Side Channel Timing Attack Analysis in Authentication Methods

TL;DR "use secure hashing methods that time buffer their return output rather than rolling-your-own method, let alone doing direct password/token string comparison"

Why did I do this?

What drove this notebook was Microsoft's discovery of vulnerabilities in NetGear's DGN-2200v1 series routers, including finding that the routers store plaintext credentials and use the C strcmp method to validate passwords. A client was trying to understand why strcmp could leak information about passwords, so I created this notebook. I'm calling this "a hash comparison appreciation exercise".

If you aren't familiar with the problem:

• strcmp tests each letter of the stored plaintext password from left to right compared to each letter of the inputted plaintext password and then fails the as soon as they are not the same.
• if you have a password like "apple" and I need to guess it, it's hard to do if I just throw the dictionary at the input - my guess must match "apple" or it fails and I haven't learned anything more than "both applied and banana are the wrong answer".
• but if I calculate how much time it takes for the input to reject each wrong attempt, then I now have some insight into which combinations take longer. Longer time to fail means "more right", and can influence subsequent attacks to get closer and closer to the right answer, and rule out wrong answers. That stored Timing value is the Side-Channel, which is why it's referred to as a "Side Channel Attack".
• this also means that I can use smaller attack space with an iterative approach, and I can rule out the bad answers faster.

The NetGear issue is an especially weird implementation. Stored credentials in plaintext on the system is never a good idea, but is compounded by using the String Comparison (strcmp) method to determine if the password is correct - the strcmp induces this flaw, but it's also a flaw to store the passwords in plaintext.

The gold standard for auth comparison is to:

• use a hashing method to manage the creds creation and validation.
• On account creation only store the hashed value (preferably with a salt), never store the plaintext string.
• On user validation hash the incoming password then compare the hashes, not decrypted plaintext strings, to determine if the credential is correct.

The issue, described by Microsoft's 365 Defender Research Team on June 30, 2021:
https://www.microsoft.com/security/blog/2021/06/30/microsoft-finds-new-netgear-firmware-vulnerabilities-that-could-lead-to-identity-theft-and-full-system-compromise/

Deriving saved router credentials via a cryptographic side-channel

At this stage, we already had complete control over the router, but we continued investigating how the authentication itself was implemented.

If a page had to be authenticated, HTTPd would require HTTP basic authentication. The username and password would be encoded as a base64 string (delimited by a colon), sent in the HTTP header, and finally verified against the saved username and password in the router’s memory. The router stores this information (along with the majority of its configuration) in NVRAM, that is, outside the filesystem that we had extracted.

However, when we examined the authentication itself, we discovered a side-channel attack that can let an attacker get the right credentials:

Note that the username and the password are compared using strcmp. The libc implementation of strcmp works by comparing character-by-character until a NUL terminator is observed or until a mismatch happens.

Demonstrating the side-channel Timing Attack analysis against authentication methods that use strcmp

Why authentication shouldn't use libc strcmp (or other non-time-compensated methods).

The basic idea is to show that, when you make lots of tries to a system, analyzing the amount of time each transaction takes to return can reveal meaningful information about a secret string used in a password or token.

We'll be using python code to demonstrate the action and analysis, but this can be performed with any language that uses similar methods.

The Breakdown

Our secret string is 'apple'.

A client presents the secret string to the server, who knows the secret string, in an operation that will fail if the secret is incorrect.

So:

Client -> 'apple' -> Server

Server:

1. receives the unencrypted credentials from the client containing the string.
2. gets the local, unencrypted copy of the secret
3. strcmp's (compares) each byte of the unencrypted raw string of the client credential to the local, unencrypted secret
   • compare each byte from l to r
   • when a byte fails, return False
   • if the byte matches move to next char
   • Return True if nothing fails or return False when a match fails.
4. Server -> True/False -> Client

The problem is that for every correct chunk of text on the LHS the "fail" takes longer to return. Also - longer strings also take longer to return.

So in comparing against "apple", "b" or "broken" or "zebra" returns a failure instantly, but "a", "aplomb", "app", "application", etc will each take slightly longer for each character matched. The times are tiny, but it's possible to model the jitter window such that these lengths don't get lost in the noise. This can drastically cut down the time of a brute force attack and enhances the chances of success. using sound hash functions which return in a constant time and not rolling-your-own reduces this risk.

Why?

A few reasons:

• if you store a hash, then both strings are always the same size.
• the mature methods also add time-based buffers into place so that long/short/alphabetically diverse strings all return in constant time. No predictable time variation = no attack.
• modern methods/tools for passwords also include strengtheners like salts. Not available for token comparison.

Epilogue

I don't think it's a waste of time for a developer to want to expand their technique on the company dime, but naive solutions that go to production in this arena are a big deal. To limit potential impact to your project or your employer/shareholders, you probably want to partner with someone who writes secure services as you approach mastery as a programmer. Or just use the well understood off the shelf solutions. Otherwise you risk turning your project or employer into a CERT advisory.

Let Tom Scott tell you more Tom Scott on Computerphile - How NOT to Store Passwords!

Further reading:

OWASP Password Storage Cheat Sheet

Tim McLean's fine "Beginners Guide To Constant Time Cryptography"

PassCamp's "What is Password Hashing: A Comprehensive Guide"