My attempt to explain a fun attack I recently learned about - no claim to anything new here! Video of presentation at https://www.youtube.com/watch?v=3R_q5XD-RJU. Thanks go to Soroush @irsdl.

1. Hash Length Extension Attacks
Jerome Smith
CamSec April 2017

2. Introduction
• An application uses this scheme as an integrity check:
hash($secret + $message)
// where + denotes concatenation
• Whenever the message is potentially subject to interference, the hash is sent
alongside
• The theory goes that any message tampering will be detected
• But this is potentially vulnerable to a “hash length extension attack”
• Secret prefix
• Attacker knows message and hash
• Vulnerable algorithm, e.g. MD5, SHA-1, SHA-256, SHA-512 (not SHA-384)
• Thanks to Soroush Dalili @irsdl

3. What makes a hash vulnerable?
• The hash algorithm chews on the input
• At the end, the internal state of the hash algorithm is the hash it spits out
• We can set the internal state of the hash algorithm to start from this point
• We can then feed in more input
• We now have a hash for a longer message that starts with the original input
• It doesn’t matter if the input began with a secret, we don’t need to know it

4. Exploitation
• Sounds great, doesn’t it?
POST /transfer HTTP/1.1
account_from=10203040&account_to=90807060&amount=100&hash=AABBCC112233
// where hash = MD5("SECERET1020304090807060100")
• So we set our MD5 state to AABBCC112233, feed in a 0 and get a new hash
• This will “validate”:
account_from=10203040&account_to=90807060&amount=1000&hash=DDEEFF445566
// where hash = MD5("SECERET10203040908070601000")
• Unfortunately it’s not that simple

5. Deep dive
• MD5 is a block-based algorithm
• Padding is used to prepare the input before it’s digested
• Different algorithms use different schemes
• Take a message M
• The input that MD5 works on is:
M + PADDING + LENGTH_OF_M
• This will be some whole number of blocks in length
• What does that mean for our attack?

6. What’s really going on
account_from=10203040&account_to=90807060&amount=1000&hash=DDEEFF445566
• We’d like hash = MD5("SECERET10203040908070601000")
• When we run our hash length extension attack, the input to the hash is really:
"SECERET1020304090807060100" + PADDING + LENGTH + "0" + PADDING + LENGTH
• The “message” we have a valid hash for is:
"SECERET1020304090807060100" + PADDING + LENGTH + "0"
• The app is checking hash($secret + $account_from + $account_to + $amount)
• We need to preserve account_from=10203040&account_to=90807060
• So that leaves amount="100" + PADDING + LENGTH + "0"
• The first PADDING + LENGTH was originally “metadata”: it’s now part of the data
• The crafted input isn’t tolerated in context

7. Demo
• That’s not to say it can never work
seller_id=1234&reference=widget&amount=145.20&hash=75b145717ad82cfefdcd74
0683e182f0
// where hash = MD5($secret + $seller_id + $reference + $amount)
= MD5($secret + "1234widget145.20")
= MD5($secret + "1234widget145.20" + PADDING + LENGTH)
• So what about
seller_id=1234&reference=widget145.20PADDINGLENGTH&amount=0.99&hash=398e6
d69a7fdf27744bd55cfdfc9fdb4
= MD5($secret + "1234widget145.20" + PADDING + LENGTH + "0.99")
• This will work if the app accepts the weird reference value
• https://github.com/iagox86/hash_extender
./hash_extender --data 1234widget145.20 --secret-min 8 --secret-max 12 --
append 0.99 --signature 75b145717ad82cfefdcd740683e182f0 --format md5

8. Final Thoughts
• Not always exploitable – but when it is, impact can be high
• Tricky to find in a pure black box test
• If the hash scheme used a delimiter, the attack would still work
• Just makes it harder to find – need to know delimiter as well
• But it would stop a simpler attack:
seller_id=1234&reference=widget&amount=145.20&hash=75b145717ad82cfefdcd
740683e182f0
seller_id=1234&reference=widget1&amount=45.20&hash=75b145717ad82cfefdcd
740683e182f0
• Secret suffix is vulnerable due to collisions
• https://rdist.root.org/2009/10/29/stop-using-unsafe-keyed-hashes-use-hmac/
• We’ve already solved the MAC problem
• “Length Extension Attacks” Burp App – not tested

9. 9
Questions?