Dark Reading is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them.Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Cloud

10/25/2018
10:31 AM
Ory Segal
Ory Segal
Commentary
Connect Directly
Twitter
LinkedIn
RSS
E-Mail vvv
50%
50%

Securing Serverless: Attacking an AWS Account via a Lambda Function

It's not every day that someone lets you freely wreak havoc on their account just to find out what happens when you do.

Part two of a two-part series. Click to read Caleb Sima’s Securing Severless: Defend or Attack?

On August 3rd 2018, I got a cryptic LinkedIn message from Caleb Sima, with only the following text:

I have to admit, at first, I didn't quite understand what Caleb was trying to tell me, however a quick glimpse at the URL, which contained the words Lambda and Shell, seemed as if he was trying to show me that he deployed a Lambda function which allows users to execute shell commands. I've seen several such projects in the past, such as Lambdash, which is why I failed to be overly impressed or engaged. A few seconds later, I decided to take a peek at the Web page, and noticed the call for a challenge:

Seeing this, I first had to figure out Caleb's motives, so I fired him another message, trying to get more understanding as to why he decided to build this:

So… Caleb decided to let people attack his AWS account through a Lambda function that enables you to run shell commands. Sounds like a worthy challenge. After all, it's not every day that someone lets you wreak havoc on their account and run attacks freely. Now I was excited!

Step 1: Gathering Reconnaissance
I started by extracting the filename of the function’s handler by running 'ls -lF' on the current directory (/var/task):

With the filename at hand (index.js), I went on to retrieve the source code of the function by running 'cat index.js' (output truncated):

At the time, the original source code was quite unimpressive to say the least. It was the classic 'aws-lambda-function-that-executes-shell-commands' function, which as mentioned earlier, I’ve seen plenty such projects in the past.

"Ok then..." I thought to myself, "I can run shell commands, what's next? How do we go from here, to inflicting some real damage for the account?"

My next attempt to gather more information was to list all the environment variables, and see if Caleb left something in there that might be useful, and so I ran the ‘env’ command (output truncated)

Step 2: Impersonating the Lambda Function
The next morning, I got to work, and decided to take another peak at the environment variables. Suddenly something dawned on me - when an AWS Lambda function executes, it uses the temporary security credentials received by assuming the IAM role the developer granted to that function. When assuming a role, the assuming entity receives 3 parameters from AWS STS (security token service):

  • AWS_SECRET_ACCESS_KEY
  • AWS_ACCESS_KEY_ID
  • AWS_SESSION_TOKEN

These 3 extremely sensitive tokens were just printed on my browser screen as a result of listing the environment variables - "how convenient...," I thought to myself.

For those of you who aren't familiar with the AWS IAM security model, this is an extremely granular and powerful security permissions model. Here's an excerpt from the AWS documentation on IAM roles:

An IAM role is similar to a user, in that it is an AWS identity with permission policies that determine what the identity can and cannot do in AWS. However, instead of being uniquely associated with one person, a role is intended to be assumable by anyone who needs it. Also, a role does not have standard long-term credentials (password or access keys) associated with it. Instead, if a user assumes a role, temporary security credentials are created dynamically and provided to the user. You can use roles to delegate access to users, applications, or services that don't normally have access to your AWS resources.

Now, given that I have the tokens generated to the function by AWS STS, I am no longer forced to work with the somewhat annoying and limiting www.lambdashell.com Web interface. I can simply use the tokens to invoke AWS CLI commands from my local machine. In order to do that, I set these environment variables locally by calling:

/> export AWS_SECRET_ACCESS_KEY = …..
/> export AWS_ACCESS_KEY_ID = ….
/> export AWS_SESSION_TOKEN = ....

