1. Account Separation and Mandatory
Access Control on AWS
Dave Walker
Specialised Solutions Architect, Security and Compliance
22/10/2015

2. Mandatory Access Control?
• Contrast with Discretionary Access Control
– u/g/o / rwx file permissions
– Under the control of the file owner
• MAC is a function of core system policy
– Immutable to all system users; sometimes also invisible to them
– …including root
• Epitomised in SELinux, descended from Orange Book B1
systems
– Sometimes extended to do multilevel / cross-domain security

3. Mandatory Access Control?
• SELinux on AWS
– RHEL, Ubuntu, SuSE, etc AMIs…
– (Don’t forget FreeBSD and other Community AMIs)
• First native MAC service on AWS: Glacier Vault Lock
– Set a Policy and fix it in place
– Even the account owner can’t change it, until its time lock expires
– Meets SEC “Books and Records” requirements (Rule 17a-4(f))
• Also FINRA Rule 4511, CFTC Regulation 1.31
• How can we make more services behave similarly?
– Cross-account access gets us close!

4. S3 Subtleties
• Versioning
• MFA Delete
– Put these together, and you get something which looks a lot like an
append-only object store
– …consider evidential integrity and weight
– Consider adding lifecycle policies to rotate into Vault Locked Glacier
• Good for long-term log retention

5. S3 Subtleties
• CloudTrail, Config, CloudWatch Logs, ELB logs, VPC Flow
Logs
– Make them write-only for production / resource accounts
• No means to read or list bucket contents
– Make them read-only for audit accounts
• Though audit user activities may need to be written to logs too
– Potentially to a different log location
• Create a separate Logging account and apply cross-account
sharing:

6. S3 Subtleties
• S3 write-only cross-account sharing
– Share write-only (no reading or listing of contents)
from owner account via bucket policy
– Writer accounts have IAM permissions to write

7. S3 Subtleties: Log Bucket Policy, Part 1
• (Actual policy won’t fit here, but…):
– Start with the cross-account bucket policy for writing CloudTrail logs, at
https://blogs.aws.amazon.com/security/post/Tx1QT0TX44KW7XM/Sha
ring-AWS-CloudTrail-Log-Files-Between-Accounts Scenario 1
– Add the Sid + Effect + Principal + Action + Resource aggregate objects
from the bucket policy for Config, at
http://docs.aws.amazon.com/config/latest/developerguide/s3-bucket-
policy.html , applying the same principles
– Add s3:GetBucketLocation permissions, to handle cross-Region logs
• (we want to log from all Regions to 1 bucket)
– Add the following for CloudWatch Logs:

8. S3 Subtleties: Log Bucket Policy, Part 2{
"Sid": "Cross-account write allow for CloudWatch Logs, mediated by control below",
"Effect": "Allow",
"Principal": ]
"AWS": "arn:aws:iam::Writer-Account-ID:root”,
<Add other accounts here>
],
"Action":[
"s3: PutObject",
"S3: GetBucketLocation"
],
"Resource":"arn: aws: s3:::myorg-logbucket/<optionalprefix>/AWSLogs/*"
},
{
"Sid":"Control to require full control grant on write",
"Effect":"Deny",
"Principal":[
"AWS":"arn: aws:iam::Writer-Account-ID:root”,
<Add other accounts here>
],
"Action": [
"s3:PutObject",
"s3:GetBucketLocation"
],
"Resource": "arn:aws:s3:::myorg-logbucket/<optional prefix>/AWSLogs/*",
"Condition": {
"StringNotEquals": {
"s3:"bucket-owner-full-control"
}
}}

9. S3 Subtleties: Log Bucket Policy, Part 3
• Audit users (in another account) will need read-only access to
your log bucket; see
https://blogs.aws.amazon.com/security/post/Tx1QT0TX44KW
7XM/Sharing-AWS-CloudTrail-Log-Files-Between-Accounts ,
again (Scenario 2)
• Good to do via a Role which has to be explicitly assumed;
again, see the URL

10. S3 Subtleties: Log Bucket Policy and IAM
• Point CloudTrail and Config in other accounts to our log
bucket for writing, when setting these accounts up
• IAM policy to add to each log-generating account to allow
cross-account writing:
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": ”Cross-account Write",
"Effect": "Allow",
"Action": [
"s3:PutObject”,
”s3:GetBucketLocation”
],
"Resource": [
"arn:aws:s3:::myorg-logbucket"
]
}
]}

