Copyright © The OWASP Foundation Permission is granted to copy, distribute and/or modify this document under the terms of the OWASP License. The OWASP Foundation OWASP Padding Oracle Attacks Satish B 20/08/2011

OWASP 2 Cryptography Attack

OWASP 3 Agenda  Cryptography Basics  Padding oracle attack  Exploitation  Padding oracle in.NET  Tools  Remedy

OWASP 4 Cryptography Basics Stream Ciphers : Key supplied to encryption algorithm to get key stream Plain text is XOR with key stream to generate cipher text Ex: Rc4 1 0 = 1P 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

OWASP 5 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.

OWASP 6 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)

OWASP 7 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.

OWASP 8 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.

OWASP 9 Padding oracle attack Initially discovered in Credits: Got famous in 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: A28ED4AAC6

OWASP 10 Padding oracle attack Client data value = BRIAN;12;1; IV=7B216A F F851D6CC68FC A28ED4AAC6

OWASP 11 Padding oracle attack Client data A28ED4AAC6

OWASP 12 Padding oracle attack A28ED4AAC6 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 response Invalid ciphertext (improper padding) – Exception Valid ciphertext and decrypts to an invalid value – Custom error Wrong padding can result in:  Error messages  Stack Traces  Time difference  Different responses

OWASP 13 Padding oracle attack oracle refers to a mechanism in cryptography that can be used to determine whether a test has passed or failed. Pass and Fail conditions can be used to decrypt without key. Decrypting without a key Valid cipher UID=7B216A F F851D6CC68FC9537 Invalid cipher UID= F851D6CC68FC9537

OWASP 14 Padding oracle attack Invalid cipher

OWASP 15 Padding oracle attack Invalid cipher UID= F851D6CC68FC9537 Intermediary Byte ^ 0×3C == 0×01, Intermediary Byte == 0×3C ^ 0×01, Intermediary Byte == 0×3D

OWASP 16 Padding oracle attack Valid cipher Plain text == Intermediary byte 0×3D ^ corresponding IV byte 0F = = 02 Now crack the 7 th byte and so on … In the end it gives Intermediate value

OWASP 17 Padding oracle attack Encrypting arbitrary values without key XOR the plaintext value with intermediary value to get IV Summary Padding oracle attack allows to encrypt and decrypt data without the key.

OWASP 18 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

OWASP 19 Padding oracle attack in.NET 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.

OWASP 20 Tools  Padbuster  Poet  Padbusterdotnet

OWASP 21 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 : After fix: adfadfdfdfe_aereradf349jkMjlrfgRr6moijfdn_Aretkjf093rpA2

OWASP 22 For more information on exploitation and usage of tools visit my site References oracle.html update-now-available.aspx relates-to.html attacks-with-padbuster/

OWASP 23 Questions?