To test the tokens, I decided to invoke the AWS STS command line utility with the option to get the current caller identity:

/> aws sts get-caller-identity

It took a second, and then the CLI tool sent me back the following:

{

  "UserId": "AROA********GL4SXW:exec",
  "Account": "1232*****446",
  "Arn": "arn:aws:sts::1232*****446:assumed-role/lambda_basic_execution/exec"

}

Nice! I can now run the AWS CLI utilities locally, and essentially impersonate the IAM role that Caleb's Lambda function is running with. However, my newly discovered satisfaction didn't last long when I saw that the function is running with what seems to be the most basic and limiting IAM role - 'lambda_basic_execution.' Darn.

The reason for my sudden mood change was the fact that the role with which the function was running, was probably limited to the standard boilerplate AWS Lambda permissions. When you create a new function from scratch, you usually start with an IAM role that only enables the function to create new AWS CloudWatch log groups and streams, and to write log lines into those streams.

When looking at the AWS Lambda Web console, it would look like this:

My hopes were dashed. Caleb did not leave any application secrets stored insecurely as environment variables - a rather sad but common mistake that many developers do. (See the Serverless Security Top 10 guide.)

"What's next?" I asked myself. I decided to go back to my actual day job, which was piling up, and leave Caleb’s challenge for a while, until I either get a brilliant idea, or some more free time.

(Column continues on next page.)

Ory Segal is a world-renowned expert in application security, with 20 years of experience in the field. Ory is the CTO and co-founder of PureSec, a start-up that enables organizations to secure serverless applications. Prior to PureSec, Ory was senior director of threat ... View Full Bio
Previous
1 of 3
Next
Comment  | 
Print  | 
More Insights
Comments
Threaded  |  Newest First  |  Oldest First
CleverM629
50%
50%
CleverM629,
User Rank: Apprentice
10/25/2018 | 12:23:58 PM
so cool
thank you for sharing
amitaymolko
50%
50%
amitaymolko,
User Rank: Apprentice
11/9/2018 | 2:19:34 AM
Bruteforce permissions script
Did you end up writing that bruteforce permissions script?
Did you find any other vulnerable services?
Data Leak Week: Billions of Sensitive Files Exposed Online
Kelly Jackson Higgins, Executive Editor at Dark Reading,  12/10/2019
Lessons from the NSA: Know Your Assets
Robert Lemos, Contributing Writer,  12/12/2019
4 Tips to Run Fast in the Face of Digital Transformation
Shane Buckley, President & Chief Operating Officer, Gigamon,  12/9/2019
Register for Dark Reading Newsletters
White Papers
Video
Cartoon
Current Issue
The Year in Security: 2019
This Tech Digest provides a wrap up and overview of the year's top cybersecurity news stories. It was a year of new twists on old threats, with fears of another WannaCry-type worm and of a possible botnet army of Wi-Fi routers. But 2019 also underscored the risk of firmware and trusted security tools harboring dangerous holes that cybercriminals and nation-state hackers could readily abuse. Read more.
Flash Poll
Rethinking Enterprise Data Defense
Rethinking Enterprise Data Defense
Frustrated with recurring intrusions and breaches, cybersecurity professionals are questioning some of the industrys conventional wisdom. Heres a look at what theyre thinking about.
Twitter Feed
Dark Reading - Bug Report
Bug Report
Enterprise Vulnerabilities
From DHS/US-CERT's National Vulnerability Database
CVE-2019-19807
PUBLISHED: 2019-12-15
In the Linux kernel before 5.3.11, sound/core/timer.c has a use-after-free caused by erroneous code refactoring, aka CID-e7af6307a8a5. This is related to snd_timer_open and snd_timer_close_locked. The timeri variable was originally intended to be for a newly created timer instance, but was used for ...
CVE-2014-8650
PUBLISHED: 2019-12-15
python-requests-Kerberos through 0.5 does not handle mutual authentication
CVE-2014-3536
PUBLISHED: 2019-12-15
CFME (CloudForms Management Engine) 5: RHN account information is logged to top_output.log during registration
CVE-2014-3643
PUBLISHED: 2019-12-15
jersey: XXE via parameter entities not disabled by the jersey SAX parser
CVE-2014-3652
PUBLISHED: 2019-12-15
JBoss KeyCloak: Open redirect vulnerability via failure to validate the redirect URL.