11. Detailed Billing: Sample Records
ItemDescription
UsageStart
Date
UsageEnd
Date
UsageQuanti
ty
Currency
Code
CostBefor
eTax
Cred
its
TaxAm
ount
TaxTy
pe
TotalCo
st
$0.000 per GB - regional data transfer under the monthly
global free tier
01.04.14
00:00
30.04.14
23:590.00000675 USD 0.00 0.0
0.0000
00 None
0.00000
0
$0.05 per GB-month of provisioned storage - US West
(Oregon)
01.04.14
00:00
30.04.14
23:59
1.126.666.5
54USD 0.56 0.0
0.0000
00 None
0.56000
0
First 1,000,000 Amazon SNS API Requests per month are
free
01.04.14
00:00
30.04.14
23:5910.0 USD 0.00 0.0
0.0000
00 None
0.00000
0
First 1,000,000 Amazon SQS Requests per month are free
01.04.14
00:00
30.04.14
23:594153.0 USD 0.00 0.0
0.0000
00 None
0.00000
0
$0.00 per GB - EU (Ireland) data transfer from US West
(Northern California)
01.04.14
00:00
30.04.14
23:590.00003292 USD 0.00 0.0
0.0000
00 None
0.00000
0
$0.000 per GB - data transfer out under the monthly
global free tier
01.04.14
00:00
30.04.14
23:590.02311019 USD 0.00 0.0
0.0000
00 None
0.00000
0
First 1,000,000 Amazon SNS API Requests per month are
free
01.04.14
00:00
30.04.14
23:5988.0 USD 0.00 0.0
0.0000
00 None
0.00000
0
$0.000 per GB - data transfer out under the monthly
global free tier
01.04.14
00:00
30.04.14
23:593.3E-7 USD 0.00 0.0
0.0000
00 None
0.00000
0

12. Linked Accounts
• Consolidate daily Detailed Billing logs into one bucket, for all
accounts
• Now put it all together…

13. The Base Account Structure
AWS Account
Root Account • No Access Keys
• MFA Enabled
• Raise Alert on Login
IAM Master • No Access Keys
• MFA Enabled
• Raise Alert on Login
Define IAM Policies
Enable IAM Managers (User or Role)
• Have Passwd Policy
• Enforce Passwd
Rotation
• Have Acct Questions
set up
• Have Info eMail set up
IAM Manager • No Access Keys
• MFA Enabled
Create IAM Users/Groups/Roles
Use Pre-Defined Policies

14. The Larger Picture
BILLING
S3 Holder
CloudTrail
Config
CW Logs
S3 Holder
BILL
CloudTrail
IAMUser
IAM User
Assume
Role
IAM User
Assume
Role
IAM User
Assume
Role
Resources
IAM ROLE
IAM ROLE
IAM ROLE
Backup Data
Backup
S3 Holder
Audit
Display Rights
STS
{
"Version": "2012-10-17",
"Statement": [ {
"Sid": ”STS-Only",
"Effect": "Allow",
"Action": [ "sts:AssumeRole" ],
"Resource": [ "*" ] }
]
}

15. There’s One More Account to Consider…
• (…and it won’t fit on the diagram)
• Service Catalogue
– Also has cross-account capability
– Repository for CloudFormation templates, golden AMIs…
– …add latest database backups and other necessary datasets, and
you have an Intellectual Property Holding Account
• Something to copy cross-Region for DR
• See http://aws.amazon.com/servicecatalog/faqs/ for cross-account access
• (Haven’t actually done this bit in practice yet)

16. Raising Alerts
• Raise (through CloudTrail, watched by a Lambda function triggered on bucket
writes) an Alert (through, eg, SNS) if:
– Any account’s root user logs in
– Any IAM-Master account logs in
– Billing/CloudTrail accounts have another S3 Bucket created
– IAM-User generates any new AWS resource
– IAM-User generates any CloudTrail events other than assume-
role and console login
– IAM-User logs in to any Resource Accounts (besides IAM-
Manager)
– Resource-Account has IAM-Users assigned (besides IAM-Master/IAM-
Manager)

17. Logs→metrics→alerts→actions
AWS Config
CloudWatch /
CloudWatch Logs
CloudWatch
alarms
AWS CloudTrail
Amazon EC2 OS logs
Amazon VPC
Flow Logs
Amazon SNS
email notification
HTTP/S
notification
SMS notifications
Mobile push
notifications
API calls
from
most
services
Monitoring
data from
AWS services
Custom
metrics

18. Also:
• Federate from IAM-User to another IdP
• Filter on LDAP DN elements; ou=, dc=
• Result: no PII in IAM!
– See page 20 of https://d0.awsstatic.com/whitepapers/aws-whitepaper-
single-sign-on-integrating-aws-open-ldap-and-shibboleth.pdf
• Set account and contact details to an email alias and a PABX
hunt group, and arrange with your AM to have your billing
invoiced to your company accounts
• …no PII in your AWS account
– unless you explicitly need to process PII in your AWS environment

19. Other Resources
• Re:Invent 2015: “Wrangling Security Events in
the Cloud” (SEC308)
– Further ways to enable service configuration
immutability
• Essentially, “config-correcting Lambda functions”