize exposure.
3. Revise Incident Response Playbooks: Update playbooks to account for eventual consistency delays, ensuring that detection rules and mitigation strategies are effective against this persistence technique.
4. Monitor CloudTrail Logs: Regularly review CloudTrail logs to detect and respond to unauthorized actions promptly.
Conclusion
The exploitation of AWS IAM’s eventual consistency model underscores the need for organizations to adapt their security strategies to address emerging threats. By understanding the nuances of IAM’s consistency model and implementing robust mitigation measures, organizations can enhance their cloud security posture and protect against sophisticated persistence techniques.
