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OWASP

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Padding Oracle Attacks

Satish BSatishb3@securitylearn.net

20/08/2011

OWASP 2

Cryptography Attack

OWASP 3

Agenda

Cryptography Basics Padding oracle attack Exploitation Padding oracle in .NET Tools Remedy

OWASP4

Cryptography Basics

Stream Ciphers :Key supplied to encryption algorithm to get key streamPlain text is XOR with key stream to generate cipher textEx: Rc4

1 0 = 1 P K = C

0 1 = 1 C P = K

Block Ciphers: Operates on fixed length group of bits or bytes (64 or 128 bit blocks)

128 bits of plain text is converted into 128 bits of cipher text Ex: AES

OWASP5

Cryptography Basics

Block Cipher : Modes

ECB mode – Electronic code book mode

Encryption of the same plain text with the same key results in the same cipher text, which is a considerable threat to security.

OWASP6

Cryptography Basics

CBC – cipher block chaining

Encryption of the same plain text with the same key results in different cipher text because of IV.Each block of plaintext is XORed with the previous ciphertext block before being encrypted.

Ci = Ek (Pi xor Ci-1)

OWASP7

Cryptography Basics

Each block of ciphertext is decrypted and XORed with the previous ciphertext block to obtain the plain text.First block of ciphertext is decrypted and XORed with IV to obtain the plain text.

OWASP8

Cryptography Basics

Block Ciphers – - Works on fixed size data - Messages are in variety of length - padding has introduced - Final block padded before encryption

PKCS#5 standard - final block of plaintext is padded with N bytes of value N.

OWASP9

Padding oracle attack

Initially discovered in 2002. Credits: http://netifera.com/research/Got famous in 2010.

What is it ? Possible to decrypt and encrypt data without key in CBC mode. Typical Scenario:

Brian logged into myapp.com Server created an encryption string specific to Brian and sent to him Accessing any page in the application sends the encrypted value to server Server decrypts and serves the content based on decrypted value

Ex:http://myapp.com/home.jsp?UID=7B216A634951170FF851D6CC68FC9537858795A28ED4AAC6

OWASP10

Padding oracle attack

Client datavalue = BRIAN;12;1;

IV=7B216A634951170FF851D6CC68FC9537858795A28ED4AAC6

OWASP11

Padding oracle attack

Client datahttp://myapp.com/home.jsp?UID=7B216A634951170FF851D6CC68FC9537858795A28ED4AAC6

OWASP12

Padding oracle attack

http://myapp.com/home.jsp?UID=7B216A634951170FF851D6CC68FC9537858795A28ED4AAC6

The application verifies whether the encrypted value is properly padded or not.

When the application passed an encrypted value it responds with one of three ways:Valid ciphertext (with proper padding) – Normal responseInvalid ciphertext (improper padding) – ExceptionValid ciphertext and decrypts to an invalid value – Custom error

Wrong padding can result in: Error messages Stack Traces Time difference Different responses

OWASP13

Padding oracle attack

oracle refers to a mechanism in cryptography that can be used to determine whethera test has passed or failed.

Pass and Fail conditions can be used to decrypt without key.

Decrypting without a key

Valid cipher http://myapp/home.jsp? UID=7B216A634951170FF851D6CC68FC9537

Invalid cipherhttp://myapp/home.jsp?UID=0000000000000000F851D6CC68FC9537

OWASP14

Padding oracle attack

Invalid cipherhttp://myapp/home.jsp?UID=0000000000000001F851D6CC68FC9537

OWASP15

Padding oracle attack

Invalid cipherhttp://myapp/home.jsp?UID=0000000000000003F851D6CC68FC9537

Intermediary Byte ^ 0×3C == 0×01,Intermediary Byte == 0×3C ^ 0×01,Intermediary Byte == 0×3D

OWASP16

Padding oracle attack

Valid cipher http://myapp/home.jsp?UID=7B216A634951170FF851D6CC68FC9537

Plain text == Intermediary byte 0×3D ^ corresponding IV byte 0F = = 02

Now crack the 7th byte and so on …

In the end it gives Intermediate value

OWASP17

Padding oracle attack

Encrypting arbitrary values without key

XOR the plaintext value with intermediary value to get IV

SummaryPadding oracle attack allows to encrypt and decrypt data without the key.

OWASP18

Padding oracle attack in .NET

Where is it applicable ?

View state

Session cookies

Any encrypted data in hidden parameters

WebResource.axd - serves embedded resources

ScriptResource.axd - serves embedded resources and files

OWASP19

Padding oracle attack in .NET

https://samplesite.com/WebResource.axd?d=llIAeUHrAWkUZEuvZB-98g2

valid cipher text decrypted to valid value - proper response (200 ok)

valid cipher text decrypted to invalid value - page not found or similar response (404)

Invalid cipher text - padding error

If the application gives different errors in the above 3 cases, it is vulnerable and easy to exploit.

OWASP20

Tools

Padbuster https://www.gdssecurity.com/l/t/d.php?k=PadBuster

Poethttp://netifera.com/research/

Padbusterdotnethttp://www.mindedsecurity.com/fileshare/padBusterdotnet.zip

OWASP21

Padding oracle attack

Why Is this working?

CBC mode only provides confidentiality. Confidentiality doe not ensure that the value is tampered or not. Integrity check has to be performed.

Solution

Implemented integrity check by adding hash to the encrypted value

Before fix :http://website.com/application/WebResource.axd?d=jzjghMVYzFihd9Uhe_arpA2

After fix:http://website.com/application/WebResource.axd?d=jadfz0GadfahafMVYzFihd9Uhadfadfdfdfe_aereradf349jkMjlrfgRr6moijfdn_Aretkjf093rpA2

OWASP22

For more information on exploitation and usage of tools visit my site

http://www.securitylearn.net

References

http://www.troyhunt.com/2010/09/fear-uncertainty-and-and-padding-oracle.html

http://weblogs.asp.net/scottgu/archive/2010/09/28/asp-net-security-update-now-available.aspx

http://eglasius.blogspot.com/2010/09/aspnet-padding-oracle-how-it-relates-to.html

http://www.gdssecurity.com/l/b/2010/09/14/automated-padding-oracle-attacks-with-padbuster/

OWASP23

Questions?