An AWS Account or an IAM user can request temporary security credentials and use them to access Amazon S3. The credentials consist of an Access Key ID, a Secret Access Key, and a Security Token.
Temporary security credentials are obtained from AWS Security Token Service (AWS STS) by sending the AssumeRole or GetSessionToken call.
With S3 Browser you can easily work with Amazon S3 via temporary security credentials, it calls AssumeRole or GetSessionToken automatically when required.
Once obtained, temporary security credentails are cached in memory and used until they expire, then the next STS call is sent.
To connect to Amazon S3 via AssumeRole
1. Start S3 Browser and click Accounts -> Add New Account.
Click Accounts -> Add New Account
The Add New Account dialog will open:
The Add New Account dialog.
2. Choose the Amazon S3 via AssumeRole account type
3. Specify the Role ARN - The Amazon Resource Name of the role to assume.
4. Select the Source Account - the account to send the AssumeRole request.
5. Turn on the Use secure transfer (SSL/TLS) checkbox if you would like to encrypt all communications with the storage.
6. Click Add new account
You can now choose the newly added account from the Accounts menu: 
Click Accounts -> Account Name to switch between accounts.
Advanced Settings
You may also configure additional settings for the Amazon S3 via AssumeRole account type.
To open Advanced Settings please click the Advanced settings.. link located at the bottom-left corner of the dialog.
Click the Advanced settings.. link
The Advanced Account Settings dialog will open:
Advanced account settings, the AssumeRole tab.
AssumeRole
1. External ID - an optional field, a unique identifier that might be required when you assume a role in another account. If the administrator of the account to which the role belongs provided you with an external ID, then specify that value here.
2. MFA Serial - an optional field, the identification number of the MFA device that is associated with the user who is making the AssumeRole call. Specify this value if the trust policy of the role being assumed includes a condition that requires MFA authentication.
3. Session duration in seconds - the duration in seconds, that temporary credentails should remain valid.
Advanced account settings, the Miscellaneous tab.
Miscellaneous
List all my buckets when account assigned - if checked, S3 Browser performs s3:ListAllMyBuckets call when account is assigned. You may uncheck this for accounts that are not allowed to list all buckets to avoid failed tasks and warnings in the log.
Check CloudFront distributions when account assigned - if checked, S3 Browser performs cloudfront:ListDistributions call when account is assigned to set special icon for buckets that are used as origin for CloudFront distributions. You may uncheck this for accounts that are not allowed to list CloudFront distributions to avoid failed tasks and warnings in the log.
External Buckets - you may edit external buckets associated with the account. Each bucket must be specified on a new line. Optional path is also supported, slash is used as delimiter, for example my-bucket/and/optional/path
