1. Amit Kulkarni February 17, 2004
Web Security
Amit Kulkarni
February 17, 2004

2. SSL / TLS
Developed by Netscape, supported by Microsoft and other developers
TLS is latest version of SSL
Works on top of TCP/IP, below higher-level protocols (e.g. HTTP, ICMP, etc.)
Based on Symmetric / Asymmetric Encryption

3. How SSL Works Using Public Key Cryptography A  B random-message
B  A {random-message}bob’s-private-key
[Alice uses Bob’s public key to decrypt the message and authenticate Bob]

4. How SSL Works (contd.) Handing out public keys A  B hello
B  A Hi, I am Bob, bob’s-certificate
A  B prove it
B  A Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
Certificate is a way to bind public key to a name.
It also has Issuer’s name, expiration date, etc.

5. How SSL Works (contd.)
How Digital Certificate guards against “Man-in-the-middle” attack
A  M hello
M  A Hi, I am Bob, bob’s-certificate
A  M prove it
M  A ???
But, asymmetric key cryptography works slower than symmetric key cryptography. There is a workaround…

6. How SSL Works (contd.) Exchanging a secret A  B hello
B  A Hi, I am Bob, bob’s-certificate
A  B prove it
B  A Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
A  B ok bob, here is a secret {secret}bob’s public-key
B  A {some message}secret-key

7. How SSL Works (contd.) One more loophole A  M hello M  B hello
B  M Hi, I am Bob, bob’s-certificate
M  A Hi, I am Bob, bob’s-certificate
A  M prove it
M  B prove it
B  M Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
M  A Alice, this is Bob
A  M ok bob, here is a secret {secret}bob’s public-key
M  B ok bob, here is a secret {secret}bob’s public-key
B  M {some message}secret-key
B  M Garble[{some message}secret-key]
Note that Alice will try to interpret this garbled message

8. How SSL Works (contd.) Use of Message Authentication Code [MAC]
MAC := Digest[some message, secret key]
A  B hello
B  A Hi, I am Bob, bob’s-certificate
A  B prove it
B  A Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
A  B ok bob, here is a secret {secret}bob’s public-key
B  A {some message, MAC}secret-key
Now Alice can detect the tampering and will stop the conversation!!

9. SSL / TLS Size of key decides strength of encryption.
(40-bit, 56-bit encryptions are considered weak. They can be cracked in about a week)
Private Certificate Authorities: Used when certain kind of trust relation exists between the client and the server
Public Certificate Authorities: Used when no other relation exists between the client and the server

10. HTTPS Secure Hypertext Transfer Protocol
Client requests a secure transaction by accessing an HTTPS URL and informs server about the encryption algorithm and key sizes it supports
Server sends back its digital certificate issued by CA
Client verifies the certificate, generates a session key, encrypts t with server’s public key and sends it back to server
Server decrypts the session key and uses it for symmetric encryption during further communication in the session

11. Instant Messaging Security Issues
IM systems can transport sensitive and confidential data over public networks in unencrypted form.
IM systems do not prevent transportation of files containing viruses
Misconfigured file sharing can vide access to system files, passwords, etc.
To monitor and prevent IM traffic network can be configured to deny access to certain domains (e.g. block yahoo.com for Yahoo messenger)

12. Vulnerabilities of Web Tools
JavaScript
Code embedded in HTML document. Downloaded to and executed on client side.
Security loopholes exist in browsers, which could be exploited by JavaScript code in following ways:
Monitoring Web Browsing
Read System files, passwords
Read browser’s preferences (like id, mail server, etc.)
Most browsers provide patches to fix JavaScript related vulnerabilities.

13. Vulnerabilities of Web Tools (contd.)
ActiveX
This is a Microsoft technology that provides tools for linking desktop applications to WWW content.
e.g. MS Office applications
An attacker could embed harmful macros into MS Office documents, which get executed without client’s knowledge.
These macros could gain access to sensitive information, edit registry settings, use client system to target other systems, etc.
Patches are available to address vulnerabilities exposed by ActiveX.

14. Vulnerabilities of Web Tools (contd.)
Cookies: Text files created by web server, stored on Web surfer’s disk drive
They contain information about user’s personal preferences, authentication information, browsing statistics, etc.
Any Web Server can access all the cookies apart from his own, and thus can gain access to sensitive information.
Browser has option not to accept cookies.

15. Vulnerabilities of Web Tools (contd.)
Buffer Overflow: Can be triggered by sending large amounts of data that receives capacity of receiving application within a given field
2 prerequisite objectives to corrupt receiving buffer and alter program’s control flow to trigger desired action:
Place the necessary code into program’s address space
Direct the application to read and execute embedded code by manipulating registers and memory
In this attacker can gain access to system prompt, examine System variables, read file system, etc.
These are not easy to coordinate
Caused by poor application programming that does not check the size of input field.

16. Vulnerabilities of Web Tools (contd.)
Signed Applets
Unsigned applets operate within a set of restrictions called “Sandbox Model” (e.g. accessing system properties, files on the hard disk, connecting to other web servers, etc.)
Digitally signed applets do not have these restrictions. (Configurable by the user)
Providing digital signature with applet code involvs generating private/ public key pair and obtaining a certificate.

17. Vulnerabilities of Web Tools (contd.)
CGI
Client passes data to the server and requests execution of a script (perl, shell script, etc)
1. Send HTML Form to client
Client
Web Server
2. Fill out form on Client
3. Submit request
4. Execute program on Server

18. Vulnerabilities of Web Tools (contd.)
CGI
Malicious user can modify the parameters to be passed to the executable script.
In this way, user can gain access to sensitive files on the server.
To protect against these, use Intrusion Detection system, create user groups with different permissions to access files, validate security of the CGI scripts before deploying them.

19. Vulnerabilities of Web Tools (contd.)
SMTP Relay
SMTP is standard Internet protocol for global communication.
Simple by design because of high utilization
SMTP relay occurs when a SMTP server has to forward mail to client using some other SMTP server. (e.g. mail from to )
Malicious user can exploit this concept and hide the real origin of the message

20. Vulnerabilities of Web Tools (contd.)
Transaction between 2 SMTP Servers
HELO mail.example.com
250 mail.anotherexample.com Hello mail.example.com
MAIL FROM:
250 Sender ok
RCPT TO:
250 Recipient ok
DATA
354 Enter mail, end with “.” on a line by itself
From:
To: .
250 0AA08757 Message accepted for delivery
A Malicious user can forge variables to hide its identity

21. Vulnerabilities of Web Tools (contd.)
An message is “stamped” by every SMTP server along its way to destination. This could be used to track the real origin of the message.
The following is a typical “Received” header
From
Received: from example.com ([ ]) by mail.anotherexample.com (8.8.5) for
DNS lookup can verify if is same as example.com

22. References How SSL Works SMTP Command Syntax
Fundamentals of Network Security, Canavan John