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Padding Oracle Attacks

Padding Oracle Attacks. Satish B Securitylearn.wordpress@gmail.com. 20/08/2011. Cryptography Attack. Agenda. Cryptography Basics Padding oracle attack Exploitation Padding oracle in .NET Tools Remedy. Cryptography Basics.

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Padding Oracle Attacks

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  1. Padding Oracle Attacks Satish B Securitylearn.wordpress@gmail.com 20/08/2011

  2. Cryptography Attack

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

  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 = 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

  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.

  6. Cryptography Basics CBC – cipher block chaining Ci = Ek (Pi xor Ci-1) 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.

  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.

  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.

  9. 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=7B216A634951170FF851D6CC68FC953785 • 8795A28ED4AAC6

  10. Padding oracle attack Server Client datavalue = BRIAN;12;1; Encryption IV=7B216A634951170F F851D6CC68FC9537 858795A28ED4AAC6

  11. Padding oracle attack Server Client datahttp://myapp.com/home.jsp?UID=7B216A634951170FF851D6CC68FC9537858795A28ED4AAC6 Decryption

  12. 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 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

  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 keyValid cipher http://myapp/home.jsp? UID=7B216A634951170FF851D6CC68FC9537 Invalid cipherhttp://myapp/home.jsp?UID=0000000000000000F851D6CC68FC9537

  14. Padding oracle attack Invalid cipher http://myapp/home.jsp?UID=0000000000000001F851D6CC68FC9537

  15. 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

  16. Padding oracle attack Valid cipher http://myapp/home.jsp?UID=7B216A634951170FF851D6CC68FC9537 Plain text == Intermediarybyte 0×3D ^ corresponding IV byte 0F = = 02 Now crack the 7th byte and so on … In the end it gives Intermediate value

  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.

  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

  19. 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.

  20. Tools • Padbuster https://www.gdssecurity.com/l/t/d.php?k=PadBuster • Poet http://netifera.com/research/ • Padbusterdotnet http://www.mindedsecurity.com/fileshare/padBusterdotnet.zip

  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 :http://website.com/application/WebResource.axd?d=jzjghMVYzFihd9Uhe_arpA2After fix: http://website.com/application/WebResource.axd?d=jadfz0GadfahafMVYzFihd9Uh adfadfdfdfe_aereradf349jkMjlrfgRr6moijfdn_Aretkjf093rpA2

  22. For more information on exploitation and usage of tools visit my site http://Securitylearn.wordpress.com 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/

  23. Questions?

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