Securing AWS Temporary Tokens

In this blog, we are going to discuss an attack vector that utilizes the STS AssumeRole and GetSessionToken API calls, and focus on what you have to do differently to detect, mitigate, and prevent abuse vs handling permanent access keys.

Imagine This

An attacker has just compromised one of your AWS credentials and gained access to your production AWS account. What do you do next? Revoke the credentials, of course. But just revoking the compromised credentials is not enough to keep the attacker out of your environment. In this blog post, we’ll explore temporary tokens and how they can provide an attacker continued access to your AWS environments, even after you have revoked compromised credentials.

Temporary tokens are provided by AWS Secure Token Service (STS) and are similar to permanent access keys in functionality and have been used to implement several common AWS features such as:

• Assuming roles, including the passing of roles to services
• Federated identities (e.g., single sign-on and cross-account access) 
• Authentication of IoT devices

As with all security design, there are tradeoffs and security concerns from the use of temporary tokens:

• Temporary tokens  can cause confusion during incident handling/response versus permanent access keys because their creation and use is logged in CloudTrail using different, somewhat confusing fields and values. As an example, there is no explicit json attribute that distinguishes a temporary token from a permanent API access key–you must infer from the access key id naming convention or other fields.
• The remediation options for Temporary Tokens can have with high impact; e.g., deletion of users
• Temporary tokens are harder to lock down since the creation of some types of temporary tokens cannot be restricted by policy.
• There is no tracking of which tokens have already been created, so auditing temporary token usage and identifying temporary tokens is difficult, as they must be derived from parsing CloudTrail logs. Further, there are no direct management functions (such as deletion) for temporary tokens. We’ll discuss in detail what mitigation steps apply later in the blog. This also makes it more difficult to assess exposure from credential risk because there is no easy way to list all temporary credentials that have been issued or are active/outstanding.

Temporary Tokens

Temporary tokens are implicitly created by AWS in the case of IoT device authentication or from assuming of roles by services. Temporary tokens can also be explicitly created by authenticated users calling STS AssumeRole or GetSessionToken, e.g.,

In addition to the access key id and secret, a temporary credential includes an expiration date and a session token, which must be included in any API calls (along with the access key and secret). 

Temporary tokens essentially have the same functionality and similar security exposure as permanent access keys, but with a few differences:

• Expiration: Temporary tokens have an expiration, ranging from 15 minutes to 36 hours. This is good from a security viewpoint, as it reduces the time window for abuse in case of lost or stolen temporary tokens.
• API Access: Temporary tokens can call any service that the original credential (that created the temporary token) has privileges for, except that:
• within STS, temporary tokens can only call the AssumeRole API call.
• within IAM, MFA is required.

These restrictions may not necessarily constrain attackers, since there are a large number of services that support AssumeRole [1], which combined with numerous techniques for escalating privileges [2], creates a large attack surface for lateral movement from temporary tokens to other privileged roles.

Attack Scenario

To highlight the challenges when temporary tokens are used in an attack, let’s look at a simple attack scenario. The environment has support personnel who must manage production S3 buckets, and the organization has created a specific role, MyBucketRole, that is assumed by support personnel whenever the need arises for S3 bucket support.

The Attacker:

1. Gains initial access to the environment with stolen credentials (access key A) which was accidentally hard-coded into a script that was uploaded to Git by a support person.
2. Generates a temporary token B using the stolen credentials (access key A) with a call to STS GetSessionToken, for persistent access i.e., backdoor access. This is not used immediately but is saved for redundant access and will be used in a secondary attack, and we will see shortly why it has different challenges than permanent keys and other temporary tokens generated by AssumeRole.
3. Uses access key A to escalate privileges by assuming a role, MyBucketRole, that has access to an S3 Bucket. This returns temporary token C, which when used will have the permissions of MyBucketRole.
4. Accesses sensitive data on the S3 Bucket using temporary token C.
5. Exfiltrates data from the S3 Bucket (e.g. S3 sync to another S3 bucket)

The initial compromise and attack vector in this scenario is the compromised credential. The use of temporary tokens does not change this risk profile but it will make the detection, mitigation, and prevention steps different or more challenging, which we cover in the next section.

Defender Viewpoint

Let’s look at this attack from the defender’s viewpoint to understand any challenges that stem from the use of temporary tokens by the attacker. We’re in the middle of the attack, we’re alerted to suspicious data access patterns during the exfiltration phase, and we’ve found out that a support user called AssumeRole and is performing data exfiltration.

Detection and Remediation

1 – Delete Compromised Key A

We immediately delete or make inactive the compromised access key A, regenerate new keys, ensure key rotation is enabled:

Additionally, we can discuss with the support user how they can better secure their API keys and send him/her to security training if necessary.

This should at least kick out our attacker, right? Not quite.

2 – Alerts on Continued Access with Temporary Token C

We find that the data exfiltration continues because the temporary token C created by AssumeRole still exists and is valid for up to 36 hours.

Any access key ids will start with “ASIA”, which is the naming convention that distinguishes temporary tokens from permanent tokens (”AKIA”). [3] There is no explicit attribute to distinguish a temporary token vs access key. There is also nothing in this event that ties the temporary token back to the principal that created it — that information is contained in a different event that captures the creation of the temporary token.

3 – Mitigate by Revoking Active Sessions for Temporary Token C

Although there is no way to list, directly delete, or deactivate temporary tokens, there is a workaround. We can revoke active sessions associated with a Role i.e., sessions started by AssumeRole. The policy can be set in the Revoke sessions tab for the role assumed by the attacker (IAM service):

It implements a policy condition that denies any API call if the token was created earlier than the time you put the policy in place [6].

We should be done now, right? Not yet.

4 – Alerts on Continued Access with New Temporary Tokens

We continue to see data exfiltration using an AssumedRole temporary token but that is different from temporary token C:

Where are these coming from?

5 – Discover Temporary Token B

The problem is temporary token B, which is repeatedly calling AssumeRole to generate new tokens:

These new temporary tokens have creation timestamps that are newer than what was set in the Revoke active sessions policy in step #3, so they are not denied.

We could do the Revoke active sessions step again (step #3) which will update the timestamp to the current time, but that doesn’t stop the attacker, who can keep calling AssumeRole. 

Should we delete temporary token B, as in step #1? Unfortunately, we can’t because it’s not a permanent key, and there are no API calls or Console actions to list or delete temporary tokens.

Should we try to contain it using Revoke active sessions, which worked in step #3? Unfortunately, Revoke active sessions works only for Roles i.e. it’s only available to control sessions from tokens created with AssumeRole. It doesn’t apply to or work for temporary tokens created by GetSessonToken. 

What now?

6 – Delete User or Change Permissions

The recommended ways to remediate temporary tokens created by GetSessonToken, are to:

1. Change/restrict the privileges (policies) for the user or role that created the token since the temporary token creates a session with the current privileges of the user/role at the time of use (not creation) [4].
2. Delete the user [5], which will immediately delete all temporary tokens associated with that user. 

The user in question is the IAM user who created the temporary token i.e. the support user. So, in order to be safe, we delete th