SSLstrip Stepan Shykerynets 23.03.2013.

SSLstrip

Stepan Shykerynets

http://wisc.org.ua/ 23.03.2013

!Attention


Private / Public key

Public key

Private key

SERVERCLIENT



Public key

Private key

SERVERCLIENT

Public key CA



Public key

Private key

SERVERCLIENT

Public key

CA



Message Public key

Encrypted message

Private key Message


SSL And Certificate Chaining


Certificate

• X509 Certificateversionserial numberissuervaliditysubjectpublic key

• Signature algorithm• Signature


Certificate Chaining

CA CertificateEmbedded in browser.All powerful.Certifies that a site certificate is authentic.

Site CertificateIdentifies a particular URL.Is known to be authentic based on CA Certificate's signature.



CA CertificateEmbedded in browser.All powerful.Certifies that a site certificate is authentic.

Site CertificateIdentifies a particular URL.Is known to be authentic based on CA Certificate's signature.

Intermediate CANot embedded in browser.Still sort of all-powerful.Certifies that a site certificate is authentic.



VeriSign

Facebook.com

Intermediate CA


Certificate Chains Can Be > 3

VeriSign

Facebook.com

Intermediate CA

Intermediate CA


How do we validate these things?

• Verify that the leaf node has the name of the site you're connecting to.

• Verify that the leaf node hasn't expired.• Check the signature.• If the signing certificate is in our list of root

CA's, stop.• Otherwise, move one up the chain and repeat.


Very tempting to use a simple recursive function.

Everyone focuses on the signature validation.

The result of a naive attempt at validation is a chain that is complete, but nothing more.


What if …VeriSign

hack.org

Intermediate CA

Intermediate CA


What if …VeriSign

hack.org

Intermediate CA

Intermediate CA

Facebook.com


What they say :

• Verify that the leaf node has the name of the site you're connecting to.

• Verify that the leaf node hasn't expired.• Check the signature.• If the signing certificate is in our list of root

CA's, stop. • Otherwise, move one up the chain and repeat.


But …

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

trusted


The missing piece


The missing piece

!


The missing piece


• Most CA's didn't explicitly set basic Constraints: CA=FALSE

• A lot of web browsers and other SSL implementations didn't bother to check it, whether the field was there or not

• Any one with a valid leaf node certificate could create and sign a leaf node certificate for any other domain.


But we have one problem


Smart browsers


People are …


People are …LAZY


http://...https://...


SSLsniff


SSLsniff

• Intercept a connection from the client side.

• Generate a certificate for the site it is connecting to.

• Sign in with any random valid leaf node certificate.

• Pass that certificate chain to the client.

• Make normal SSL connection to the server.

• Pass data between client and server, decrypting and encrypting on each end.


Web browsing

• SSL is almost never encountered directly.• It is either encountered as a result of:

A 302 redirect from HTTP URL to an HTTPS URL.An HTTPS link that a user click on from an HTTP page.


SSLstrip

• Watch HTTP traffic go by.• Switch <a href="https://> to <a href="http://> and keep a map of what you've changed.• Switch Location: https:// to Location: http:// and keep a map of what you've changed.


SSLstrip

• Watch HTTP traffic go by.• When we seen an HTTP request for URL that we've stripped, proxy that out as HTTPS to the server.•Watch the HTTPS traffic go by, log everything that we want, and keep a map of all relative, CSS and JS links that go by.


SSLstrip

•The server never knows the difference. Everything looks secure on their end.•The client doesn't display any of the disastrous warnings that we want to avoid.•We see all the traffic.



Let's simplify it…

Time to action


BackTrack : Giving Machine Guns to Monkeys since 2006


Tools

• SSLstrip• ARPspoof• Ettercap


Commands

• #bt echo "1" > /proc/sys/net/ipv4/ip_forward• # bt iptables -t nat -A PREROUTING -p tcp --

destination-port 80 -j REDIRECT --to-port 10000• #bt sslstrip -a -l 10000 -w log.txt –f• # arpspoof -i <yourNetworkdDevice> -t <yourTarget>

<theRoutersIpAddress>• # ettercap -T –q –i <yourNetworkdDevice>


Protection

• Google services• High protection network equipment• Checking site certificates


Stepan Shykerynets

[email protected]

SSLstrip Stepan Shykerynets 23.03.2013.

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