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Be very careful with assigning IAM roles to EC2 instances. Many web applications have some kind of implicit proxying, e.g. a function to download an image from a user-defined URL. You might have remembered to block 127.0.0.*, but did you remember 169.254.169.254? Are you aware why 169.254.169.254 is relevant to IAM roles? Did you consider hostnames pointed to to 169.254.169.254? Did you consider that your HTTP client might do a separate DNS look-up? etc.

There are other subtleties which make roles hard to work with. The same policies can have different effects for roles and users (e.g., permission to copy from other buckets).

IAM Roles can be useful, especially for bootstrapping (e.g. retrieving an encrypted key store at start-up), but only use them if you know what you're doing.

Conversely, tips like disabling SSH have negligible security benefit if you're using the default EC2 setup (private key-based login). It's really quite useful to see what's going on in an individual server when you're developing a service.

Also, it does matter whether you put a CDN in front of S3. Even when requesting a file from EC2, CloudFront is typically an order of magnitude faster than S3. Even when using the website endpoint, S3 is not designed for web sites and will serve 500s relatively frequently, and does not scale instantly.




Great point with regards to IAM roles. The applications I've worked on don't download things from user-defined URLs, so this never even occurred to me.

Is the purpose of blocking 169.254.169.254 important because it could potentially give users access to the instance metadata service for your instance? I'd be interested to hear more information on securing EC2 with regards IAM roles, you seem to have lots of experience in that area.

The disabling SSH tip wasn't really about security (I agree that it has negligible security benefit), it's more about quickly highlighting parts of your infrastructure that aren't automated. It's often tempting to just quickly SSH in and fix this one little thing, and disabling it will force you to automate the fix instead.

The CDN info has been mentioned elsewhere too, lots of things I didn't know. I'll be updated the article soon to add all of the points that have been made. Thanks for the tips!


The way IAM roles work is terrifyingly simple. IAM generates temporary access key identifier and secret access key with the configured permissions and EC2 makes them available to your instance via the instance metadata as JSON at http://169.254.169.254/latest/meta-data/iam/security-credent... . The AWS SDK periodically retrieves and parses the JSON to get the new credentials. That's it. I'm not entirely sure whether the credentials can be used from a different IP or not, but given a proxying function that does not really matter.

I make sure all HTTP requests in my (Java) application go through a DNS resolver that throws an exception if: ip.isLoopbackAddress() || ip.isMulticastAddress() || ip.isAnyLocalAddress() || ip.isLinkLocalAddress()

The last clause captures 169.254.169.254. Of course, many libraries use their own HTTP client, so it's easy to make a mistake.

I'm trying to bring my usage of IAM roles down to 0 as a matter of policy. Currently, I'm only using an IAM role to retrieve an encrypted Java key store from S3 (key provided via CloudFormation) and encrypted AWS credentials for other functions (keys contained in the key store). I'd be happier to bootstrap using CloudFormation with credentials that are removed from the instance after start-up.

Thanks for making updates. There are definitely some great tips in there.


I agree. Not to toot own horn too much, but I use SSH for everything, including backups, automated jobs, live log monitoring, etc. disclaimer: founder Userify




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