Transcript Web Security - Texas Tech University

Web Security
Amit Kulkarni
February 17, 2004
SSL / TLS
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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
How SSL Works
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Using Public Key Cryptography
AB
BA
random-message
{random-message}bob’s-private-key
[Alice uses Bob’s public key to decrypt the message and
authenticate Bob]
How SSL Works (contd.)
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Handing out public keys
AB
BA
AB
BA
hello
Hi, I am Bob, bob’s-certificate
prove it
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.
How SSL Works (contd.)
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How Digital Certificate guards against “Man-inthe-middle” attack
AM
MA
AM
MA
hello
Hi, I am Bob, bob’s-certificate
prove it
???
But, asymmetric key cryptography works slower than symmetric
key cryptography. There is a workaround…
How SSL Works (contd.)
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Exchanging a secret
AB
BA
AB
BA
AB
BA
hello
Hi, I am Bob, bob’s-certificate
prove it
Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
ok bob, here is a secret {secret}bob’s public-key
{some message}secret-key
How SSL Works (contd.)
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One more loophole
AM
MB
BM
MA
AM
MB
BM
hello
hello
Hi, I am Bob, bob’s-certificate
Hi, I am Bob, bob’s-certificate
prove it
prove it
Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
MA
Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
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
How SSL Works (contd.)
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Use of Message Authentication Code [MAC]
MAC := Digest[some message, secret key]
AB
BA
AB
BA
AB
BA
hello
Hi, I am Bob, bob’s-certificate
prove it
Alice, this is Bob
{digest[Alice, This is Bob]}bob’s-private-key
ok bob, here is a secret {secret}bob’s public-key
{some message, MAC}secret-key
Now Alice can detect the tampering and will stop the conversation!!
SSL / TLS
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Size of key decides strength of encryption.
(40-bit, 56-bit encryptions are considered weak. They can be cracked in
about a week)
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Private Certificate Authorities: Used when certain kind
of trust relation exists between the client and the server
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Public Certificate Authorities: Used when no other
relation exists between the client and the server
HTTPS
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Secure Hypertext Transfer Protocol
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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
Instant Messaging
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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)
Vulnerabilities of Web Tools
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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:
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Monitoring Web Browsing
 Read System files, passwords
 Read browser’s preferences (like email id, mail server, etc.)
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Most browsers provide patches to fix JavaScript
related vulnerabilities.
Vulnerabilities of Web Tools (contd.)
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ActiveX
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This is a Microsoft technology that provides tools for linking
desktop applications to WWW content.
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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.
Vulnerabilities of Web Tools (contd.)
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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.
Vulnerabilities of Web Tools (contd.)
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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:
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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.
Vulnerabilities of Web Tools (contd.)
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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.
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Vulnerabilities of Web Tools (contd.)
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CGI
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Client passes data to the server and requests
execution of a script (perl, shell script, etc)
1. Send HTML Form to client
2. Fill out form on Client
Web Server
3. Submit request
4. Execute program on
Server
Client
Vulnerabilities of Web Tools (contd.)
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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.
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Vulnerabilities of Web Tools (contd.)
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SMTP Relay
SMTP is standard Internet protocol for global e-mail
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 [email protected] to
[email protected] )
 Malicious user can exploit this concept and hide the
real origin of the message
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Vulnerabilities of Web Tools (contd.)
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Transaction between 2 SMTP Servers
HELO mail.example.com
250 mail.anotherexample.com Hello mail.example.com
MAIL FROM: [email protected]
250 [email protected] Sender ok
RCPT TO: [email protected]
250 [email protected] 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
Vulnerabilities of Web Tools (contd.)
An email 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 [email protected]
Received: from example.com ([172.16.35.44]) by
mail.anotherexample.com (8.8.5) for
<[email protected]>
DNS lookup can verify if 172.16.35.44 is same as
example.com
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References
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How SSL Works
SMTP Command Syntax
Fundamentals of Network Security, Canavan
John
http://www.howstuffworks.com