More Trick For Defeating SSL

DEFCON 17Moxie Marlinspike

1. Introduction 2. Background Knowledge

SSL/TLS protocol 3. sslstrip 4. sslsniff

A. Basic Constraints vulnerability B. Null-Prefix Attack C. bypassing OCSP

5. Conclusion

Outline

Demonstrate some new tricks for defeating SSL/TLS in places where sslstrip does not reach.

Introduction

Background KnowledgeSSL/TLS Protocol

abbreviation for Transport Layer Security and it’s successor Secure Socket Layer

Provide communication security over the Internet.

Even when the network is being MITM attack.

SSL/TLS Introduction

Network Stack

Handshake Process

Handshake Process

SSLstrip

demonstration of the HTTPS stripping attacks It will transparently hijack HTTP traffic on a

network, watch for HTTPS links and redirects, then map those links into either look-alike HTTP links or homograph-similar HTTPS links

SSLstrip Introduction[1]

Bridge www.facebook.com bridge https://www.facebook.com?

302 redirect Hyper link

How it work

302 Redirect

Detail – Normal Scenario

User type:example.com

http://example.com

https://abc.example.com

Server reply302 redirect tohttps://abc.example.com

SSL/TLS handshake

Serve reply200 ok

User BrowserServer

Detail – Normal Scenario

Detail – Attack Scenario

User/browser Attacker Server

http://example.com http://example.com

Server reply302 redirect tohttps://abc.example.com

Strip https to http

http://abc.example.com

Server reply302 redirect tohttp://abc.example.com

http://abc.example.com

Record url

url match https://abc.example.com

SSL/TLS handshake

Application DataStripped Application Data

Strip https to http

Result(without strip)

Result(with strip)

the browser query https://abc.example.com directly. Bookmark User typing

Other protocol smtps Ftps Sftp….

What can’t sslstirp do

SSLsniff -Basic Constraints vulnerability

Certificate Chaining

Certificate Chaining

Verify that the name of the leaf node is the same as the site you're connecting to.

Verify that the leaf certificate has not expired.

Check the signature If the signing CA is in our list of trusted root

CAs, stop. Otherwise, move one up the chain and repeat.

How we verify

Verify that the name of the leaf node is the same as the site you're connecting to.

Verify that the leaf certificate has not expired.

Check the signature If the signing CA is in our list of trusted root

CAs, stop. Otherwise, move one up the chain and repeat.

What they say

All the signatures are valid. Nothing has expired. The chain is in tact. The root CA is embedded in the browser and

trusted.

Something must be wrong, but...

But we just created a valid certificate for PayPal, and we're not PayPal?

The missing piece

Most CAs didn't explicitly set basicConstraints: CA=False

Whether the field was there or not, most SSL implementations didn't bother to check it.

Anyone with a valid leaf node certificate could create and sign a leaf node certificate for any other domain.

When presented with a complete chain, IE, Outlook, Konqueror, OpenSSL, and others considered it valid...

Back in the day

Microsoft claimed that it was impossible to exploit.

So The Author published the tool that exploits it.

And then in 2002...

SSLsniff detail

User/browser Attacker

https://abc.example.com

SSL/TLS handshake

Application Data

https://abc.example.com

1. Generate a certificate for the site it is connected to2. Sign it with any random valid leaf node certificate.3. Pass that certificate chain to the client.

SSL/TLS handshake

Application Data

1. Get the Data from server2. Encrypt it with our private key3. Send to user

SSLsniff –Null Prefix Attack

Author’s PPT

X509 Certificate Version Serial Number Issuer Validity Subject PublicKey

Signature Algorithm Signature

What's with certificates, anyways?

Identify some subjectsGet the public key

Issue by some Issuer

Issuer Signature

Secrecy - Encryption algorithm Authenticity - Digital Signature Integrity - Checksum

The Big Three

SSL Handshake Beginnings